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4  -v 


A    TEXT-BOOK    OF 

GEOLOGY. 

FOR  USE  IN  MINING  SCHOOLS,  COLLEGES, 
AND  SECONDARY  SCHOOLS. 


BY 


JAMES   PARK, 


PROFESSOR  OP  MINING  IN  THE  UNIVERSITY  OF  OTAGO  ; 

DIRECTOR  OF  THE  OTAGO  SCHOOL  OF  MINES  ; 
CORRESPONDING  MEMBER  OF  COUNCIL  OF  THE  INSTITUTION  OF  MINING  AND  METALLURGY 

FELLOW  OF  THE  GEOLOGICAL  SOCIETY  OF  LONDON; 

LATE  PRESIDENT  OF  THE  NEW  ZEALAND  INSTITUTE  OF  MINING  ENGINEERS; 
SOMETIME  GEOLOGIST  IN  N.Z.  GEOLOGICAL  SURVEY. 


TOtb  ^frontispiece,  70  plates,  ant)  264  otber 
illustrations. 


LONDON: 

CHARLES   GRIFFIN  &   COMPANY,   LIMITED, 

EXETER   STREET,    STRAND 

1914 

[All  Rights  Reserved.] 


PREFACE. 

THIS  volume  comprises  a  systematic  course  of  lectures  carefully 
revised  and  expanded  so  as  to  cover  the  requirements  in  Geology 
as  now  defined  for  Engineering,  Mining,  and  Agricultural  Schools 
and  Colleges. 

The  first  principles  of  Geology  are,  so  to  speak,  the  ABC  of  the 
science.  They  are  mostly  based  on  the  simple  processes  that  are 
now  going  on  around  us  ;  and  when  we  know  Man  we  are  able  to 
read  the  story  of  the  earth  wherever  we  may  chance  to  find  our- 
selves. But  the  student  who  confines  his  observations  and  studies 
to  his  own  immediate  neighbourhood  is  in  danger  of  acquiring  a 
false  sense  of  proportion,  and  may  in  time  unconsciously  come  to 
believe  that  the  things  and  processes  he  sees  in  his  own  terrain  are 
typical  of  the  whole  globe.  When  he  afterwards  comes  to  travel 
further  afield  he  may  find  himself  compelled  to  modify  his  standards 
and  renounce  many  of  his  early  conceptions.  The  corrective  of 
local  prejudices  and  a  narrow  horizon  is  extensive  reading  and  still 
more  extensive  travel. 

A  word  to  the  student.  Make  an  early  acquaintance  with  the 
facts  of  geology  as  presented  in  the  field.  Miss  no  chance  of  travel 
or  exploration  with  the  experienced  geologist.  Cultivate  the 
faculty  of  exact  observation,  and  be  mindful  not  to  form  hasty 
conclusions.  Remember  that  things  are  not  always  what  they 
seem.  Make  extensive  collections  of  rocks  and  fossils,  and  be 
careful  to  keep  the  fossils  of  each  horizon  apart.  Take  every 
care  to  preserve  your  fossils  from  injury.  The  dictum  of  the  Hon. 
Walter  Mantell  that  "  a  fossil  that  is  worth  collecting  is  worth 
its  paper  "  is  as  true  now  as  when  made  to  the  Author  forty  years 
ago,  and  applies  equally  well  to  rock  specimens.  Do  not  attempt 
to  describe  new  species  ;  leave  that  to  the  specialist.  Even  when 
you  have  gained  some  note  as  a  writer  on  geology,  refrain  from 
coining  new  terms. 

The  scientific  study  of  scenery  is  one  of  the  most  fascinating 
branches  of  geology.  It  embraces  the  morphological  description  of 
the  surface  features,  and  the  investigation  of  the  causes  which  have 
brought  these  forms  about.  To  be  successful  in  diagnosis  you 


VI  A    TEXT-BOOK    OF    GEOLOGY. 

need  to  be  equipped  with  a  good  knowledge  of  geological  causation. 
In  the  review  of  the  topography  of  a  given  terrain,  always  bear  in 
mind  that  every  natural  feature  is  the  result  of  some  definite 
happening  or  combination  of  happenings.  Take  the  familiar 
crags  and  hollows,  peaks  and  valleys,  headlands  and  bays.  These 
owe  their  existence  for  the  most  part  to  the  relative  hardness  of  the 
rocks  in  which  they  are  carved.  Here,  again,  beware  of  forming 
hasty  conclusions,  and  remember  that  circumstances  may  alter 
cases.  Coastal  embayments  are  not  invariably  due  to  projecting 
headlands  of  hard  rock.  The  spacious  and  beautiful  Golden  Bay 
in  North  Nelson,  New  Zealand,  is  guarded  on  one  side  by  a  narrow 
barrier  of  loose  sand  twenty  miles  long,  against  which  the  fury  of 
the  cyclonic  gales  of  the  South  Pacific  beats  unavailingly.  Long 
lines  of  escarpment  are  not  necessarily  the  result  of  faulting. 
Invariably  they  are  the  result  of  uplift  of  sedimentary  strata  of 
varying  degrees  of  hardness,  followed  by  denudation,  and  the 
excavation  of  longitudinal  hollows  along  the  softer  zones  of  rock. 
The  mere  circumstance  that  the  prominent  ridges  and  peaks  of  a 
mountain  complex  are  nearly  of  uniform  height  cannot  be  taken  as 
primdfdcie  evidence  that  these  ridges  and  peaks  are  the  remnants 
of  an  ancient  peneplain.  When  you  are  tempted  to  invoke  the  aid 
of  a  dissected  peneplain,  consider  the  effect  that  may  be  produced 
when  a  great  succession  of  sedimentary  rocks  of  alternating  hard 
and  soft  strata  are  thrown  into  a  series  of  isoclinal  folds,  the  arches 
of  which  are  truncated  and  cut  by  deep  transverse  gorges  and  wide 
longitudinal  valleys.  Since  the  rocks  in  each  fold  were  originally 
uplifted  to  the  same  height  and  offer  the  same  resistance  to  denuda- 
tion, the  harder  bands  of  rock  will  obviously  form  a  series  of  more 
or  less  parallel  ranges,  the  prominent  peaks  of  which  will  stand  at 
about  the  same  height. 

Do  not  select  a  single  specimen  of  a  rock  for  analysis  and  regard 
it  as  representative  of  the  whole  mass,  as  the  results  may  be  alto- 
gether misleading.  All  sedimentary  and  igneous  rocks  vary 
considerably  in  composition  in  different  parts,  the  former  because 
they  are  mechanically  formed,  the  latter  on  account  of  the  develop- 
ment of  large  phenocrysts  or  the  proximity  of  sedimentary  or 
other  rocks.  Select  fresh  unweathered  examples  whenever 
procurable,  and,  except  it  be  an  analysis  of  a  particular  fragment 
that  is  required  as  an  aid  to  microscopical  examination,  select  the 
average  sample  or  samples  with  as  much  laborious  care  as  you 
would  sample  a  coal-seam  or  an  ore- vein.  Unless  you  are  a 
specialist,  which  is  unlikely,  do  not  attempt  the  analysis  of  your 
rock  samples  ;  do  not  think  that  because  you  have  waded  through 
the  analysis  of  a  rock  specimen  or  two  in  your  graduate  course  that 
you  are  competent  to  undertake  the  systematic  analysis  of  a  rock. 


PREFACE.  V.li 

Rock  analysis  is  work  calling  for  special  skill  and  great  experience. 
But  the  greatest  skill  in  the  laboratory  may  be  stultified  by  negligent 
sampling  in  the  field  ;  and,  conversely,  the  most  painstaking  work 
in  the  field  may  be  rendered  worthless  by  lack  of  experience  in  the 
laboratory. 

I  desire  to  acknowledge  my  indebtedness  to  the  Director  of  the 
Geological  Survey  of  the  United  States  for  permission  to  utilise  the 
illustrations  of  the  Survey's  publications,  a  privilege  of  which  I 
have  fully  availed  myself  ;  to  Dr  Tempest  Anderson  for  the  use  of 
Plate  XVI.  and  Figure  123  ;  to  Mr  E.  F.  Pittman,  Government 
Geologist  for  New  South  Wales,  for  the  use  of  Plate  XXXVIIlA.  ; 
and  to  my  Publishers,  who  have  courteously  placed  at  my  disposal 
many  of  the  figures  scattered  throughout  the  text,  as  well  as  the 
beautiful  plates  of  fossils  illustrating  Chapters  XXII.  to  XXXIII. 


JAMES   PARK. 


UNIVERSITY  OF  OTAGO,  DUNEDIN,  N.Z., 
January  1914. 


CONTENTS 


PART  I. 


PAGE 

CHAPTER   I. 


SOME  FIRST  PRINCIPLES 


CHAPTEE  II. 
THE  SCOPE  OF  GEOLOGY 13 

CHAPTER   III. 
THE  DENUDATION  OF  THE  LAND  .> 15 

CHAPTER   IV. 
THE  WORK  OF  STREAMS  AND  RIVERS 30 

CHAPTER  V. 
SNOW  AND  GLACIERS 54 

CHAPTER  VI. 

THE  GEOLOGICAL  WORK  OF  THE  SEA  .....      79 

CHAPTER  VII. 

ROCK-BUILDING Ill 

CHAPTER  VIII. 

ROCK  STRUCTURES 125 

ix 


X  A    TEXT-BOOK    OF    GEOLOGY. 

PAGE 

CHAPTER   IX. 
EARTH-MOVEMENTS 132 

CHAPTER   X. 
JOINTS,  FAULTS,  CLEAVAGE 151 

CHAPTER   XL 
COMPOSITION  OF  EARTH'S  CRUST  ......     174 

CHAPTER   XII. 

ROCK-FORMING  MINERALS      .        ...        •        .        .        ,     188 

CHAPTER  XIII. 
SEDIMENTARY  ROCKS     .        ...        .        .        .        .     199 

CHAPTER   XIV. 
VOLCANOES  AND  VOLCANIC  ACTION       .        .        '.        .        .219 

CHAPTER   XV. 

IGNEOUS  ROCKS     .        .        .        .        ;        .        .        .        .     236 

CHAPTER   XVI. 
PLUTONIC,  HYPABYSSAL,  AND  VOLCANIC  ROCKS  .        .        .    248 

CHAPTER   XVII. 
METAMORPHISM  AND  METAMORPHIC  ROCKS  ....     259 

CHAPTER  XVIII. 

FOSSILS  :    THEIR  OCCURRENCE,  PRESERVATION,  CLASSIFICA- 
TION, AND  USES     .        .        .     .    .        .        .  ;.  f.    268 

CHAPTER   XIX. 

CONFORMITY  AND  UNCONFORMITY      ...  .        .        .     291 


CONTENTS.  XI 

PART   II. 

PAGE 

CHAPTER   XX. 

HISTORY  OF  THE  EARTH        .  300 

CHAPTER   XXI. 
Eozoic  ERA .        .306 

CHAPTER   XXII. 
PALAEOZOIC  ERA .    317 

CHAPTER   XXIII. 

ORDOVICIAN  SYSTEM      .        .        .        .  .        .        .     325 

CHAPTER   XXIV. 

SILURIAN  SYSTEM ,    333 

CHAPTER   XXV. 
DEVONIAN  SYSTEM 342 

CHAPTER   XXVI. 
CARBONIFEROUS  SYSTEM 349 

CHAPTER   XXVII. 
PERMIAN  SYSTEM 363 

CHAPTER   XXVIII. 

MESOZOIC  ERA  :    TRIASSIC  SYSTEM 372 

CHAPTER   XXIX. 
JURASSIC  SYSTEM .    386 

CHAPTER   XXX. 

CRETACEOUS  SYSTEM  ....    405 


Xll  A    TEXT-BOOK    OF    GEOLOGY. 

PAGE 

CHAPTER   XXXI. 
CAINOZOIC  OR  TERTIARY  ERA 428 

CHAPTER   XXXII. 
MIOCENE  AND  PLIOCENE 447 

CHAPTER  XXXIII. 
PLEISTOCENE  AND  RECENT 461 

CHAPTER   XXXIV. 

DEVELOPMENT  OF  SURFACE  FEATURES  485 


PART  III. 

CHAPTER   XXXV. 

ECONOMIC  GEOLOGY       .        .        .        .        .        .        .        .    504 

CHAPTER   XXXVI. 
ELEMENTS  OF  FIELD  GEOLOGY  AND  GEOLOGICAL  SURVEYING    537 

APPENDICES   .        .        .        .        .        .        .  .  .    551 

BIBLIOGRAPHY        .  .        .  .        .        .        .     555 

INDEX    .      '  .        .        .  559 


LIST  OF  PLATES. 


Grand  Canon  of  Colorado — looking  east         .  .  Frontispiece 

PLATE  TO  FACE  PAGE 

I.  A.  Wind-ripples  in  Coarse  Sand,  Cromwell  Dunes     .  .       18 

B.  Wandering  Dune,  showing  Ridge               .             .  .18 

II.  Rock-erosion,  Reef  Point,  North- West  Auckland      .  .       20 

III.  Cross-bedded  Sandstone  at  Red  Buttes,  Wyoming    .  .       20 
Pulpit  Rock— Erosion  by  Wind-borne  Sand  .            .  .20 

IV.  Sand-worn  Pebbles  of  Augite-andesite           .             .  .20 
V.  Horizontal  Joints  and  Concentric  Weathering  in  Granite  .       22 

VI.  Spheroidal  Weathering  of  Ordovician  Chert,  Missouri  .       22 

VII.  Mafeking  Grotto,  Jenolan  Caves,  New  South  Wales  .       26 

VIII.  Alluvial  Cones                          .            .            .            .  .51 

IX,  Glaciated  Rock  Surfaces         .            .            .            .  .66 

X.  A.  Showing  Dissection  of  Alluvial  Plain       .            .  .70 
B.  Hanging  Valley     ......       70 

XI.  Sea-Cliff  of  Turner  Glacier,  Alaska     .            .            .  .84 

XII.  Ripple-marks  in  Burke  Formation  on  Tiger  Peak     .  .     120 

XIII.  A.  Horizontal  Pliocene  Strata,  New  Zealand            .  .     122 
B.  Inclined  Strata       .             .             .             .             .  .122 

XIV.  Raised  Wave-cut  Marine  Terrace       .            .            .  .132 
XV.  Quartzite  and  Chert  Breccia— Utah  .            .            .  .200 

XVI.  White  Island,  New  Zealand   .            .            .            .  .226 

XVII.  Cloud  Forms— Eruption  of  Colimo  Volcano,  Mexico,  1910  .     231 

XVIII.  A.  Diorite,  Yellowstone  National  Park          .            .  .252 

B.  Diorite-porphyry  with  Phenocrysts  of  Plagioclase  .     252 

XIX.  Cambrian  Fossils        .            .            .            .            .  .320 

XX.  Syncline  of  Ordovician  Slate,  Vermont          .             .  .     327 

XXI.  Puckered  Mica-Schist  from  Taconic  Range   .            .  .327 

XXII.  Schist  Conglomerate,  Michigan  ....     327 

XXIII.  Ordovician  Fossils  .     328 


XIV  A    TEXT-BOOK    OF    GEOLOGY. 

PLATE  TO   FACE   PAGE 

XXIV.  Ordovician  Fossil  Crustacea   .                         .  .328 

XXV.  Ordovician  and  Silurian  Fossils          .             .  .  .328 

XXVI.  Silurian  Fossils            .             .            .             .  .  .336 

XXVII.  Silurian  Fossils            .            .             .            .  .  .340 

XXVIII.  Silurian  Fossils            .             .            .            .  .  .340 

XXIX.  Devonian  Fossils         .            .            .            .  .  .345 

XXX.  Devonian  Fossils         .            .             .             .  .  .346 

XXXI.  Devonian  and  Carboniferous  Fossils  ....     346 

XXXII.  Fossils  of  the  Carboniferous  Limestone         .  .  .350 

XXXIII.  Carboniferous  Fossils 350 

XXXIV.  Carboniferous  Fossils .            .            .  .  .350 
XXXV.  Fossils  of  the  Carboniferous  Limestone         .  .  .350 

XXXVI.  Fossils  of  the  Carboniferous  Coal-Measures  .  .  .350 

XXXVII.  Representative  Types  of  Glossopteris  Flora  .  .  .366 

XXXVIII.  Permian  Fossils          .            .            .            .  .  .366 

XXXVIIlA.  Dolerite  Dyke,  Nobbys,  N.S.W.         .  .  .370 

XXXVIIlB.  Typical  Triassic  Fossils  of  Alpine  Facies       .  .  .380 

XXXIX.  Fossils  of  the  Lower  and  Middle  Lias        -    .  .  .390 

XL.  Fossils  of  the  Lower  and  Middle  Lias            .  .     392 

XLI.  Fossils  of  the  Lower  and  Middle  Lias            .  ..   "         .     392 

XLII.  Characteristic  Jurassic  Ammonites   .             .  .  .     392 

XLIII.  Jurassic  Ammonites  and  Structural  Parts     .  .  .     392 

XLIV.  Lower  Jurassic  Fossils           .            .            .  .  .393 

XLV.  Lower  Jurassic  Fossils.     (Inferior  Oolite)      .  .  .394 

XLVI.  Middle  Jurassic  Fossils           .            .            .  .  .397 

XL VII.  Upper  Jurassic  Fossils            .            .            .  .397 

XLVIII.  Characteristic  Cretaceous  Ammonites            .  .  .     408 

XLIX.  Purbeck  and  Wealden  Fossils            .             .  _ .  .410 

L.  Cretaceous  Fossils.     (Neocomian)     .            .  .  .411 

LI.  Cretaceous  Fossils.     (Various)          .            .  .  .411 

LII.  Cretaceous  Fossils.     ( Upper  Greensand  and  Chalk) .  .     413 

LIII.  Cretaceous  Fossils.     (Chalk)              .             .  .  .417 

LIV.  Restoration  of  Triceratops  Prorsus.     Cretaceous,  Wyoming     421 

LV.  Eocene  Fossils            .             .             .            .  .  .436 

LVI.  Eocene  Fossils            .             .             .             .  .  .439 

LVII.  Oligocene  Fossils 443 

LVIII.  Foraminifera.     Waitemata  Beds,  N.Z.     Lower  Tertiary     .     446 


LIST    OF   PLATES.  XV 

PLATE  TO   FACE  PAGE 

LIX.  Fossil  Fish  from  the  Esmeralda  Formation,  Miocene  .     450 

LX.  Miocene  Gasteropoda  .....     451 

LXI.  Miocene  Lamellibranchs         .....     451 
LXII.  Pliocene  Fossils.     (Crag)       .  .  .  .  .456 

LXIII.  Section  of  Glacial  Drift.     New  Jersey          .  .  .473 

LXIV.  Mastodon  Americanus  .....     473 

LXV.  Post-Pliocene  Fossils  .  ...     474 

LXVI.  Palaeolithic  Implements         .  .     478 

LXVII.  Hills  carved  from  Cretaceous  Beds  .  .  .  .488 

LXVIII.  Representation  of  Willis'  Experiments — Artificial  Produc- 
tion of  Mountain  Folds  .....     489 
LXIX.  Showing  Effects  of  Subaerial  Erosion  in  Arid  Regions         .     493 
LXX.  Cambrian  Green  Slate  Quarry  ....     509 


A  TEXT-BOOK  OF  GEOLOGY, 


PART  I. 


CHAPTEE   I. 
SOME  FIRST  PRINCIPLES. 

BEFORE  proceeding  with  the  detailed  study  of  the  crust  of  the 
Earth  and  the  processes  which  have  modified  its  surface,  it  is 
necessary  as  a  first  step  to  take  a  bird's-eye  view  of  the  whole 
history  of  the  globe  from  its  first  beginning  up  to  the  present  time. 
By  pursuing  this  course  we  shall  acquire  a  better  understanding 
of  the  facts  subsequently  presented  ;  for  it  is  obvious  that  when 
the  ground-plan  of  the  structure,  so  to  speak,  has  been  reviewed 
and  intelligently  grasped,  the  filling  in  of  the  details  will  be  a 
matter  of  comparative  ease. 

The  Earth  when  viewed  in  its  widest  sense  is  found  to  consist 
of  three  concentric  envelopes,  namely  (1)  the  Atmosphere',1  (2) 
the  Hydrosphere ; 2  and  (3)  the  Lithosphere.3 

The  Atmosphere  is  the  outer  gaseous  envelope,  the  Hydrosphere 
the  watery  envelope,  and  the  Lithosphere  the  solid  rocky  crust  on 
which  we  live. 

The  central  core  enclosed  by  the  Lithosphere  is  called  the  Bary- 
sphere  *  from  its  supposed  greater  density. 

.  The  water-surface  of  the  globe  comprises  about  145  million 
square  miles,  and  the  land-surface  about  52  million  square  miles. 

GENERAL   OUTLINE   OF   GEOLOGICAL   HISTORY. 

The  Purpose  of  Geology. — Geology  is  the  science  which  deals 
with  the  materials  forming  the  crust  or  outer  shell  of  the  globe. 

1  Gr.  atmos  =  vapour,  and  sphaira  =  a,  sphere. 

2  Gr.  hudor  =  water,  and  sphaira. 

3  Gr.  Uthos=&  stone,  and  sphaira. 

4  Gr.  barus= heavy,  and  sphaira. 

1  1 


%  *''         lJtVv-.       A   TEXT-BOOK   OF   GEOLOGY. 

It  is  thus  a  science  of  observation  with,  a  laboratory  embracing  the 
open  field  and  mountain  slope,  the  river- valley  and  rocky  strand. 

From  a  study  of  the  conditions  that  govern  the  deposition  or 
formation  of  sediments  in  our  existing  seas  and  lakes,  and  from 
a  knowledge  of  the  habits  and  environment  of  the  animals  and 
plants  that  now  people  and  clothe  the  Earth,  the  geologist  attempts 
to  follow  the  orderly  succession  of  the  conditions  that  existed  in 
bygone  ages.  For  the  construction  of  this  mental  picture  he 
mainly  relies  on  the  structure  and  composition  of  the  rocks,  and 
onvthe  fossil  remains  of  the  plants  and  animals  which  the  rocks 
enclose. 

In  other  words,  the  geologist  applies  the  present  to  read  the  past, 
and  in  doing  so  he  is  surely  on  safe  ground,  for  the  present  is  merely 
a  continuance  of  the  past.  He  recognises  that  the  same  air  and 
the  same  precipitation  of  watery  vapour  in  the  form  of  rain  have 
existed  since  the  beginning  of  geological  time,  that  water  in  the 
form  of  running  streams  has  always  played  a  dominant  rdle  in 
wearing  away  the  solid  land,  and  that  seas  and  lakes  as  occupying 
the  hollows  and  depressions  have  always,  as  now,  been  the  places 
where  the  rocky  detritus  carried  down  by  the  streams  has  been 
sorted  and  spread  out. 

The  Origin  of  the  Earth. — The  Earth  is  a  planet  belonging  to 
our  solar  system.  From  the  researches  of  the  astronomer  we  learn 
that  many  of  the  so-called  fixed  stars  are  suns,  each  moving  in  its 
own  orbit,  and  each,  like  our  own  Sun,  attended  by  a  system  of 
dark  satellites  or  planets. 

An  investigation  of  the  heavenly  bodies  has  shown  that  some 
exist  in  the  form  of  intensely  heated  incandescent  gases,  some 
as  globular  masses  of  highly  heated  glowing  liquid  matter,  and 
others,  like  our  own  Earth,  as  dark  solid  bodies.  According  to 
the  nebular  hypothesis,  it  is  believed  that  the  solid  bodies  were 
at  one  time  masses  of  incandescent  gases,  and  that  they  became 
first  liquid  and  then  solid  through  the  radiation  of  their  heat 
into  space. 

When  the  heated  globular  body  had  become  sufficiently  cool,  a 
solid  crust,  at  first  thin  and  brittle,  formed  on  the  surface.  As  the 
loss  of  heat  continued,  the  glassy  crust  became  thicker  and  thicker, 
and  in  its  endeavours  to  adapt  itself  to  the  shrinking  dimensions 
of  the  molten  interior  mass,  became  wrinkled  into  ridges  and 
valleys,  like  the  skin  of  a  dried- up  apple. 

It  is  almost  certain  that  through  the  cracks  and  fissures  thus 
formed,  floods  of  uprising  molten  matter  would  be  spread  over 
the  thin  crust,  portions  of  which  would,  from  this  cause,  collapse 
and  become  engulfed,  leaving  pools  and  lakes  of  liquid  magma  over 
which  a  new  crust  would  gradually  form. 


SOME    FIRST   PRINCIPLES.  3 

In  course  of  time  the  scarred  and  gnarled  igneous  crust  became 
cool  enough  to  permit  the  condensation  of  the  watery  vapours 
that  enveloped  the  Earth.  A  portion  of  the  waters  settled  in  the 
hollows  and  formed  the  first  seas  and  lakes  that  ever  existed  on 
the  face  of  the  globe  ;  while  another  portion  penetrated  the  dry 
land,  thereby  forming  the  springs  from  which  the  first  streams  and 
rivers  took  their  source. 

The  restless  waters  of  the  new-born  seas  at  once  began  to  wear 
away  the  dry  land  along  their  shores,  and  the  streams  draining 
the  valleys  to  deepen  and  widen  their  channels.  The  denuded 
material  was  spread  out  in  layers  and  beds  on  the  rocky  floor  of 
the  seas,  thus  marking  the  beginning  of  the  conditions  of  sedi- 
mentation that  have  prevailed  without  interruption  up  to  the 
present  day. 

It  was  not  until  the  precipitation  of  the  dense  aqueous  vapours 
had  taken  place  and  the  waters  were  gathered  together  into  seas 
that  life  appeared  on  the  globe. 

Beginning  of  Geological  Time. — Geological  time  dates  back  to 
the  first  beginning  of  the  physical  conditions  that  now  prevail 
upon  the  Earth  ;  that  is,  to  the  time  when  detrital  matter  derived 
from  the  denudation  of  the  dry  land  first  began  to  be  spread  out 
in  the  form  of  beds  or  layers  on  the  floor  of  the  new-born  seas 
and  lakes.  These  ancient  sediments  formed  the  first  records  of 
geological  time. 

The  Action  of  Water  in  Destroying  and  Re-forming. — From  that 
date  up  till  now,  water  has  continued  to  be  the  most  powerful 
agency  in  sculpturing  and  modifying  the  surface  of  the  Earth.  In 
wasting  and  eroding  the  dry  land,  in  transporting  the  eroded 
material,  in  sorting  and  spreading  it  out,  the  action  of  water  has 
been  unceasing  throughout  all  time  up  to  the  present  day. 

The  amount  of  matter  forming  the  Earth  is  practically  a  fixed 
quantity  ;  hence  it  is  obvious  that  all  the  deposits  and  beds  now 
exposed  in  the  dry  land  must  have  been  derived  from  the  destruc- 
tion of  the  first  igneous  crust,  or  of  sedimentary  rocks  of  later  date. 

Ever  since  the  beginning  of  geological  time  the  dry  land  has  been 
denuded  by  water,  yielding  the  material  to  form  new  deposits  in 
seas  and  lakes.  Through  the  progressive  crumpling  of  the  crust 
these  deposits  in  course  of  time  became  raised  above  sea-level, 
forming  dry  land  which,  in  its  turn,  was  subjected  to  the  agents 
of  erosion,  thus  yielding  material  to  form  newer  deposits  or  strata. 
This  action  is  still  going  on,  the  older  formations  providing  the 
material  for  the  younger. 

From  this  it  will  be  seen  that  the  same  material  has  appeared 
re-sorted  in  different  forms,  in  different  geological  ages.  It  is  now 
easy  to  understand  how  some  of  the  older  formations  have  been 


4  A   TEXT-BOOK   OF   GEOLOGY. 

entirely  removed  by  this  everlasting  denudation,  or  are  represented 
only  by  isolated  remnants  of  small  extent. 

No  portion  of  the  original  igneous  crust,  or  even  of  the  first- 
formed  sediments,  has  ever  been  found  ;  but  shreds  and  patches 
may  still  exist,  buried  beneath  the  deposits  of  later  times. 

The  Ocean  Basin  not  Permanent. — The  existing  dry  land  of  the 
globe  is  found  to  be  mainly  composed  of  aqueous  or  sedimentary 
rocks,  from  which  it  is  known  that  the  present  distribution  of  land 
and  water  is  not  that  which  always  existed.  On  the  contrary, 
by  slow  movements  of  the  crust  extending  over  countless  ages, 
known  to  geologists  as  secular  movements,  some  portions  of  the 
crust  have  been  elevated,  while  others  have  been  depressed  or 
submerged.  In  this  way  the  seas  and  dry  land  have  been  changing 
places,  so  to  speak,  all  through  geological  time  ;  the  effect  of  this 
wandering  of  the  seas  has  been  to  cover  the  whole  of  the  first 
igneous  crust  of  the  Earth  with  sedimentary  rocks. 

The  bulk  of  the  sedimentary  formations  were  formed  on  the 
floor  of  the  sea,  but  strata  containing  freshwater  shells  and  fishes, 
and  sometimes  beds  of  rock-salt,  tell  us  of  the  former  existence  of 
continents,  inland  lakes,  and  mediterranean  seas  of  which  no 
vestige  now  remains.  From  a  study  of  the  rock-formations  and 
the  fossils  which  they  enclose,  much  may  be  gleaned  of  the 
physical  geography  and  life  of  past  geological  times. 

The  Earth's  Crust  mostly  Sedimentary. — An  examination  of  the 
fabric  of  the  outer  shell  shows  that  it  is  principally  composed  of 
stratified  rocks — that  is,  rocks  occurring  in  parallel  beds  or  layers. 
A  study  of  the  materials  forming  these  rocks,  and  of  their  fossil 
contents,  shows  us  that  they  have  been  formed  by  the  gradual 
deposit  of  sediments  on  the  floor  of  some  sea  or  lake,  or  in  some 
cases  by  precipitation  from  solutions,  or  in  others  by  the  growth 
and  accumulation  of  animal  or  vegetable  organisms. 

The  physical  structure  of  sedimentary  or  aqueous  rocks— as  they 
are  sometimes  called — is  dependent  on  three  main  factors,  namely  : 

(1)  The  texture  of  the  material. 

(2)  The  character  of  the  cementing  medium. 

(3)  The  amount  of  induration,  alteration,  or  metamorphism  to 

which  the  material  has  been  subjected. 

The  term  texture  refers  to  the  coarseness  or  fineness  of  the  con- 
stituent grains  or  pebbles. 

Streams  and  rivulets,  as  well  as  the  ebbing  and  flowing  tide  of 
the  sea,  have  through  all  the  ages  possessed  the  same  eroding, 
transporting,  and  sorting  power,  and  what  we  now  see  going  on  in 
our  valleys  and  along  our  shores  is  a  fair  example  of  what  took 
place  in  earliest  geological  time. 


SOME    FIRST   PRINCIPLES.  5 

The  denudation  or  wearing  away  of  the  dry  land  was  mainly  the 
work  of  running  water,  while  the  sorting  and  spreading  out  of  the 
denuded  material  was  effected  by  the  laving  action  of  the  waves 
of  the  sea  as  they  advanced  and  retreated  on  the  ancient  strands. 

The  gravels  were  piled  along  the  shore  in  the  shallow  water ;  the 
smaller  pebbles  were  carried  into  deeper  water ;  while  the  sands 
and  finer  particles  were  borne  further  seaward,  the  latter  forming 
beds  of  mud  at  the  extreme  limit  of  the  deposit. 

The  gravels  along  the  sea  littoral,  when  consolidated,  formed 
what  are  termed  conglomerates  ;  the  water-borne  sands  formed 
sandstones ;  the  more  distant  muds  became  mudstones  and  shales  ; 
while  the  shell-banks  and  coral  reefs  became  limestones. 

Folding  and  Tilting  of  Sedimentaries. — The  older  sedimentary 
strata  have  been  of  necessity  subject  to  all  the  later  movements 
that  have  affected  the  crust  of  the  Earth.  They  have  been  indurated 
by  the  great  weight  of  superincumbent  strata,  and  plicated  or 
corrugated  by  entanglement  in  great  crustal  folds.  Hence  the 
strata  do  not  always  occupy  the  horizontal  position  in  which  they 
were  originally  laid  down,  but  are  inclined  or  tilted  at  various 
angles,  being  arranged  in  folds  with  gentle  or  steep  slopes. 

Alteration  of  Sedimentaries. — Many  of  the  older  rocks  have  been 
altered  or  metamorphosed  by  the  rearrangement  of  their  con- 
stituent minerals.  Thus  limestones  have  been  changed  to  marbles, 
sandstones  to  quartzite,  mudstones  and  shales  to  slates  and  schists. 

The  agencies  principally  concerned  in  the  metamorphism  of 
sedimentary  rocks  have  been  pressure,  which  induces  the  schistose 
and  slaty  structures  ;  heat  and  circulating  waters,  which  cause  a  re- 
arrangement of  the  constituents,  whereby  a  crystalline  structure 
may  be  formed.  Hence  metamorphic  rocks  are  often  spoken  of  as 
schistose  or  crystalline. 

Origin  of  Igneous  Rocks. — Throughout  all  geological  time  the 
outer  crust  or  shell  of  the  Earth  has  been  subject  to  the  intrusion 
and  overflow  of  molten  magmas  from  below. 

Whether  the  interior  is  in  (a)  a  molten  state,  or  (6)  exists  in  a 
highly  heated,  but  enormously  compressed,  condition  ready  to 
assume  the  liquid  form  whenever  and  wherever  the  stress  is  re- 
lieved, or  (c)  whether  the  lavas  that  are  from  time  to  time  erupted 
come  from  huge  subterranean  caverns  of  molten  rock  that  have 
escaped  the  general  cooling  of  the  crust,  is  at  present  not  known  to 
geologists. 

Notwithstanding  the  frequency  and  violence  of  igneous  intru- 
sions in  past  times,  the  fact  remains  that  probably  nine-tenths  of 
the  rocks  forming  the  known  crust  of  the  Earth  are  of  sedimentary 
or  aqueous  origin. 

R61e  of  Igneous  Intrusions. — Although  subordinate  in  extent  and 


6  A    TEXT-BOOK    OF    GEOLOGY. 

mass,  the  eruptive  rocks  have  played  an  important  part  in  the 
occurrence  and  distribution  of  ore-bodies  and  mineral  deposits. 
Not  only  are  they  metalliferous  themselves,  but  in  many  cases  their 
intrusion  has  caused  a  fracturing  of  the  rocks  which  they  pene- 
trated, thus  permitting  the  invasion  of  the  fissured  country  by 
metalliferous  gases  and  waters  that  emanated  from  the  intruding 
mass  itself. 

Thus  we  find  that  the  igneous  intrusion  frequently  played  a 
double  role  in  the  formation  of  ores  : — 

(a)  By  fracturing  and  fissuring  the  rocks. 

(6)  By    supplying   the    metalliferous    gases   and    waters   which 
deposited  their  mineral  contents  in  the  fissures. 

Intrusive  igneous  rocks  have  also  played  an  important  part  in 
folding,  crumpling,  and  tilting  the  sedimentary  strata  which  they 
have  broken  through,  or  with  which  they  have  come  in  contact. 
Moreover,  in  many  parts  of  the  globe  volcanic  flows  and  fragmen- 
tary ejecta  have  been  piled  up  so  as  to  form  mountain-chains  or 
isolated  mountains  that  frequently  attain  a  great  height. 

Alteration  of  Igneous  Rocks. — All  igneous  rocks,  like  sedimen- 
taries,  are  subject  to  alteration.  Superheated  steam  has  frequently 
caused  a  rearrangement  of  the  constituents  ;  while  the  circulation 
of  thermal  waters  has  led  to  the  elimination  or  removal  of  some 
constituents  and  the  substitution  of  others  which  are  thus  secondary. 
It  is  also  found  that  intense  pressure  may  cause  altered  lavas  and 
tuffs  to  assume  a  schistose  structure  not  unlike  that  induced  in 
metamorphosed  sedimentary  rocks. 

Interior  of  the  Earth. — Of  the  interior  condition  of  the  globe 
almost  nothing  is  known,  except  that  the  density  is  greater,  and 
that  the  temperature  increases  with  the  depth,  although  not  at  a 
uniform  rate  in  different  places. 

The  mean  density  or  specific  gravity  of  the  whole  globe  has  been 
determined  to  be  about  5*5,  and  that  of  the  materials  forming  the 
outer  portion  of  the  crust  to  which  man  has  access,  about  3.  The 
inference  to  be  drawn  from  this  is  that  the  interior  or  Barysphere 
must  be  composed  of  materials  possessing  a  greater  mean  density 
than  those  forming  the  outer  shell  or  Lithosphere. 

It  has  been  contended  by  some  writers  that  the  interior  must 
possess  a  nucleus  of  iron,  or  of  iron  alloyed  with  nickel  and  other 
heavy  metals. 

The  Planetismal  Hypothesis. — This  view  of  the  origin  of  the 
Earth  as  elaborated  by  Chamberlin  and  others  is  merely  a  modi- 
fication of  the  Nebular  Hypothesis.  It  assumes  that  the  great 
incandescent  nebula  x  which  originally  composed  the  solar  system 
1  Lat.  Nebula =a  cloud,  fog,  or  mist. 


SOME    FIRST   PRINCIPLES.  7 

cooled  relatively  rapidly,  the  cooled  gases  taking  the  form  of 
myriads  of  solid  meteorites  to  which  the  name  planetismals  1  has 
been  applied.  Under  the  influence  of  gravity  the  cold  meteorites 
segregated  into  knots  that  eventually  became  the  nuclei  of  the  sun, 
the  planets  and  their  satellites.  During  this  aggregation,  the 
meteorites  are  believed  to  have  bombarded  one  another  with  great 
violence,  thereby  becoming  hot. 

The  main  source  of  the  high  temperature  of  the  sun  and  of  the 
earth  before  it  cooled  was  not  due  to  the  heat  generated  by  the 
collision  of  the  constituent  meteorites,  but  to  the  contraction  and 
consolidation  of  the  mass  after  the  meteorites  had  come  into 
contact. 

All  solid  bodies  possess  a  potential  energy  proportional  to  their 
distance  from,  or  height  above,  the  common  centre  of  gravity  of 
the  globular  mass  to  which  they  belong.  Thus  a  block  of  stone 
resting  on  the  edge  of  a  high  tower  possesses  a  large  store  of  this 
energy  of  situation,  which  it  loses  as  soon  as  it  falls  to  the  ground. 
The  energy  is  not  lost,  but  merely  transformed  into  heat. 

The  packing  or  crowding  of  a  swarm  of  meteorites  under  the 
influence  of  gravity,  which  always  acts  towards  the  centre  of  the 
mass,  is  accompanied  by  the  generation  of  great  heat.  Meteoric 
matter  is  a  good  conductor  of  heat,  and  hence  the  whole  mass  would 
soon  assume  a  uniform  temperature.  As  the  packing  continued,  the 
globular  mass  would,  in  time,  reach  its  maximum  density,  and  there- 
after gradually  became  cool  by  the  radiation  of  heat  from  its  surface. 

The  heat  of  contraction  might  not  inconceivably  be  sufiicient  to 
melt  the  materials  in  the  upper  layers,  but  the  lower  layers  would 
remain  solid  as  the  pressure  of  the  superincumbent  mass  would  be 
sufficient  to  prevent  expansion,  without  which  liquefaction  cannot 
take  place. 

The  more  fusible  materials  in  the  upper  layers,  in  accordance 
with  the  law  of  liquation,  would  rise  to  the  surface,  and  in  process 
of  time  solidify  as  a  stony  crust.  The  tendency  of  this  process  of 
differentiation  would  be  to  divide  the  constituent  materials  of  the 
Earth  into  three  distinct  zones  corresponding  to  the  heavy  metal, 
lighter  sulphide  regulus,  and  still  lighter  stony  slag  formed  in  a 
reverberatory  furnace. 

The  central  core  and  regulus  form  the  barysphere  ;  while  the 
stony  slag  or  crust,  now  modified  and  re-sorted  by  the  action  of 
subaerial  agencies,  constitutes  the  lithosphere  or  stony  envelope. 

Age  of  the  Earth. — Lord  Kelvin,  in  the  early  'seventies,  basing  his 

estimate  on  the  rate  of  terrestrial  radiation,  concluded  that  the  age 

of  the  Earth  did  not  exceed  400  million  years.     But  the  rocky  crust 

of  the  Earth  contains  radium,  and  radium  in  disintegrating  gives  off 

1  Meaning  infinitely  small  planets. 


8  A   TEXT-BOOK   OF   GEOLOGY. 

heat.  Hence  it  is  contended  that  the  rate  of  cooling  of  the  globe 
must  be  slower  than  that  of  a  molten  globe  containing  no  radium. 
On  these  new  premises  Professor  Jolly  has  advanced  the  view  that 
the  age  of  the  Earth  may  be  perhaps  five  times  greater  than  Lord 
Kelvin's  estimate. 

Succession  of  Life  in  Geological  Time. — Examination  has  shown 
that  the  earlier  strata  contain  a  few  indistinct  and  badly  preserved 
remains  of  plants  and  animals  of  a  very  primitive  type. 

Beds  or  formations  higher  in  the  succession  are  found  to  contain 
a  larger  and  more  varied  assemblage  of  plant  and  animal  life, 
many  of  a  highly  complex  structure,  including  molluscs,  fishes, 
huge  bird-like  lizards,  saurians,  palms,  and  tree-ferns. 

The  higher,  i.e.  younger,  deposits  contain,  besides  molluscs  and 
fishes,  the  remains  of  many  mammals  which  have  representatives 
living  at  the  present  time.  In  other  words,  there  has  been  a  gradual 
succession  of  life  throughout  geological  time  from  the  lowly  to 
the  more  highly  organised  forms,  this  succession  of  life  being 
characterised  by  a  singular  persistency  of  the  primitive  types. 

The  Origin  of  Life.— The  problem  of  the  origin  of  life  is  still 
unsolved.  We  do  not  know  when  life  first  appeared  on  the  Earth  nor 
what  form  it  took.  If  we  are  right  in  our  conception  of  the  Earth 
as  a  cooling  satellite  of  the  Sun,  we  are  probably  not  far  from  the 
truth  in  believing  that  when  life  first  appeared  the  conditions  and 
environment  were  such  as  to  render  its  perpetuation  difficult  and 
precarious.  Perhaps  the  first  ray  of  life  glowed  feebly  for  a  time, 
flickered,  died,  and  became  rekindled  many  score  of  times  before 
it  eventually  succeeded  in  establishing  itself  in  the  saline  waters 
gathered  in  the  hollows  of  the  still  steaming  crust. 

The  first  assemblage  of  life  of  which  we  have  any  knowledge 
appeared  in  the  Cambrian  epoch.  It  comprised  representatives 
of  most  of  the  great  groups  of  marine  invertebrates,  and  burst  on 
the  geological  horizon  with  the  suddenness  of  a  meteor  in  a 
September  sky.  From  what  we  now  know  of  biological  processes, 
it  is  obvious  that  this  highly  specialised  congeries  of  life  was 
preceded  by  a  pre-Cambrian  ancestry,  of  which  no  certain  trace 
has  yet  been  found. 

It  is  probable  that  the  primordial  germs  of  life  were  tiny  nuclei 
of  jelly-like  colloidal  matter  possessing  no  higher  volition  than  the 
"  Brownian  dance  "  of  motes  in  a  beam  of  sunlight.  As  time  rolled 
on,  the  stream  of  life  gradually  increased  in  volume  by  the  continual 
accession  of  more  and  more  complex  forms,  eventually  culminating 
in  the  advent  of  man.  We  can  easily  conceive  that  this  stately 
procession  of  life  could  only  come  into  existence  as  the  result  of 
increasing  food  supply,  increasing  sunshine,  wider  seas,  and  more 
settled  climatic  conditions. 


SOME    FIRST   PRINCIPLES.  9 

The  primitive  forms  came  into  existence  when  the  conditions  of 
life  were  adverse.  As  the  conditions  became  more  and  more  pro- 
pitious, higher  and  higher  forms  appeared. 

The  primitive  and  higher  types  are  still  coeval. 

The  highly  organised  forms  are  more  sensitive  to  climatic  and 
other  changes  than  the  hardier  Radiolarians  and  other  lowly  types. 
Hence,  as  the  Earth  becomes  decadent  and  the  conditions  of  life  less 
and  less  favourable,  the  first  types  to  disappear  will  be  those  that 
were  the  last  to  come  into  existence  ;  and  the  last  to  survive  will 
be  the  simple  primordial  forms.  In  other  words,  life  will  disappear 
in  the  inverse  order  of  its  appearance. 

Geological  Time  marked  by  Distinctive  Life. — Close  investigation 
has  shown  that  certain  organic  forms  occur  only  in  certain  beds  or 
strata.  Such  fossils  are  termed  characteristic  or  distinctive  forms. 
Geologists  have  taken  advantage  of  these  to  divide  geological  time 
into  periods,  just  as  historic  time  is  divided  into  periods  by  succeed- 
ing dynasties  or  empires.  These  periods  are  purely  empirical  or 
artificial,  and  are  merely  used  for  convenience  of  description  and 
study. 

Tetrahedral  Hypothesis.1— If  we  examine  a  terrestrial  globe  we 
cannot  fail  to  observe  that  the  great  mass  of  the  dry  land  lies  in 
the  Northern  Hemisphere,  and  the  greatest  expanse  of  sea  in 
the  Southern  Hemisphere.  Moreover,  we  shall  at  once  see  that  the 
continental  units  and  seas  are  frequently  triangular  in  shape, 
the  former  presenting  their  bases  to  the  north  and  tapering  to 
the  south. 

Further,  we  shall  find  that  the  continents  and  seas  are  antipodal 
or  opposite  to  one  another. 

The  unequal  distribution  of  land  in  the  two  Hemispheres,  the 
dominant  triangular  shape  of  the  geographical  units,  and  the  anti- 
podal distribution  of  the  land  and  seas  cannot  be  set  down  to  mere 
coincidence  or  fortuitous  happening,  but  to  the  operation  of  well- 
defined  physical  laws. 

As  so  clearly  demonstrated  by  Lothian  Green,  the  arrangement 
of  the  continental  units  approximates  the  shape  of  the  tetrahedron,2 
which  is  a  figure  bounded  by  four  triangular  planes.  The  great 
oceans  lie  on  the  flattened  or  depressed  triangular  faces  of  the  figure. 

Now  a  sphere  is  the  figure  which  presents  the  smallest  surface  for 
its  volume,  and  the  tetrahedron  the  greatest. 

So  long  as  a  globular  mass  possesses  heat  it  will  continue  to 
shrink,  and  the  inner  portion,  on  account  of  its  greater  heat,  will 

1  A  very  clear  and  graphic  exposition  of  the  Planetismal  and  Tetrahedal 
hypotheses  will  be  found  in   The  Making  of  the  Earth,  by  Professor  J.  W. 
Gregory,  F.R.S.,  Home  University  Series.     Price  Is.,  London,  1912. 

2  Gr.  7Wra  =  four,  and  hedra  =  &  base  or  plane. 


10  A   TEXT-BOOK   OF   GEOLOGY. 

contract  more  rapidly  than  the  outer  rigid  shell.  As  the  cooling 
and  internal  shrinking  proceed,  the  globular  mass  will,  in  time,  be 
encumbered  with  an  excess  of  surface  which  will  be  most  easily 
disposed  of  by  assuming  the  form  of  the  tetrahedron. 

When  a  metal  tube  collapses  under  compressive  stress,  as  may  be 
easily  demonstrated  in  a  compression-testing  machine,  it  becomes 
triquetral,  that  is,  bounded  by  three  concave  sides.  As  viewed  in 
cross-section,  each  of  the  three  projecting  lobes  is  seen  to  be  opposite 
a  depression. 

The  antipodal  arrangement  of  the  land  and  seas  is  the  natural 
corollary  of  the  tetrahedral  form  assumed  by  a  rotating  globular 
mass. 

GEOLOGICAL   HISTORY   OF   EARTH  SUMMARISED. 

We  may  summarise  the  successive  stages  through  which  the 
Earth  has  passed  up  to  the  beginning  of  the  conditions  that  now 
prevail  as  follows  :— 

(1)  In  the  beginning  the  Earth  was  a  mass  of  nebular  incandescent 

gases  swinging  through  space. 

(2)  Through  loss  of  heat  by  radiation  the  gases  eventually  became 

condensed  into  a  highly  heated  viscous  globular  body. 

(3)  By  continued  loss  of  heat  a  solid  crust  formed  on  the  surface 

of  the  liquid  globe. 

(4)  In  process  of  time  the  crust  became  thicker  and  thicker,  and 

in  its  attempts  to  adapt  itself  to  the  smaller  dimensions  of 
the  rapidly  contracting  heated  interior,  became  crumpled 
and  wrinkled  like  the  skin  of  a  dried  apple. 

(5)  When  the  cooling  had  sufficiently  advanced,  the  aqueous 

vapours  which  enveloped  the  Earth  became  condensed, 
and  the  waters  settled  on  the  land,  forming  streams  and 
rivers  which  denuded  or  wore  away  the  rocky  crust. 

(6)  The  streams  flowed  into  the  hollows  or  depressions  in  which 

were  formed  the  first  seas  and  lakes  that  ever  existed. 

(7)  The  muds,  sands,  and  gravels  carried  down  by  the  streams 

were  sorted  and  spread  out  on  the  floor  and  along  the 
strand  of  the  seas,  forming  aqueous  or  sedimentary  deposits 
that  were  thus  the  first  records  of  geological  time. 

(8)  It  was  probably  soon   after   streams   and   seas   came   into 

existence  that  life  first  appeared  on  the  globe. 

(9)  Since  the  beginning  of   geological   time   the   dry  land   has 

always  been  subject  to  denudation  or  erosion  by  the  action 
of  moving  water.  The  formation  of  aqueous  deposits 
has,  therefore,  been  continuous  throughout  all  geological 
time  in  those  portions  of  the  globe  occupied  by  seas  and 


SOME    FIRST   PRINCIPLES.  11 

lakes ;  but  as  the  areas  of  denudation  and  deposition  have 
been  constantly  changing  places,  deposition  has  never 
been  continuous  in  any  one  area. 

(10)  Through  the  slow  secular  crumpling  of  the  Earth's  crust 

causing  elevation  in  one  portion  and  subsidence  in  another, 
the  older  formations  in  course  of  time  became  dry  land, 
and  thus  provided  the  material  to  form  newer  and  younger 
formations. 

(11)  In  the  continuous  cycle  of  erosion  and  deposition  that  has 

always  prevailed,  the  same  material  has  appeared  re-sorted 
in  different  forms  of  aqueous  rocks. 

(12)  Since  the  beginning   of   geological  time   the   sedimentary 

strata  which  comprise  the  great  bulk  of  the  known  crust 
have  been  intruded  by  igneous  dykes,  or  broken  through 
and  covered  in  places  with  streams  of  lava  and  volcanic 
ash. 

(13)  The  strata  originally  laid  down  in  a  horizontal  position  have 

been  folded  by  slow  secular  crustal  movements,  and 
frequently  broken,  crushed,  and  tilted  at  various  angles 
by  igneous  intrusions. 

(14)  Sedimentary  and  igneous  rocks  alike  are  subject  to  alteration 

or  metamorphism,  forming  the  class  of  rocks  known  as 
metamorphic. 

(1.5)  The  fossil  remains  enclosed  in  the  rocks  show  a  gradual 
evolution  from  the  lowly  to  the  more  highly  organised 
types  now  inhabiting  the  globe,  but  the  primitive  forms 
have  been  persistent  through  all  time. 

(16)  The  Nebular  Hypothesis  supposes  that  the  Earth  was  a 
nebula  of  incandescent  gases,  the  heat  of  which  gradually 
radiated  into  space  until  the  planet  became  a  globular  mass 
of  glowing  molten  matter  in  which  the  heavier  metallic  con- 
stituents segregated  themselves,  under  the  influence  of 
gravity,  into  a  heavy  central  core,  forming  the  barysphere  ; 
while  the  lighter  material  arranged  itself  as  an  outer  con- 
centric shell  constituting  the  lithosphere. 

In  course  of  time  the  glowing  mass  cooled  sufficiently  to 
allow  the  outer  envelope  to  form  a  solid  crust.  And  as  the 
heated  interior  of  the  Earth  contracted  more  rapidly  than 
the  outside  crust,  the  surface  became  crumpled  and  wrinkled 
in  its  endeavours  to  accommodate  itself  to  the  rapidly 
diminishing  dimensions  of  the  interior. 

When  the  outer  envelope  had  sufficiently  cooled,  the 
aqueous  vapours,  which  up  till  now  covered  the  surface  in 
a  dense  impenetrable  cloud,  became  condensed  and  soon 
settled  in  the  hollows.  Thereafter,  the  newly-formed  seas 


12  A   TEXT-BOOK   OF   GEOLOGY. 

and  the  other  agents  of  denudation  began  the  cycle  of  pro- 
cesses of  denudation  and  re-sorting,  destruction,  sorting  and 
reconstruction,  which  have  continued  without  intermission 
through  all  the  geological  ages. 

(17)  The  Planetismal  Hypothesis  assumes  that  the  primeval 
gaseous  nebula  of  our  solar  system  cooled  rapidly  and 
resolved  itself  into  a  vast  cloud  of  solid  meteorites  called 
planetismals,  which,  under  the  operation  of  gravity,  be- 
came segregated  into  knots.  The  largest  knot  formed  the 
nucleus  of  the  Sun,  the  smaller  knots,  the  nuclei  of  the 
planets. 

The  packing  and  contraction  of  the  meteorites  generated 
sufficient  heat  to  cause  the  fusion  and  permit  the  differen- 
tiation of  the  constituents  into  a  central  barysphere  and 
an  outer  lithosphere. 

Thereafter  the  crumpling,  denudation,  and  re-sorting  of 
the  stony  crust  proceeded  as  postulated  in  the  nebular 
hypothesis. 


CHAPTER   II. 
THE   SCOPE  OF  GEOLOGY. 

THE  whole  scope  of  geological  investigation  is  contained  in  two 
principal  divisions,  namely  : — 

(a)  General  Geology. 
(6)  Economic  Geology. 

General  Geology,1  with  which  we  are  mainly  concerned,  covers 
a  wide  field  of  scientific  research.  It  deals  with  the  origin  and 
structure  of  the  rocky  materials  forming  the  crust  of  the  Earth, 
with  the  manner  in  which  the  strata  are  arranged  or  disposed,  and 
with  the  agencies  which  have  brought  about  the  present  configura- 
tion of  the  surface.  It  also  concerns  itself  with  the  chronological 
succession  of  the  various  groups  of  rocks,  and  attempts  to  classify 
the  strata  in  accordance  with  their  fossil  contents. 

Economic  Geology,  also  known  as  Mining  or  Applied  Geology,  is 
a  highly  specialised  branch  of  geology  that  possesses  a  peculiar 
interest  to  the  miner  and  mining  engineer.  It  concerns  itself  with 
the  origin,  mode  of  occurrence,  and  classification  of  mineral  deposits 
of  all  kinds,  and  with  water  supply.  Its  study  is  seldom  attempted 
until  a  knowledge  of  the  fundamental  principles  of  General  Geology 
have  first  been  acquired. 

Different  Branches  of  General  Geology. — For  methodical  study 
General  Geology  is  most  conveniently  considered  under  the  follow- 
ing subdivisions  : — 

(a)  Petrology,  which  deals  more  particularly  with  the  character 
and  structure  of  the  rocky  material  forming  the  crust. 

(6)  Dynamical  Geology,  which  investigates  the  agencies  that  form, 
denude,  and  re-form  these  materials,  as  well  as  the  processes 
which  tend  to  modify  or  change  the  shape  and  configuration 
of  the  crust. 

(c)  Structural  Geology,  which  concerns  itself  with  the  arrange 
rnent  of  the  rocky  materials. 

1  Gr.  gre  =  the  earth,  and  logos  =  description,  discussion. 
13 


14  A   TEXT-BOOK   OF   GEOLOGY. 

(d)  Palceontology,  which  deals  with  the  plant  and  animal  remains 

embedded  in  the  rocks. 

(e)  Stratigraphical  Geology,  which  attempts  to  unravel  the  order 

in   which  the  rocks  have  appeared,  and  to  interpret  the 
geological  history  of  the  globe. 

Historical. — As  a  true  science  geology  dates  from  the  close  of 
the  eighteenth  century.  Before  that  time  there  were,  even  among 
scientists,  many  theories  relating  to  the  origin  of  earthquakes, 
volcanoes,  fossils,  and  other  phenomena  that  to  us,  with  our  better 
knowledge,  seem  curious  and  sometimes  whimsical. 

Among  the  founders  of  the  science  as  we  now  know  it,  the  names 
of  the  contemporary  workers,  Abraham  Gottlob  Werner,  Professor 
of  Mining  at  Freiberg  ;  James  Hutton,  M.D.,  of  Edinburgh  ;  and 
William  Smith,  an  English  land-surveyor,  stand  pre-eminent. 

Smith  was  the  first  to  show  by  actual  field  observation  that 
stratified  rocks  could  be  identified  and  arranged  in  chronological 
sequence  according  to  their  fossil  contents.  By  his  epoch-making 
work  on  the  Jurassic  rocks  of  South- West  England,  he  laid  the 
foundation  of  Stratigraphical  Geology.  His  famous  "  Geological 
Map  of  England  and  Wales,"  published  in  1815,  was  the  first 
attempt  to  represent  the  geological  relationships  of  the  different 
rock- formations  over  an  extensive  tract  of  country,  and  sub- 
sequently it  became  the  model  of  all  geological  maps. 

Werner,  a  persuasive  and  eloquent  teacher,  maintained  that  the 
organic  remains  found  in  different  rock-formations  bore  a  constant 
relation  to  the  age  of  the  deposits.  He  affirmed  that  all  rocks 
above  the  basal  granites,  gneisses,  and  metamorphic  rocks  were 
of  aqueous  origin,  including  the  trap  rocks  ;  and  this  contention 
formed  the  cardinal  doctrine  of  the  school  known  as  Neptunists. 

Button  recognised  the  aqueous  origin  of  sandstones,  shales,  and 
limestones,  and  in  his  philosophical  writings  forcibly  discussed  the 
consolidation,  uplift,  tilting,  and  bending  of  strata.  He  considered 
crustal  movements  as  due  to  extreme  heat  and  expansion  supple- 
mented by  volcanic  disturbance  and  earthquakes.  His  views 
found  many  disciples  and  formed  the  basis  of  the  theses  of  the 
school  of  Vulcanists  or  Plutonists. 

The  fundamental  truths  of  geology  were  subsequently  sorted 
and  crystallised  by  Charles  Lyell,  a  Scotsman,  and  in  1830-33 
embodied  in  his  monumental  Principles  of  Geology,  which  from  the 
first  met  with  extraordinary  success  and  at  once  placed  the  author 
in  the  front  rank  of  geologists. 


CHAPTER   III. 
THE  DENUDATION  OF  THE   LAND. 

Denudation  Defined. — By  denudation  l  is  meant  the  wearing  away, 
wasting,  or  breaking  up  of  the  surface,  whereby  the  general  level  of 
the  land  is  lowered.  It  therefore  embraces  the  work  of  all  the 
agents  of  wear  and  tear. 

Erosion  refers  to  the  more  active  and  obvious  wear  and  tear 
carried  on  by  the  sea,  by  streams,  rivers,  and  glaciers,  and  it  is 
embraced  within  the  general  term  denudation. 

The  principal  agents  of  denudation  are  : — 

(a)  Air  and  wind.  (d)  Streams  and  rivers. 

(6)  Rain.  (e)   Glaciers. 

(c)   Frost.  (/)   The  sea. 

Denudation  that  takes  place  above  sea-level  is  termed  sub- 
aerial,  and  that  which  takes  place  below  sea-level,  marine. 

In  a  general  way  we  may  say  that  air,  rain,  and/rosZ  act  upon  the 
dry  land,  decomposing,  softening,  and  breaking  up  the  surface  of 
the  rocks.  The  joint  action  of  these  agents  is  generally  spoken  of 
as  iveathering. 

The  material  loosened  by  weathering  gradually  finds  its  way 
under  the  influence  of  gravity  to  lower  and  lower  levels,  until  at 
last  it  gets  within  the  reach  of  running  water  in  the  form  of 
streams  and  rivers  by  which  it  is  transported  to  the  sea. 

Scope  of  Denuding  Agents.— Streams  and  rivers  erode  or  cut 
away  the  bottom  and  sides  of  their  channels  ;  while  the  sea  erodes 
or  eats  away  the  edge  of  the  dry  land  that  borders  its  shores. 

The  action  of  air,  rain,  and  frost  is  silent,  slow,  and  almost  imper- 
ceptible ;  that  of  rivers  and  the  sea  relatively  rapid  and  obvious. 
The  deep  river-gorge,  the  undermined  and  tumbling  sea-cliff,  are 

1  In  its  literal  sense  it  means  to  expose,  or  lay  bare,  rocks  that  lie  below 
the  surface.  This  is  what  prolonged  denudation  actually  does  perform. 
Denude  comes  from  Lat.  de  =  down,  and  nudus  =  naked. 

15 


16  A   TEXT-BOOK    OF    GEOLOGY. 

evidences  of  active  erosion  that  cannot  fail  to  attract  the  notice 
of  even  the  most  casual  rambler  among  the  mountains  or  on  the 
seashore. 

The  eroding  action  of  glaciers,  like  that  of  air  and  rain,  is  silent 
and  perhaps  relatively  slow  ;  but  its  effects  are  nearly  always 
quite  obvious  in  the  form  of  rounded  contours,  striated,  and 
furrowed  rocks. 

THE  WORK  OF  AIR. 

The  atmosphere  consists  of  a  mechanical  mixture  of  about  four 
volumes  of  nitrogen  and  one  volume  of  oxygen  (N79-l,020;9), 
with  traces  of  carbon  dioxide  gas  (C02),  water-vapour,  ammonia, 
ozone,  and  other  gases.  The  proportion  of  carbon  dioxide  is  about 
3-5  parts  in  10,000.  The  action  of  the  air  is  chemical,  mechanical, 
and  physical.  Its  chemical  activity  as  a  denuding  agent  is  mainly 
due  to  certain  inherent  properties  possessed  by  carbon  dioxide  and 
oxygen. 

Activity  of  Carbon  Dioxide. — Let  us  first  consider  the  case  of 
carbon  dioxide.  This  gas,  like  a  lump  of  sugar,  is  dissolved  by 
water  and  water- vapour.  Even  at  ordinary  atmospheric  pressures 
water  can  dissolve  its  own  volume  of  the  gas.  Now  when  water 
or  moist  air  containing  carbonic  acid  (C02,H20)  comes  in  contact 
with  a  carbonate  mineral  or  a  calcareous  rock,  the  carbon  dioxide 
in  the  water  unites  with  the  carbonate  of  lime  in  the  rock,  forming 
a  bicarbonate  of  lime,  which  is  soluble  in  water  and  therefore 
easily  removed. 

In  this  way  the  surface  of  a  limestone  or  calcareous  sandstone  is 
eaten  away  ;  and  as  you  may  observe  for  yourself  by  examining 
a  limestone  cliff  or  ledge,  the  grains  of  sand  which  are  not  acted 
on  by  the  carbonic  acid  stand  up  in  sharp  relief  on  the  surface 
of  the  rock,  as  also  do  sharks'  teeth  that  may  be  present. 

When'  a  calcareous  sandstone  is  acted  on,  the  removal  of  the 
binding  medium  or  cement  allows  the  grains  of  sand  to  become 
free,  when  they  are  then  easily  carried  away  by  the  wind,  rain,  or 
moving  water. 

This  eating  away  of  the  rock  is  due  to  chemical  solution  ;  hence 
the  term  corrosion  is  frequently  used  to  denote  chemical  denudation. 

Carbonic  acid  also  acts  as  a  powerful  agent  in  weathering  or 
decomposing  all  rocks  containing  silicates  of  alumina,  potash,  or 
soda.  Both  potash  and  soda  possess  a  greater  liking  or  affinity 
for  carbon  dioxide  than  for  silica,  with  the  result  that  they  combine 
with  carbon-dioxide  acid,  forming  soluble  carbonates,  thereby 
liberating  the  silicate  of  alumina  and  other  undissolved  constituents 
that  may  be  present. 


THE  DENUDATION  OF  THE  LAND.  17 

Perhaps  one  of  the  best  examples  of  this  mode  of  rock-decom- 
position is  that  seen  in  the  rotting  of  granite.  The  three  essential 
constituents  of  this  rock  are  quartz,  felspar,  and  mica.  The 
felspar  is  a  silicate  of  alumina  and  potash.  The  potash  unites 
with  the  atmospheric  C02,  forming  a  soluble  carbonate  of  potash, 
while  the  silicate  of  alumina  remains  behind  to  be  afterwards 
washed  away  by  the  rain.  With  one  important  constituent 
removed,  the  surface  of  the  rock  crumbles  away,  liberating  the 
quartz  grains  and  the  mica  scales,  which  are  then  carried  away  by 
the  wind,  rain,  or  moving  objects. 

When  a  rock  loses  its  cohesion  by  the  removal  of  a  constituent, 
or  by  the  dissolving  out  of  the  cementing  medium,  so  that  the 
remaining  constituents  become  liberated  or  crumble  into  sand, 
it  is  said  to  be  disintegrated. 

Chemical  Work  of  Oxygen. —  Oxygen  is  an  active  weathering 
agent,  but  a  less  powerful  one  than  carbonic  acid.  In.  the  case  of 
silicates  it  frequently  begins  to  act  after  the  carbonic  acid  has 
effected  the  initial  decomposition  of  the  mineral.  When  the 
silicate  mineral  contains  iron  protoxide  (FeO),  as  is  frequently  the 
case>  the  FeO  is  liberated  and  unites  with  the  atmospheric  oxygen 
and  water,  forming  the  hydrated  brown  oxide  called  limonite, 
to  which  the  rusty-brown  colour  of  all  weathered  rock-surfaces 
is  due. 

Oxygen  also  acts  energetically  in  conjunction  with  moisture  in 
the  decomposition  of  metallic  sulphides  that  happen  to  be  present 
in  rocks.  The  most  prevalent  sulphide  is  pyrite  (FeS2),  the  disul- 
phide  of  iron  which  occurs  in  all  kinds  of  sedimentary  and  igneous 
rocks.  This  sulphide  is  oxidised  with  liberation  of  sulphuric  acid, 
which  at  once  attacks  the  aluminous  rocks  and  minerals  it  comes 
in  contact  with,  forming  sulphates,  many  of  which  are  soluble  in 
water  and  hence  easily  removed.  In  this  way  the  disintegration 
of  a  rock  may  proceed  at  a  comparatively  rapid  rate. 

THE  WORK  OF  WIND. 

Sandhills  or  Dunes. — Moving  air  in  the  form  of  wind  sweeps  over 
the  land,  carrying  before  it  the  particles  of  dust  and  sand  loosened 
by  the  agents  % of  decomposition  and  disintegration.  Along  the 
sea-coast  and  in  deserts,  the  sands  are  blown  into  hummocks  and 
ridges  that  frequently  attain  a  height  of  100  feet  or  more.  Such 
hummocks  and  hills  of  sand  are  called  dunes  (Plate  I.). 

Blown  sands  are  frequently  piled  up  in  lines  of  dunes  fronting  a 
sandy  beach.  Where  the  dunes  obstruct  the  natural  drainage  to 
the  sea  it  is  not  unusual  to  find  chains  of  shallow  lagoons  on  their 
inland  side  running  parallel  with  the  coast-line. 

17  2 


18  A    TEXT-BOOK    OF    GEOLOGY. 

In  the  rainless  Sahara  and  other  arid  regions  there  are  places 
where  vast  accumulations  of  loose  wind-borne  sand  and  dust  fill  all 
the  depressions  and  frequently  rise  to  the  crests  of  the  ranges.  In 
some  parts  of  tropical  Australia,  and  in  the  dry  belts  of  Central 
Otago  in  New  Zealand  and  the  south-west  States  of  America,  there 
are  extensive  wastes  of  drifting  sand  blown  by  the  prevailing 
winds  into  continually  shifting  drifts  and  ridges. 

In  moist  climates  the  travel  of  wind-blown  sand  is  relatively 
slow,  and  by  the  planting  of  sand- binding  grasses  and  shrubs  it 
can  frequently  be  checked  ;  but  in  arid  regions  a  powerful  dust- 
storm  of  even  short  duration,  as  the  author  has  found,  is  capable 
of  displacing  vast  quantities  of  sand  and  dust  that  overwhelm 
everything  in  their  course.  In  desert  regions  the  wind  is  therefore 
an  important  sorting  and  transport  agent  ;  but  the  manner  in 
which  it  operates  is  fundamentally  different  from  that  of  streams 
and  rivers.  These  always  flow  in  one  direction,  and  hence  they 
carry  their  load  from  a  higher  to  a  lower  level — that  is  seaward  ; 
whereas  the  desert  winds  travel  backward  and  forward  across  the 
arid  wastes,  moving  the  sand  and  dust  from  place  to  place  within 
the  arid  zone  itself.  In  this  way  sand  may  accumulate  in  desert 
places  until  it  occupies  the  whole  landscape,  thereby  creating  in 
the  observer's  mind  the  erroneous  impression  that  the  denudation 
of  arid  regions  is  excessively  rapid. 

Sand-Ripples. — Ripples,  somewhat  similar  in  appearance  to 
those  formed  by  wave-movement,  are  frequently  formed  on  dunes 
by  the  action  of  the  wind.  It  has  been  proved  experimentally 
that  ripples  are  not  formed  where  the  sand-grains  are  of  uniform 
size,  but  only  where  there  is  a  mixture  of  fine  and  coarse  grains. 
This  depends  on  the  principle  that  where  the  wind  strikes  on  an 
obstacle  an  eddy  is  formed  on  its  lee-side.  Rippling  takes  place 
when  this  eddy  in  the  lee  of  the  larger  grains  is  of  sufficient  strength 
to  lift  the  smaller  grains. 

On  the  windward  side  of  the  large  grains  a  long  gentle  slope  is 
formed,  up  which  the  grains  travel.  At  the  summit  the  larger 
grains  are  arrested  by  the  eddy  and  build  up  the  ridge  of  the  ripple, 
while  the  vertical  motion  of  the  eddy  scours  out  a  trough  in  the 
loose  sand  at  the  foot  of  the  steep  slope  (Plate  I.). 

The  ripples  are  continually  moving  forward,  the  larger  grains 
falling  over  the  crest  of  the  ridge,  thereby  assisting  to  build  up  the 
advancing  steep  lee-slope  on  which  the  grains  assume  the  natural 
angle  of  rest. 

Two  series  of  ripples  may  be  formed  in  desert  regions  where  the 
prevailing  winds  have  winnowed  out  the  finer  particles,  leaving  only 
the  coarser  sands.  The  larger  or  primary  ripples  occur  in  parallel 
lines,  and  resemble  miniature  sea- waves.  They  are  formed  of  the 


PLATE  I. 


A.  WIND-RIPPLES,  PRIMARY  AND  SECONDARY,  IN  COARSE  SAND. 
CROMWELL  DUNES.     (After  Cockayne.) 


B.  GENERAL  VIEW  OF  WANDERING  DUNE,  FORMERLY  GOOD  GRAZING-LAND, 
SHOWING  RIDGE  AT  SUMMIT  OF  SAND-FALL.     (After  Cockayne.) 


THE  DENUDATION  OF  THE  LAND. 


19 


coarsest  particles  such  as  can  only  be  moved  by  the  strongest  winds. 
The  smaller  or  secondary  ripples  move  forward  under  the  influence 
of  the  gentle  breezes.  They  may  lie  parallel  to,  or  run  obliquely 
across,  the  trend  of  the  primary  sand- waves  according  to  the 
direction  of  the  wind. 

Sand-ripples  and  sand-waves  always  lie  at  right  angles  to  the 
direction  of  the  wind  that  produces  them. 

Mechanical  Effects  of  Wind. — The  erosive  effects  of  wind-borne 
sand  is  everywhere  present  in  arid  regions.  The  fretting  or  abrad- 
ing action  of  the  travelling  sand  produces  effects  resembling  those 
of  a  giant  sand-blast.  The  sand  wears  away  the  rough  edges  of 


FIG.  1. — Showing  sand -worn,  mushroom-shaped  rock  of  millstone- 
grit,  Yorkshire.     (After  Phillips.) 

all  the  rock  hummocks  that  lie  in  the  path  of  the  prevailing  winds. 
Rock-faces  are  grooved  and  corrugated,  or  worn  into  fantastic 
shapes  according  to  the  varying  hardness  and  resistance  offered 
by  different  portions  of  the  rock.  In  some  situations,  cliffs 
and  stacks  are  undercut ;  and  in  places  where  wind  eddies  are 
formed,  miniature  cirques  and  rock-basins  may  be  eroded  by  the 
swirling  sands  (Plates  II.  and  III.). 

Notable  examples  of  the  erosive  effects  of  travelling  sand  may 
be  seen  in  Lower  Egypt,  Western  Arabia,  on  the  Great  Western 
Plateau  of  Australia,  and  in  Southern  California. 

The  sand-erosion  suffered  by  the  Sphinx  and  some  ruined 
temples  in  Egypt  would  tend  to  show  that  the  action  of  moving 
sand  is  relatively  rapid. 

In  places  where  pebbles  lie  on  a  wind-swept  rocky  platform,  the 


20  A   TEXT-BOOK    OF    GEOLOGY. 

pebbles  in  time  become  worn  into  tent-shaped  forms  by  the  sand 
travelling  first  from  one  side  and  then  from  the  other. 

An  instructive  example  of  sand  action  is  represented  in  fig.  1. 
Another  is  to  be  seen  on  the  pebble-scattered  limestone  plat- 
form on  the  sea-coast  at  Nukumaru,  New  Zealand,  where 
hundreds  of  sand- worn  pebbles  are  to  be  seen  in  every  stage 
of  erosion  (Plate  IV.). 

Effects  of  Changes  of  Temperature. — This  is  a  powerful  agency 
of  denudation  in  regions  where  there  is  a  considerable  daily  range 
of  temperature.  In  the  interior  of  arid  continents  there  is 
frequently  a  range  of  40°  or  50°  Fahr.  as  between  the  day  and  night 
temperatures.  This  rapid  change  of  temperature  through  alternate 
expansion  and  contraction  introduces  enormous  stresses  in  the 
surface  skin  of  the  rocks.  The  effect  of  these  stresses  is  to  cause 
the  surface  of  the  rocks  to  peel  off  in  thin  irregular  flakes.  In  this 
way  cliffs  are  slowly  disintegrated  and  the  surface  of  arid  plains 
loosened. 

The  action  is  similar  to  that  which  takes  place  when  a  plate  of 
steel  is  exposed  to  the  oxidising  influence  of  moist  air.  A  film  of 
rust,  that  is  oxide  of  iron,  forms  on  the  surface.  In  a  short  time 
the  alternate  expansion  and  contraction  of  the  plate,  due  to  changes 
of  temperature,  cause  the  rust  to  peel  off  in  irregular  scales.  This 
exposes  a  fresh  surface  to  the  oxidising  agent.  A  new  skin  of  rust 
forms,  soon  to  be  displaced  in  the  same  way  as  the  first.  Thus, 
in  course  of  time,  the  plate  becomes  corroded  and  pitted  ;  and  the 
thinner  the  plate  becomes  the  more  rapidly  does  the  oxidation 
proceed. 

The  primary  condition  of  aridity  is  restricted  rainfall,  which 
may  be  modified  by  latitude  and  altitude,  topographical  barriers, 
and  prevailing  winds.  In  such  regions,  the  ratio  of  the  annual 
rainfall  to  the  possible  evaporation  is  an  important  feature. 

The  low  relief  of  arid  desert  regions  and  the  vast  accumulations 
of  loose  sandy  material  that  generally  abound  on  them  would 
tend  to  indicate  that  surface-stress  due  to  changes  of  temperature 
must  rank  among  the  most  active  of  the  processes  of  disintegration. 

THE  WORK  OP  RAIN. 

The  work  of  rain  is  both  chemical  and  mechanical.  By  its 
chemical  action  it  decomposes  and  softens  the  surface  of  rocks, 
and  by  its  mechanical  action  it  washes  away  the  loosened  particles 
to  a  lower  level.  It  also  decomposes  and  oxidises  rocks  as  far  as  it 
can  penetrate. 

Chemical  Effects  of  Rain. — Water  is  sometimes  spoken  of  as 
the  universal  solvent.  Even  when  quite  pure  it  can  readily  dis- 


THE  DENUDATION  OF  THE  LAND.  21 

solve  rock-salt  and  many  sulphate  minerals.  When  it  contains 
dissolved  salts  or  gases  its  power  as  a  solvent  is  greatly  increased. 

The  chemical  effect  of  rain-water  is  only  distinguished  from 
that  of  moist  air  by  its  greater  activity.  Its  dissolving  and  de- 
composing action,  like  that  of  moist  air,  is  mainly  dependent 
on  the  carbonic  acid  and  oxygen  which  it  gathers  from  the  air 
as  it  falls. 

Eain  acts  with  greater  energy  than  moist  air  because  it  brings 
to  bear  on  a  given  place  a  larger  quantity  of  carbonic  acid  and 
oxygen.  Besides,  by  its  mechanical  effect  it  washes  away  the 
loosened  particles,  thus  exposing  fresh  surfaces  to  be  acted  on  by 
the  contained  gases.  Moreover,  the  field  of  action  of  rain  is  wider 
than  that  of  moist  air  ;  for  not  only  does  rain  act  on  the  surface 
of  the  rocks,  but  it  also  soaks  into  the  pores  and  interstices,  decom- 
posing and  weathering  the  constituent  minerals  as  far  as  it  can 
reach.  It  is  in  this  way  that  granites,  which  crop  out  on  moorlands 
and  other  low-lying  situations  where  the  natural  drainage  is  slow, 
frequently  become  decomposed  to  a  depth  of  many  feet.  This 
decomposition,  as  we  have  already  seen,  is  the  work  of  the  carbonic 
acid,  which  attacks  the  felspar — the  silicate  of  alumina  and  potash — 
with  great  energy.  The  potash  unites  with  the  carbonic  acid, 
forming  a  carbonate  of  potash  which  is  soluble  in  water.  With 
one  important  constituent  broken  up.  the  other  constituents  are 
loosened.  Outcrops  of  granite  that  have  been  disintegrated  in 
this  way  can  be  easily  excavated  with  a  pick,  and  in  some  cases 
dug  out  with  a  spade. 

The  milky  white  clay  that  is  found  mixed  with  the  loosened 
quartz  grains  and  mica  scales  is  the  silicate  of  alumina  liberated 
from  the  decomposed  felspar.  It  is  the  mineral  which  forms  the 
commercially  valuable  deposits  of  Kaolin  so  often  found  in  the 
vicinity  of  granite  outcrops. 

Rain-water  is  always  a  carrier  of  carbonic  acid  ;  hence,  when 
it  finds  its  way  into  cracks  and  joints  in  limestone,  the  rock  is 
slowly  dissolved  and  in  this  way  the  cracks  become  wider  and 
larger.  The  caves  and  underground  tunnels  and  passages  that  are 
so  prevalent  in  limestone  formations  are  merely  cracks  or  joints 
that  have  been  enlarged  by  the  action  of  surface-water  containing 
carbonic  acid. 

Rain  is  also  a  conveyor  of  oxygen  gas.  Hence  we  find  that 
wherever  surface-water  has  penetrated,  the  rocks  are  always  more 
or  less  oxidised  and  decomposed.  The  most  obvious  effect  of  this 
decomposition  is  the  staining  of  the  rock  a  yellow  or  rusty-brown 
colour,  due  to  the  oxidising  of  the  iron  protoxide  and  sulphides  as 
previously  described.  As  may  be  observed  in  many  quarries  and 
railway-cuttings,  the  oxidised  yellow-coloured  portion  of  a  rock  is 


22  A    TEXT-BOOK    OF    GEOLOGY. 

always  softer  and  more  friable  than  the  underlying  unoxidised 
blue  portion. 

In  the  course  of  mining  operations,  rocks  are  sometimes  found  to 
be  oxidised  to  a  depth  of  50  or  even  100  feet  below  the  surface. 
In  some  of  the  Kimberley  diamond  mines  in  South  Africa,  the 
oxidised  zone,  or  what  is  locally  known  as  the  yellow  ground, 
descends  to  a  depth  of  100  feet.  Below  the  yellow  ground 
comes  the  unoxidised  rock  called  blue  ground. 

It  should  here  be  noted  that  the  oxidation  of  ferrous  oxide  in 
the  presence  of  moisture  results  in  the  formation  of  the  hydrous 
ferric  oxide  called  limonite,  which,  as  before  stated,  imparts  its 
characteristic  yellow  and  rusty-brown  colours  to  rocks  within  the 
zone  of  weathering. 

Weathering  and  oxidation  are  found  to  proceed  most  rapidly 
along  cracks  and  stratification  planes,  because  it  is  along  these  that 
surface-water  can  most  easily  find  its  way.  When  the  rock  is 
crossed  by  two  systems  of  joints  crossing  each  other  at  nearly 
right  angles,  in  the  earlier  stages  of  weathering,  the  only  signs 
of  oxidation  are  confined  to  the  walls  of  the  cracks.  As  the  weather- 
ing proceeds  the  unoxidised  portion  gets  smaller  and  smaller  until 
only  a  core  of  unaltered  rock  is  left.  When  the  oxidation  is  com- 
plete no  unoxidised  core  of  solid  rock  remains. 

Spheroidal  Weathering. — A  rock-mass  that  is  intersected  by  two 
systems  of  joints  lying  at  right  angles  to  one  another  is  obviously 
divided  into  a  series  of  cubes  or  cuboidal  blocks.  It  is  found  that 
when  some  ferruginous  sandstones,  claystones,  and  basalts  are 
jointed  in  this  way.  the  weathering  proceeds  in  concentric  layers 
around  each  block,  the  layers  frequently  presenting  various  shades 
of  yellow  or  brown.  When  the  blocks  are  exposed  on  the  face  of  a 
cliff  or  cutting,  the  different  layers  are  found  to  exfoliate  or  peel 
off  like  the  successive  coats  of  an  onion.  This  process  of  weathering 
is  termed  spheroidal  weathering  (Plates  V.  and  VI.).  In  the 
case  of  greywacke,  granite,  basalt,  andesite,  phonolite,  and  most 
igneous  rocks,  it  is  not  uncommon  to  find  a  core  of  solid  undecom- 
posed  rock  in  the  centre  of  the  spheroid. 

Effect  of  Rain  on  Sulphides. — The  oxidising  effect  of  rain-water 
is  very  noticeable  in  the  case  of  sulphide  ore-deposits.  By  long- 
continued  exposure  to  the  action  of  descending  surface-waters, 
the  outcrops  of  iron,  copper,  and  silver  sulphide  lodes  are  frequently 
oxidised  and  so  altered  as  to  bear  little  resemblance  to  the  unaltered 
lode-matter,  which  is  generally  found  at  a  greater  depth.  The 
iron  sulphides  are  first  oxidised  to  sulphates  and  then  to  oxides, 
while  the  copper  is  removed  by  the  water  as  soluble  sulphates,  or 
is  oxidised  to  carbonates  which  stain  the  rock  green  and  blue. 

The  far-reaching  effect  of  rain-water  is  well  seen  in  the  Broken 


To  face  page  22.] 


[PLATE  VI. 


SPHEROIDAL  WEATHERING  or  ORDOVICIAN  CHERT,  MISSOURI. 
(After  Ball  and  Smith.) 


THE  DENUDATION  OF  THE  LAND.  23 

Hill  lead  and  silver  mines  in  New  South  Wales,  and  at  the  cele- 
brated Mount  Morgan  mine  in  Queensland,  where  the  ores  are 
oxidised  to  a  depth  of  over  200  feet  below  the  surface. 

Hydration. — Many  minerals  when  exposed  to  the  action  of 
moisture  possess  the  property  of  absorbing  a  certain  definite 
proportion  of  water,  a  process  which  is  chemically  termed  hydration. 
Thus,  the  mineral  olivine,  when  hydrated,  becomes  serpentine  ; 
and  anhydride,1  the  anhydrous  sulphate  of  lime,  changes  into 
gypsum,  the  hydrous  sulphate,  the  change  being  accompanied  by 
an  increase  of  volume  amounting  to  33  per  cent.  Hydration  is 
one  of  the  results  of  weathering,  and  is  confined  to  the  zone  of 
oxidation. 

When  the  hydration  is  accompanied  by  increase  of  bulk  the  pro- 
cess may  cause  disruption,  fracturing,  or  disintegration  of  the 
adjacent  rocks  or  rock-surfaces. 

Mechanical  Effects  of  Rain. — We  will  now  consider  the  mechanical 
effects  of  rain  as  distinguished  from  that  of  running  water  in  the 
form  of  streams  and  rivers. 

The  principal  effect  of  a  pelting  rain  is  to  displace  the  particles 
of  rock  loosened  by  the  chemical  action  of  the  atmospheric  carbonic 
acid.  Under  the  influence  of  gravity  the  particles  tend  to  fall  to 
a  lower  level  where  they  will  accumulate  in  favourable  situations  ; 
or  perhaps  they  may  find  their  way  into  some  small  trickling  stream 
by  which  they  are  slowly  rolled  downwards  until  they  finally  reach 
a  river  which  carries  them  towards  the  sea. 

Earth-Pillars.— Another  well-known  effect  of  rain  is  the  produc- 
tion of  what  are  termed  earth-pillars.  Miniature  examples  of  these 
may  be  seen  after  heavy  rain  in  many  a  newly  ploughed  field,  or  on 
the  sloping  bank  of  a  newly  formed  road-cutting.  A  small  pebble 
or  flake  of  stone  acts  as  a  protecting  cap  or  umbrella,  so  that,  while 
the  surrounding  soil  or  clay  is  washed  away  by  the  rain,  the  portions 
protected  by  a  cap  of  stone  remain  for  a  time  forming  cone-shaped 
pillars. 

Gigantic  earth- pillars,  in  some  cases  attaining  a  height  of  20  feet 
or  more,  are  frequently  formed  in  the  glacial,  boulder  clays  and 
moraines  of  Scotland,  Switzerland,  New  Zealand,  and  other  glaci- 
ated countries. 

Formation  of  Soil. — The  angle  of  rest  of  wet  clay  is  16°  ;  of  sand, 
22°  ;  and  of  splintered  rock  and  shingle,  40°.  It  is  therefore 
obvious  that  on  all  surfaces  flatter  than  the  angle  of  rest,  the  pro- 
ducts of  weathering  and  disintegration  will  tend  to  accumulate 
where  they  were  formed,  except  perhaps  on  the  face  of  crags  and 
scarps  where  the  rocky  face  is  exposed  to  driving  winds  and  pelting 
rain,  or  the  drag  of  winter  snows. 

1  Gr.  a = without,  and  hudor  =  water. 


24 


A    TEXT-BOOK    OF    GEOLOGY. 


On  the  loosened  weathered  crust  such  lowly  forms  of  plant  life 
as  lichens  and  mosses  soon  establish  themselves,  their  roots  and 
rootlets  penetrating  into  all  the  crevices  of  the  disintegrated  rock 
surface.  The  decaying  vegetation  produces  humic  and  other 
organic  acids  which  disintegrate  the  surface  still  further.  Thus, 
as  time  goes  on,  the  particles  of  rock  become  mixed  with  decaying 
vegetable  matter,  forming  a  dark-brown  vegetable  humus  or  soil. 

The  thin  layer  of  soil  (fig.  2)  thus  formed  soon  attracts  grasses, 
shrubs,  and  trees  which,  owing  to  their  more  vigorous  growth, 
send  their  roots  deeper  into  the  broken  crust,  and  by  their  decay 
provide  a  larger  supply  of  organic  matter.  In  this  way  the  layer 
of  soil  becomes  deeper  and  richer,  and  frequently  darker  in  colour. 
Moreover,  in  favourable  places,  earth-worms  carry  on  their  opera- 
tions, crumbling  up  and  enriching  the  soil  with  their  castings. 

Below  the  soil  there  lies  the  subsoil,  which  consists  principally 


FIG.  2. — Showing  graduation  from  (c)  rock  to  (6)  subsoil,  and 
thence  into  (a)  vegetable  soil. 

of  comminuted  rock  and  clayey  material,  frequently  possessing 
a  yellow  or  brown  colour  due  to  the  oxidation  and  hydration  of 
the  iron  ;  and  below  the  subsoil  lies  the  decomposed  or  partially 
decomposed  rock. 

The  character  and  fertility  of  the  soil  depend  on  the  composition 
and  nature  of  the  rock  out  of  which  it  has  been  formed. 

Decomposed  mica-schist  and  calcareous  sandstones  produce 
light  soils  of  great  fertility  ;  basalts,  limestones,  and  marls  give 
soils  that  are  commonly  heavy  and  fertile  ;  andesites,  soils  heavy 
and  poor  ;  granite  and  rhyolite,  soils  light  and  poor. 

Soils  Mechanically  Formed. — Besides  soils  formed  in  situ  by  the 
chemical  corrosion  of  the  rocks  by  carbonic  and  other  acids,  many 
soils  owe  their  existence  to  the  mechanical  effects  of  rain  and  run- 
ning water.  The  rain  washes  the  finer  particles  of  rock  into  hollows 
and  depressions,  or  carries  them  within  the  influence  of  some  stream, 
by  which  they  are  borne  seaward.  In  times  of  flood  when  the 
stream  or  river  overflows  its  banks,  the  mud-laden  waters  deposit 
a  layer  of  silt  over  the  adjacent  lands.  According  to  the  duration 


THE  DENUDATION  OF  THE  LAND.  25 

of  the  inundation  and  the  amount  of  matter  held  in  suspension, 
so  is  the  thickness  of  the  deposit.  It  is  in  this  way  that  the  rich 
alluvial  flats  at  the  estuaries  of  rivers  and  in  river  valleys  are 
formed. 

Alluvial  flats  are  to  be  seen  in  almost  every  country  ;  but  perhaps 
there  is  no  better  example  of  the  mechanical  formation  of  soil, 
or  one  of  more  historic  interest  or  economic  importance,  than 
that  of  the  Nile,  the  seasonal  inundation  of  which  deposits  a  fresh 
layer  of  silt  over  the  surface  of  all  the  alluvial  lands  bordering 
the  river. 

ACTION  OF  SPRINGS. 

Accurate  gaugings  of  the  discharge  of  streams  has  shown  that 
only  a  certain  proportion  of  the  rainfall  within  a  given  watershed 
is  discharged  to  the  sea.  The  run-off,  as  it  is  termed,  is  dependent 
on  the  amount  of  evaporation,,  the  steepness  of  the  contours,  the 
presence  of  forests,  and  the  character  of  the  rocks  within  the 
drainage  area.  In  arid  regions  the  run-off  may  not  amount  to 
more  than  10  per  cent,  of  the  rainfall,  and  in  only  a  few  cases  does 
it  anywhere  exceed  40  per  cent.  This  means  that  a  large  quantity 
of  the  rain-water  soaks  into  the  soil  and  rocks. 

Many  rocks  are  so  open  or  porous  in  texture  that  they  are  what 
is  termed  pervious,  and  rain-water  slowly  sinks  into  them  until  an 
impervious  bed  or  stratum  is  reached.  When  this  happens  the 
water  flows  along  the  impervious  stratum,  and  if  this  stratum 
comes  to  the  surface  the  water  issues  as  a  spring. 

Calcareous  Waters.— In  its  slow  percolation  through  the  pores 
of  the  rocks,  the  water  dissolves  certain  constituents  and  thus 
becomes  more  or  less  charged  with  mineral  matter.  For  example, 
water  that  flows  through  a  limestone  formation  is  found  to  be  hard, 
this  hardness  being  due  to  the  dissolved  bicarbonate  of  lime  con- 
tained in  the  water. 

What  is  termed  the  temporary  hardness  of  water  is  represented 
by  the  bicarbonate  of  lime  that  is  precipitated  as  carbonate  of 
lime,  when  the  water  is  boiled.  The  boiling  disengages  the  molecule 
of  C02,  which  enabled  the  water  to  dissolve  the  carbonate  of  lime, 
and  thus  permits  the  carbonate  to  be  deposited  as  a  solid  incrusta- 
tion in  the  vessel.  The  permanent  hardness  of  water  is  the 
hardness  that  remains  after  the  carbonate  of  lime  has  been  pre- 
cipitated by  boiling.  It  is  mostly  caused  by  sulphate  of  lime, 
which  is  not  thrown  down  by  boiling. 

Waters  possessing  a  high  degree  of  temporary  hardness  are 
injurious  to  steam  boilers  on  account  of  the  hard  incrustations 
they  deposit. 

Where   calcareous    waters   reach   the   surface   they   frequently 


26  A   TEXT-BOOK    OF   GEOLOGY. 

deposit  a  white  crust  of  carbonate  of  lime  round  the  objects  over 
which  the  water  flows.  This  calcareous  sinter,  or  travertine  as  this 
deposit  is  called,  is  generally  porous  in  structure,  and  often  contains 
the  petrified  remains  of  mosses,  twigs,  and  various  plants  that  grew 
within  reach  of  the  spring. 

Stalactites  and  Stalagmites. — When  rain-water  in  its  underground 
journey  through  limestone  has  widened  out  a  fissure  to  the  dimen- 
sions of  a  cave,  the  slow  drip  of  calcareous  water  from  the  roof 
allows  the  feebly  attached  carbon  dioxide  to  escape  once  more  into 
the  air,  and  in  this  way  the  carbonate  of  lime  is  deposited  as  a  thin 
ring.  As  drop  succeeds  drop,  the  ring  of  carbonate  grows  thicker 
and  longer,  in  time  forming  a  long  tube  which,  by  subsequent 
deposit  inside,  becomes  solid.  As  the  process  goes  on,  so  the 
pendent  deposit  grows  longer  until  it  forms  what  is  termed  a 
stalactite,  which  in  form  somewhat  resembles  an  icicle  of  frozen 
water. 

The  drops  of  water  fall  on  the  floor  of  the  cave  and  deposit 
more  carbonate  of  lime.  In  this  way  there  is  built  up  a  solid 
pillar  or  stalagmite  that  in  many  cases  unites  with  the  depending 
stalactite,  forming  a  continuous  pillar  reaching  from  the  floor  to 
the  roof.  Stalactitic  calcareous  deposits  always  possess  a  beautiful 
radiating  fibrous  structure. 

Caves  and  underground  caverns  are  common  in  limestone 
regions  in  all  parts  of  the  globe.  Among  the  best  known  are  the 
Mammoth  Caves  in  Kentucky,  Wyandotte  Caves  in  Southern 
Indiana,  Peak  Caves  in  Derbyshire,  Dachstein  in  Upper  Austria, 
Jenolan  Caves  in  New  South  Wales  (Plate  VII.),  and  Waitomo 
Caves  in  New  Zealand.  Many  streams  and  rivers  flow  for  miles 
in  underground  channels  or  caverns,  the  extent  of  which  has  not 
yet  been  disclosed. 

Ferruginous  or  Chalybeate  Springs. — Rain-water  in  its  passage 
through  rocks  containing  sulphides  frequently  becomes  charged 
with  iron  salts.  When  the  water  issues  at  the  surface  the  iron, 
through  the  action  of  the  atmospheric  carbon  dioxide,  is  converted 
into  the  ferrous  carbonate.  The  carbonate  is  rapidly  oxidised  by 
the  oxygen  of  the  air  into  the  hydrous  oxide  which  falls  as  a  yellow 
or  foxy  brown  precipitate.  In  this  way  are  formed  the  limonite 
(hydrous  peroxide  of  iron)  veins  so  frequently  found  traversing 
ferruginous  sandstones  and  altered  igneous  rocks.  The  variety 
of  the  hydrous  peroxide  known  as  bog-iron  ore  is  formed  in  the 
bottom  of  swamps  and  lagoons  by  the  same  series  of  reactions, 
aided  by  the  operations  of  certain  species  of  bacteria. 

Brine  Springs. — The  underground  waters  that  in  the  course  of 
their  journey  come  in  contact  with  rock-salt  or  with  rocks  im- 
pregnated with  that  mineral  become  strongly  saline,  and  where 


[PLATE  VII. 


Photo,  by  0.  Trickett.] 

MAFEKTNG  GROTTO,  JENOLAN  CAVES,  NEW  SOUTH  WALES. 


THE  DENUDATION  OF  THE  LAND.  27 

they  appear  as  springs,  bring  large  quantities  of  the  dissolved  salt 
to  the  surface.  Brine  derived  from  artificial  wells  made  by  boring 
is  a  valuable  source  of  salt  (chloride  of  sodium)  in  Cheshire,  in 
England,  and  Rex  in  Switzerland. 

Mineral  Springs. — These  are  found  both  cold  and  hot.  Some 
are  alkaline,  containing  carbonates  of  soda  and  potash,  and 
bicarbonate  of  lime ;  others  are  acid,  containing  hydrochloric  or 
sulphuric  acid  mostly  combined  with  lime,  magnesia,  soda  and 
potash.  Free  hydrochloric  and  sulphuric  acids  are  frequently 
present  in  large  amount  in  the  hot  mineral  waters  that  abound  in 
some  volcanic  regions. 

In  regions  of  expiring  volcanic  activity  hot  mineralised  springs 
are  quite  common.  Notable  examples  of  these  are  found  in  the 
Yellowstone  National  Park  in  the  United  States,  in  the  North 
Island  of  New  Zealand,  and  volcanic  regions  of  Japan. 

Geysers  and  hot  springs  very  frequently  deposit  silica  or  siliceous 
sinter  around  their  vents  and  on  the  walls  of  their  passages.  In 
this  way  enormous  deposits  of  sinter  have  been  formed  in  Iceland, 
Yellowstone  National  Park,  and  Rotorua,  New  Zealand. 

The  silica  exists  in  the  water  in  the  form  of  soluble  alkaline 
silicates,  and  it  is  deposited  on  reaching  the  surface,  partly  owing 
to  the  decrease  of  temperature  and  pressure,  and  partly  owing  to 
the  atmospheric  carbon  dioxide  uniting  with  the  alkalies  whereby 
the  silica  is  liberated. 

Oil  Springs. — Petroleum  is  sometimes  brought  to  the  surface  by 
springs  and  spread  as  a  film  over  sheets  of  stagnant  water.  All 
the  productive  oil-wells  are,  however,  made  by  boring  holes  to  a 
porous  stratum  saturated  with  the  mineral  oil.  Some  of  the 
gushers  in  the  Texas,  Baku,  and  Maikop  oilfields  have  yielded 
many  thousands  of  barrels  of  oil  per  day. 

THE  WORK  OF  FROST. 

In  countries  where  the  temperature  falls  below  freezing  in  winter, 
frost  is  always  an  active  agent  in  disintegrating  and  disrupting 
rocks.  The  principle  underlying  this  is  the  circumstance  that 
water  in  the  act  of  freezing  expands  in  volume,  particularly  that 
which  contains  dissolved  gases.  When  the  expansion  takes  place 
in  a  sealed  vessel  or  bomb,  the  pressure  exerted  by  this  expansion 
is  almost  irresistible,  amounting  to  2000  Ibs.  per  square  inch. 

Rocks  and  soils  are  always  porous  and  contain  a  good  deal  of 
water.  When  this  water  freezes,  the  particles  are  pushed  a  little 
apart.  As  the  result  of  alternate  thawing  and  freezing,  the  particles 
are  forced  further  and  further  apart  until  they  are  finally  broken 
off  the  parent  rock.  In  this  way  the  surface  of  porous  sandstones 


28  A   TEXT-BOOK    OF    GEOLOGY. 

and  sandy  limestones  is  disintegrated,  crumbling  away  in  small 
flakes. 

The  destructive  effect  of  frost  is  strongly  marked  among  the 
higher  mountains  where  the  winters  are  severe.  Water  finding 
its  way  into  the  cracks  and  fissures  of  the  rocks  exerts  such  enor- 
mous disrupting  force  that  even  large  slabs  are  broken  from  the 
solid  formation.  In  many  regions  the  crests  of  the  mountains  have 
thus  become  covered  with  a  waste  of  angular  slabs  broken  up  by 
the  frosts  of  many  winters. 

Mountain  slopes  are  frequently  covered  with  a  mantle  of  loose 
angular  fragments  reaching  in  places  from  the  crest  to  the  base, 
forming  what  is  known  as  a  scree t  talus,  or  shingle  slide.  Where 
the  rock  is  of  a  friable  character,  such  as  a  claystone  or  slaty 
shale,  easily  acted  on  by  frost,  the  scree  may  extend  along  the 
slope  of  the  range  for  many  miles  ;  but  where  the  rock  is  of  a 
more  resistant  character,  the  scree  generally  takes  the  form  of  a 
cone  which  tapers  to  smaller  dimensions  as  it  reaches  upward. 

The  apron  of  tumbled  rock  fragments  and  blocks  which  accumu- 
lates at  the  base  of  most  cliffs  and  escarpments  is  called  a  talus. 

SUMMARY. 

From  what  has  been  said  in  the  foregoing  pages  we  find  that 
the  general  effect  of  the  different  agents  of  denudation  is  to  waste 
and  degrade  the  surface  and  edge  of  the  dry  land. 

The  agents  of  subaerial  denudation  range  themselves  in  two 
main  groups,  x namely :  those  which  operate  slowly  and  almost 
imperceptibly,  but  none  the  less  surely  ;  and  those  that  work 
energetically,  but  in  a  narrower  field. 

The  first  group  includes  air,  rain,  and  frost  ;  the  second  group, 
streams,  rivers,  and  the  sea.  In  this  chapter  we  have  onlv  dealt 
with  the  first  group,  and  in  a  general  way  we  may  summarise  their 
work  as  follows  : — 

(1)  Moist  air  and  rain  decompose  the  surface  of  rocks  by  dis- 

solving or  breaking  up  certain  constituents,  or  by  removing 
the  cementing  matrix. 

(2)  The   principal   agent   in   this   process   of   decomposition   is 

atmospheric  carbon  dioxide  acting  in  conjunction  with 
water. 

(3)  The  minerals  principally  acted  on  by  aqueous  solutions  of 

carbonic  acid  are  aluminous  silicates  containing  such 
bases  as  potash,  soda,  lime,  or  iron.  These  silicates  are 
found  in  all  igneous  rocks,  and  in  many  sandstones  and 
schistose  rocks. 

(4)  The  rocks  removed  or  broken  up  by  the  direct  dissolving 


THE  DENUDATION  OF  THE  LAND.  29 

action  of  carbonic  acid  are  limestones  of  all  kinds  and 
calcareous  sandstones. 

(5)  The  decomposition  of  a  constituent  mineral  or  the  removal 

of  the  cementing  medium  permits  the  rock  to  crumble  up 
or  become  disintegrated. 

(6)  The   yellow   and   rusty-brown   colour   of   soils,    clays,    and 

weathered  rocks  is  due  to  the  oxidation  of  the  iron  present 
in  the  silicates,  or  to  the  oxidation  of  sulphides,  or  of 
magnetite,  the  black  magnetic  oxide  of  iron. 

The  oxygen  contents  of  the  three  principal  oxides  of  iron  are  : — 

Ratio. 

Iron.     Oxygen. 

Protoxide  .         .         .         .^  ..   t  -       .     FeO  1  1 

Magnetite  (Protoperoxide)          .         .     Fe304         1  1-25 

Red  or  brown  Haematite  (Peroxide)   .     Fe203         1  1-50 

In  the  presence  of  moisture  the  atmospheric  oxygen  soon  con- 
verts the  protoxide  and  magnetite  into  the  peroxide.  In  this  way 
rocks  are  weathered  wherever  surface  water  can  find  its  way. 

(7)  In  the  interior  of  arid  or  rainless  regions,  the  changes  of 

temperature  as  between  day  and  night  disintegrate  the 
surface  of  the  rocks  by  alternate  expansion  and  contraction, 
in  the  same  way  as  scales  of  rust  are  thrown  off  steel  plates 
and  rails. 

(8)  The  wind  piles  up  loose  sand  into  dunes  and  ridges  along  the 

sea-coast,  and  in  continental  desert  areas. 

(9)  Caves  are  formed  in  limestones  owing  to  the  enlarging  of 

fissures  by  the  dissolving  action  of  the  carbonic  acid 
carried  in  solution  by  rain-water. 

(10)  Frost  causes  the  breaking  up  of  rocks  by  the  expansive 

force  exerted  by  water  when  it  freezes. 

(11)  Underground  water  when  it  appears  at  the  surface  forms 

springs.  Calcareous  waters  deposit  carbonate  of  lime  in 
caves,  forming  stalactites  and  stalagmites. 

(12)  Ferruginous  waters  deposit  peroxide  of  iron  where  they 

issue  at  the  surface,  and  also  in  swamps  and  lagoons, 
forming  bog-iron  ore. 

(13)  Geysers  and  hot  mineralised  springs  are  abundant  in  regions 

of  expiring  volcanic  activity. 

(14)  Hot  springs  containing  silica  in  solution  deposit  the  silica 

where  they  issue  at  the  surface,  forming  layers  of  siliceous 
sinter. 


CHAPTER   IV. 
THE   WORK  OF   STREAMS  AND   RIVERS. 

WHEN  rain  falls  a  portion  soaks  into  the  pores  and  interstices  of 
the  rocks  and  soil,  while  the  remainder  flows  over  the  surface  in 
hesitating  trickling  streamlets.  On  their  downward  course  a 
number  of  these  streamlets  unite  and  form  brooklets  which,  lower 
down,  grow  in  size  and  volume  until  they  become  large  brooks. 
Finally,  the  larger  brooks  unite  and  form  rivers  which  may  dis- 
charge their  waters  into  the  sea  or  a  lake. 

The  sea  or  lake  is  the  lowest  level  which  the  river  can  find,  and 
is  hence  termed  the  base-level. 

The  flowing  water  descends,  or  falls,  under  the  influence  of 
gravity  ;  and  in  its  haste  to  reach  its  base-level  it  follows  the  line 
of  least  resistance.  Hence  in  its  downward  course  it  bumps 
heavily  against  every  obstruction  that  lies  in  its  path.  The  finer 
particles  it  picks  up  and  carries  away  bodily  in  a  state  of  suspen- 
sion. The  heavier  grains  are  partly  pushed  and  partly  carried  along 
the  bottom  in  a  state  of  semi-suspension  ;  while  the  pebbles  and 
boulders  too  heavy  to  be  lifted  are  rolled  onward,  one  over  another. 
Against  the  rocks  that  are  too  heavy  to  be  moved  the  water  frets 
and  chafes  continually  until  at  last  the  obstruction  is  removed, 
the  removal  being  effected  mainly  by  mechanical  erosion,  but  also 
partly  by  chemical  dissolution  of  the  rock,  or  of  some  of  its  con- 
stituents. The  erosion  of  the  land  and  the  transport  of  material 
are  happenings  merely  incidental  to  the  passage  of  rain-water  to 
the  sea. 

GEOLOGICAL   WORK   OF   STREAMS   AND   RIVERS. 

Running  water  in  its  journey  to  the  sea  performs  a  double  role. 
It  acts  both  as  an  agent  of  erosion  and  transport. 

Erosive  Work  of  Streams. — The  erosive  work  of  streams  is 
partly  chemical  and  partly  mechanical. 

The  waters  of  all  streams  contain  dissolved  carbonic  acid  and 
oxygen  which  act  slowly  on  all  the  rock  surfaces  with  which  they 

30 


THE   WORK   OF   STREAMS   AND    RIVERS.  31 

come  in  contact.  The  rate  of  dissolution  of  the  rocks  is  imper- 
ceptible and  too  small  to  measure,  except  in  the  case  of  lime- 
stones and  calcareous  sandstones,  which,  in  river  courses,  are 
frequently  worn  into  wide  cavities  or  underground  channels. 

Chemical  analyses  have  shown  that  all  river-waters  contain  a 
certain  proportion  of  dissolved  mineral  matter  generally  varying 
from  10  to  40  parts  in  100,000.  Of  this  dissolved  matter  bi- 
carbonate of  lime  constitutes  the  major  part. 

Water  is  an  almost  perfect  lubricant ;  hence  its  erosive  or  abrasive 
action,  mechanically  considered,  is  practically  nil.  But  when 
running  water  transports  particles,  grains,  or  pebbles  of  solid 
matter,  it  becomes  a  powerful  agent  of  erosion;  its  work  being 
strikingly  seen  in  many  deep  water-courses  and  profound  gorges. 

The  excavating  and  erosive  power  of  rivers  depends  on  (1)  the 
rate  of  flow  ;  (2)  the  character  of  the  transported  detritus  ;  and  (3) 


FIG.  3. — Showing  graduation  of  river-drift. 

(a)  Zone  of  angular  blocks.  (d)  Zone  of  well-rounded  gravels. 

(6)         „        semi-angular  blocks.       (e)         „       water-worn  sands, 
(c)         .,        rounded  boulders.  (/ )        „        silt  and  mud. 

the  character  and  arrangement  of  the  rocks  through  which  the 
channel  is  excavated. 

The  Influence  of  Rapid  Flow. — When  the  flow  is  rapid  the  ex- 
cavating power  is  relatively  greater  than  when  the  flow  is  slow  ; 
for  not  only  do  the  travelling  sands,  pebbles,  etc.,  abrade  with 
greater  force,  but  a  larger  quantity  of  them  is  brought  into  action 
against  a  given  place  in  a  specified  time.  The  pebbles  and  loose 
stones  that  are  rolled  onward  along  the  bottom  rub  one  another  as 
well  as  the  rocky  channel,  until  they  are  reduced  to  the  condition 
of  fine  sand  or  mud.  By  this  rubbing  and  grinding  action  the 
sides  and  bottom  of  the  river-bed  are  widened  and  deepened. 
Pebbles  and  boulders  that  have  been  rolled  along  the  bottom  of  a 
river  are  always  smooth  and  generally  possess  a  rounded  or  roughly 
oval  shape. 

The  rocky  material  in  a  river  that  traverses  a  broken  mountainous 
country  is  commonly  rough  and  angular  near  the  source  (fig.  3), 
but  it  becomes  smoother,  rounder,  and  smaller  in  size  the  further 


32  A   TEXT-BOOK   OF   GEOLOGY. 

it  is  transported.  In  the  upper  part  of  the  valley  the  channel  is 
frequently  piled  up  with  large  angular  slabs  and  masses  of  rock 
that  have  fallen  down  from  the  heights  above  where  they  have 
been  broken  off  by  the  action  of  frost  or  rain.  The  waters  plunge 
below,  around,  and  over  the  obstructing  masses  against  which  they 
wage  an  unceasing  war.  Lower  down  the  valley  the  angular 
blocks  give  place  to  semi-angular  blocks  of  smaller  dimensions. 
Still  lower  down,  only  rounded  boulders  are  seen  ;  and  in  the 
lower  reaches  these  are  progressively  succeeded  by  gravels,  sand, 
and  mud. 

Rivers  of  great  length  that  traverse  wide  stretches  of  flat  land  in 
their  lower  course,  such  as  the  Mississippi  in  North  America,  the 
Amazon  in  South  America,  or  the  Yang-tse-kiang  in  China,  trans- 
port only  fine  sand  and  silt  to  the  sea.  On  the  other  hand,  rivers 
with  short  courses  and  steep  gradients,  such  as  those  draining  the 
Southern  Alps  of  New  Zealand,  discharge  enormous  quantities  of 
coarse  sand  and  gravel  into  the  adjacent  open  seas. 

The  rocky  debris  in  a  river-bed  is  subjected  to  so  much  attrition 
and  grinding  that  only  the  harder  material  is  able  to  survive  for 
any  considerable  distance  from  the  source.  The  softer  rocks  are 
soon  broken  up  into  small  fragments  or  reduced  to  the  size  of 
small  pebbles,  and  these  after  a  time  are  comminuted  to  the 
condition  of  mud  or  silt. 

If  we  examine  the  rocky  material  in  the  bed  of  a  stream  rising 
in  a  region  composed  of  granite,  mica-schist,  slate,  and  limestone, 
we  shall  find  that  near  the  source  angular  masses  of  all  these  rocks 
will  be  piled  up  in  the  channel.  As  we  proceed  lower  down  the 
stream,  the  proportion  of  granite  boulders  will  gradually  increase 
until  in  a  few  miles  they  greatly  predominate.  Still  further  down, 
schist,  slate,  and  limestone  pebbles  and  slabs  will  become  fewer 
and  fewer  until  granite  only  is  represented  in  the  river-gravels, 
together  with  quartz,  pebbles,  and  sand  derived  from  the  broken- 
up  mica-schist  or  from  fragments  of  disintegrated  granite. 

Thus  we  find  that  while  an  examination  of  the  gravels  in  the 
lower  reaches  of  a  stream  will  give  us  evidence  of  the  existence  of 
certain  rocks  within  the  drainage  area  of  the  stream,  it  may  utterly 
fail  to  give  a  complete  view  of  all  the  rocks  actually  present  within 
the  watershed.  There  may  exist  at  the  source  or  in  the  upper 
reaches  chalk  or  other  soft  limestone,  shales,  or  even  a  whole  series 
of  Tertiary  formations,  none  of  which  may  be  represented  in  the 
detritus  in  the  lower  reaches. 

The  presence  of  blocks  or  boulders  of  a  certain  rock  among  the 
gravels  of  a  river  cannot  be  always  taken  as  conclusive  evidence 
that  the  rock  exists  in  situ,  i.e.  in  place,  within  the  drainage  area 
of  the  river  in  question.  It  is  not  infrequently  found  that  in  regions 


THE  WORK  OF  STREAMS  AND  RIVERS.         33 

at  one  time  covered  with  glaciers,  blocks  of  stone  have  been  trans- 
ported from  one  watershed  to  the  other,  and  thus  find  a  resting- 
place  among  rocks  to  which  they  are  strangers.  Such  ice-borne 
blocks,  or  erratics  as  they  are  termed,  are  not  uncommon  in  the 
glaciated  portions  of  Great  Britain,  Northern  Europe,  New  Zealand, 
and  elsewhere. 

The  Erosive  Effects  of  Floods. — All  streams  and  rivers  are  subject 
to  seasonal  or  periodic  floods.  These  floods  may  be  due  to  the 
melting  of  snow  on  the  higher  ranges  during  spring  and  summer, 
or  to  abnormal  rainfall  at  any  time  of  the  year.  By  thus  increasing 
the  volume  and  depth  of  flow  the  transporting  and  eroding  power 
of  the  current  is  enormously  increased. 

The  velocity  of  a  river,  on  the  same  slope  and  with  the  same 
cross-section,  varies  as  the  cube  root  of  its  volume  ;  and  its  trans- 
porting power  varies  as  the  sixth  power  of  the  velocity.  That  is 
to  say,  if  the  volume  of  a  stream  is  increased  eight  times,  its  velocity 
will  be  doubled  as  3^/8 =2  ;  and  its  carrying  power  be  increased 
sixty-four  times  as  26  =  64. 

The  influence  which  changes  of  velocity  exercise  on  the  trans- 
porting power  of  a  stream  or  river  is  almost  incredible,  but  will 
be  in  some  measure  realised  from  the  following  statement : — 

Velocity.  Carrying  power. 

1  1 

2  64 

3  729 

4  4096 

5  15625 

Let  us  take  an  actual  case.  The  normal  flow  of  the  Shotover 
River  in  New  Zealand  amounts  to  350  cubic  feet  per  second,  but 
the  flood  volume  is  about  9500  cubic  feet  per  second,  equal  to  an 
increase  of  twenty-seven  times.  Hence  the  normal  velocity  is 
trebled,  as  \/27  =3.  In  other  words,  the  transporting  power  of  the 
river  is  increased  over  seven  hundred  times  ;  and  during  flood- 
times  it  can  carry  masses  of  rock  weighing  a  ton  as  easily  as 
3-lb.  pebbles  when  the  flow  is  normal. 

Blocks  and  boulders  that  a  stream  could  not  even  move  at 
times  of  normal  flow  may  be  carried  down  the  channel  for  many 
miles,  there  to  be  left  stranded  as  obstructions  in  the  channel  until 
a  greater  flood  moves  them  still  further  down. 

During  floods  the  banks  are  rapidly  undermined  and  crumble  away, 
and  in  this  way  the  river-bed  is  widened  or  new  channels  formed. 

In  1842,  a  vast  landslide,  caused  by  a  flood,  blocked  the  Indus 
below  Bunji,  submerging  the  valleys  for  a  length  of  36  miles.  In 

3 


34  A    TEXT-BOOK    OF    GEOLOGY. 

1896,  a  glacier  blocked  the  Suru  Valley  in  the  Himalayas,  and  the 
imprisoned  water  when  it  burst  through  devastated  the  country 
below  for  40  miles. 

In  country  possessing  steep  slopes  the  rain  soaks  into  the  ground, 
and,  accumulating  on  a  clayey  face  or  impervious  rock,  causes  land- 
slips or  landslides  to  take  place.  The  tumbled  rocky  debris  forming 
the  slide  may  reach  down  to  the  river  torrent,  by  which  it  is  soon 
swept  away ;  or  it  may  fall  bodily  across  the  river-channel,  damming 
back  the  water  for  a  time.  When  the  pent-up  flood  at  last  breaks 
away  it  carries  everything  before  it.  Even  small  brooks  that 
normally  are  incapable  of  moving  anything  larger  than  a  grain  of 
sand,  in  times  of  flood  may  become  raging  torrents  capable  of  dis- 
placing millions  of  tons  of  rocky  debris  in  the  course  of  a  few  hours. 

The  bed  of  many  streams  and  rivers  is  covered  with  a  protecting 
screen  of  gravel,  sand,  or  mud  of  varying  depth.  That  is,  it  may 
be  only  a  few  inches  deep,  or  as  much  as  50  feet  in  the  case  of 
large  rivers. 

At  times  of  normal  flow  this  protecting  cover  is  almost  stationary, 
but  during  floods  it  is  rolled  down  stream,  its  place  being  taken  by 
fresh  material  transported  from  the  higher  reaches. 

The  majority  of  streams  and  rivers,  for  at  least  a  portion  of  their 
course,  flow  over  a  rocky  bed,  free  or  nearly  free  from  travelling 
gravel.  When  this  happens  the  floor  of  the  channel  frequently 
exhibits  many  inequalities,  and  this  is  particularly  the  case  where 
the  stream  flows  over  rocks  of  different  degrees  of  hardness  and 
toughness,  the  softer  rocks  being  worn  into  depressions,  while  the 
harder  form  bars  and  obstructing  reefs. 

Formation  of  Pot-Holes. — A  striking,  but  not  uncommon,  feature 
of  many  rocky  river-beds  is  the  presence  of  pot-holes  of  various 
shapes  and  dimensions.  These  holes,  which  are  generally  round 
and  cauldron-like  in  form,  are  more  common  where  the  bed  is  steep 
than  elsewhere.  They  are  formed  by  the  rocking  and  grinding 
action  of  a  hard  boulder  moved  by  the  swirling  eddies  and  acting 
for  a  long  time  at  one  point.  When  the  pot-hole  has  become 
large  enough,  it  is  liable  to  be  invaded  by  one  or  more  new  boulders 
which  by  their  united  action  may  enlarge  the  hole  until  it  is 
many  feet  deep  and  many  yards  wide. 

Erosive  Effects  of  Transported  Material. — The  sands,  gravels, 
and  boulders  carried  onward  by  the  current  of  a  stream,  besides 
acting  on  one  another,  also  grind  away  the  floor  and  sides  of  the 
channel,  which  is  thereby  gradually  deepened  and  widened.  In 
other  words,  their  erosive  1  effect  is  both  vertical  and  lateral,  and 

1  Corrasion  is  a  variant  of  corrosion  that  has  been  used  by  some  writers 
to  denote  the  vertical  excavation  performed  by  a  stream.  The  term  does 
not  seem  preferable  to  the  word  erosion  commonly  used  by  English  geologists. 


THE   WORK   OF   STREAMS   AND    RIVERS.  35 

the  harder  and  tougher  they  are,  the  greater  will  be  their  abrasive 
power.  The  erosive  effect  of  fine  matter  held  in  suspension  is  very 
feeble,  the  greatest  amount  of  excavation  being  effected  by  the 
semi-suspended  sands,  and  by  the  gravels  that  are  rolled  and  pushed 
along  the  floor. 

Erosion  of  Different  Rocks. — The  excavation  of  soft  rocks  is 
more  easily  accomplished  than  that  of  hard  rocks  ;  for  this  reason 
a  stream  will  frequently  bend  from  its  normal  direction  to  follow 
along  the  course  of  a  soft  and  easily  eroded  formation.  In  places 
where  a  river  is  compelled  to  cut  through  a  stretch  of  hard  rocks 
its  channel  is  generally  deep  and  narrow,  the  flowing  water  in 
seeking  its  base-level  showing  the  same  economy  in  excavation  as 
the  miner  in  cutting  his  water-race  through  the  same  class  of 
ground. 

Where  the  rock-formation  is  soft  and  comparatively  easy  to  ex- 
cavate, the  channel  is  nearly  always  relatively  wide  and  shallow. 


FIG.  4. — (a)  Joint  forming  initial  water-course  and  afterwards 
widened  into  a  broad  valley.  The  dotted  lines  show  the  pro- 
gressive widening  of  the  valley. 

Factors  in  Selection  of  River-Course. — The  causes  which  have  led 
to  the  selection  of  the  course  followed  by  rivers  in  their  descent 
to  the  sea  are  various.  In  the  main  they  may  all  be  said  to  result 
from  the  natural  tendency  of  the  water  to  find  its  base-level  by 
the  shortest  and  easiest  route,  for  brook  and  river  alike  will 
select  the  route  that  offers  the  least  resistance  to  their  downward 
course. 

If  left  to  itself,  a  stream  will  always  excavate  its  course  in  a  soft 
formation  in  preference  to  a  hard  one  ;  and  follow  a  line  or  zone 
of  shattered  rock  rather  than  cut  a  channel  through  a  compact 
unbroken  formation.  In  every  case  water  will  follow  an  opening 
or  crack  already  formed  rather  than  cut  a  new  channel  for  itself. 

Observations  in  the  field  in  many  lands  have  shown  that  the 
main  valleys  of  many  rivers  follow  the  course  of  powerful  faults  or 
crustal  dislocations  ;  or  follow  lines  of  subsidence  resulting  from 


36 


A    TEXT-BOOK    OF    GEOLOGY. 


the  operation  of  a  system  of  parallel  faults  ;   or  run  in  depressions 
left  by  the  uplift  of  parallel  mountain  blocks. 

As  we  shall  later  find,  many  rocks  are  traversed  by  one  and  some- 
times two  series  of  more  or  less  parallel  cracks,  known  as  joints. 
Water  finds  its  way  along  these  quite  easily.  In  process  of  time 
the  joints  become  enlarged  by  erosion  into  water-courses,  and  in 


FIG.  5. — Gorge  in  plateau. 

this  way  we  have  the  beginning  of  what  may  afterwards  open  up 
into  a  broad  valley  (fig.  4). 

For  the  most  part  the  main  trunk  valley  follows  the  course  of  a 
great  fault  or  dislocation,  while  the  direction  and  situation  of  the 
subsidiary  or  side  valleys  has  been  determined  by  the  presence  of 


FIG.  6.— Profile  of  Shotover  Valley,  N.Z. 
(a)  Shotover  R.  (&)  Shotover  Fault  (c)  Mica-schist. 

smaller  lateral  faults,  joint  planes,  or  the  existence  of  zones  of 
soft  rock, 

Gorges. — Where  water  flows  through  a  rift  in  compact  rock, 
and  the  gradient  is  steep,  it  soon,  geologically  speaking,  excavates 
for  itself  a  narrow  rocky  channel.  Where  the  channel  is  deep,  with 
steep  sides,  it  is  termed  a  gorge. 

The  gorge  may  be  excavated  through  a  plateau  or  tableland 
(fig.  5),  or  in  the  floor  of  a  broad  ancient  glacial  valley  ;  or  it  may 
cut  through  a  mountain-chain. 


THE   WORK    OF   STREAMS   AND   RIVERS.  37 

In  recently  glaciated  countries  there  are  to  be  found  many  fine 
examples  of  gorges  excavated  along  the  floor  of  ancient  glacial 
valleys  (fig.  6). 

Canons.— Canons  are  profound  river-gorges  with  steep  walls. 
They  are  a  feature  of  deeply  dissected  plateaux  where  the  uplift 
and  consequent  river  erosion  have  been  rapid.  Their  depth  is 
always  great  in  proportion  to  their  width. 

The  most  remarkable  known  example  is  the  Grand  Canon  of 
Colorado,  which  extends  from  the  flanks  of  the  Rocky  Mountains 
to  the  head  of  the  Gulf  of  California.  The  river  has  cut  its  channel 
for  500  miles  across  the  desert  plateau,  which  consists  of  practi- 
cally horizontal  sedimentary  strata  of  various  ages,  Palaeozoic, 
younger  Mesozoic,  and  earlier  Cainozoic,  piled  on  a  floor  of  ancient 
granite. 

The  grandest  and  most  picturesque  part  of  the  canon  is  chiefly 
cut  through  Carboniferous  and  Permian  strata.  The  maximum 
depth  is  6000  feet  below  the  surface  of  the  plateaux.  (See 
frontispiece.) 

Two  phases  of  development  may  be  traced  when  the  canon  is 
viewed  in  cross-section,  namely,  an  upper  normal  valley  with  a  flat 
floor,  and  the  canon  proper  cut  in  the  floor  of  the  upper  valley. 
The  upper  valley  is  many  miles  wide  and  bounded  by  alternating 
steep  wall  and  moderate  slope,  their  edges  in  many  places  notched 
with  short  ravines.  The  inner  canon  is  a  narrow  trench,  bounded 
by  deeper  and  steeper  walls  that  are  in  places  3000  feet  high.  It 
was  cut  by  the  river  owing  to  an  acceleration  in  the  rate  of  uplift 
after  the  excavation  of  the  upper  valley. 

The  Grand  Canon  is  a  stupendous  example  of  river  erosion,  and 
has  no  parallel  in  any  part  of  the  globe.  Notwithstanding  its  vast 
depth,  the  steepness  of  the  walls  is  an  evidence  that  it  is  the  work  of 
a  river  still  in  the  youthful  phase  of  its  existence. 

The  Highlands  of  New  South  Wales,  comprising  the  uplands  of 
New  England,  the  Blue  Mountains,  and  the  Darling  Downs,  owe 
their  existence  to  the  dissection  of  an  ancient  plateau  by  rivers 
which  have  cut  deep  valleys  and  canons,  the  latter  with  steep  walls 
600  feet  in  height. 

Waterfalls. — Where  a  stream  or  river  has  cut  its  channel  in  a 
rock-formation  consisting  of  alternating  layers  of  varying  hardness, 
it  frequently  happens  that  a  waterfall  is  formed.  The  softer  rock 
is  eroded  at  a  greater  rate  than  the  harder,  with  the  result  that  the 
stream-bed  is  excavated  into  platforms  at  different  levels.  Where 
the  descent  from  one  platform  to  another  is  vertical  a  waterfall 
is  formed,  the  water  falling  bodily  from  one  level  to  the  other 
(fig.  7)  ;  and  where  the  descent  is  steep  but  not  vertical,  there  is 
frequently  a  number  of  small  waterfalls  or  cascades.  In  places 


38 


A    TEXT-BOOK    OF    GEOLOGY. 


where  the  slope  is  comparatively  low,  the  rapidly  flowing  current  is 
broken  up  into  what  is  termed  a  cataract  or  rapid. 

Recession  of  Waterfalls. — Where  the  strata  are  lying  horizontal, 


FIG.  7.— Profile  of  waterfall, 
(a)  Waterfall.  (6)  Cascade.  (c)  Rapids. 

or  nearly  so,  and  the  cornice  over  which  the  water  tumbles  is 
much  harder  than  the  underlying  beds,  the  waterfall  slowly  recedes 
upstream.  This  recession  is  due  to  the  undercutting  of  the 


FIG.  8.— Falls  of  Niagara. 

cornice,  which  gradually  crumbles  away  under  the  weight  of  the 
flowing  water.  In  this  way  the  gorge  or  ravine  of  the  river  is 
lengthened. 

One  of  the  most  striking  examples  of  recession  is  that  afforded 
by  the  Niagara  Falls  (fig.  8),  which  have  receded  a  distance  of  seven 


THE    WORK    OF   STREAMS    AND    RIVERS. 


39 


miles  in  late  geological  times,  the  rate  of  erosion  amounting  to  about 
4J  feet  a  year. 

Although  recession  is  most  marked  in  the  case  of  horizontal 
strata  capped  with  a  hard  cornice,  it  is  certain  that  it  takes  place 
in  all  waterfalls  independently  of  the  inclination  of  the  strata. 
Moreover,  in  some  volcanic  regions  many  fine  examples  of  retreat- 
ing waterfalls  (fig.  9)  are  seen  in  places  where  rivers  flow  across 
streams  of  lava  that  alternate  with  beds  of  loose  or  only  partially 
consolidated  ash. 

The  Winding  of  Streams  and  Rivers. — The  tendency  of  a  rapid 
stream  is  to  flow  in  a  straight  course,  and  in  order  to  attain  this 
end  it  will  act  with  great  energy  on  any  obstructions  that  lie  in  its 
path  until  they  have  been  removed.  When  the  stream  emerges 
from  the  highlands,  where  its  gradient  is  steep,  and  passes  on  to 
flat  or  undulating  ground,  where  its  rate  of  flow  is  slow  and  its 
excavating  power  therefore  relatively  feeble,  it  is  generally  found 
to  pursue  a  tortuous  course  frequently  meandering  about  the  plain 


X      "^~ — X          \  R 

FIG.  9. — Section  of  river  gorge,  showing  progress  of  recession. 

in  a  series  of  bends  and  loops  that  sometimes  overlap  or  almost 
touch  each  other. 

The  stream,  having  only  a  sluggish  flow  and  little  eroding  force, 
avoids  every  obstruction  it  meets,  whether  it  is  a  hard  band  of 
gravel  or  a  boulder  ;  and  in  this  way  it  bends  first  one  way  and  then 
the  other. 

At  all  times  the  main  current  is  directed  against  the  concave 
bank,  from  which  it  is  deflected  to  the  opposite  bank.  The  greatest 
erosion,  therefore,  takes  places  on  the  concave  bank,  which  during 
times  of  abnormal  flood  is  frequently  undermined  and  thus  crumbles 
away.  In  course  of  time  the  bend  becomes  sharper  and  larger, 
until  in  many  cases  the  area  of  land  between  two  loops  is  completely 
removed.  In  this  way  comparatively  insignificant  streams  are 
frequently  found  to  have  excavated  for  themselves  channels  of 
great  width.  The  process  of  excavation  will  be  readily  understood 
by  a  reference  to  the  next  figure. 

The  current  is  directed  against  the  bank  as  indicated  by  the 
arrows  (fig.  10),  so  that  in  course  of  time  the  space  enclosed  within 
the  stream  and  the  dotted  lines  is  worn  away.  When  this  has 


40 


A   TEXT-BOOK   OF    GEOLOGY. 


taken  place  the  bends  are  seen  to  be  sharper.  As  time  goes  on  the 
whole  of  the  spaces  marked  A  (fig.  10)  lying  between  the  loops 
are  removed,  thus  forming  a  wide  river-bed.  That  is,  the  bends 
gradually  widen  and  travel  downstream  until  the  ground  separat- 
ing them  is  eventually  cut  away. 

The  velocity  of  the  current  is  greatest  against  the  concave  bank 
and  least  on  the  convex  side  a.  As  a  result  of  this  the  current 
drops  a  portion  of  its  load  on  the  convex  side  at  a,  where  it  accumu- 
lates and  forms  a  sand  or  shingle  bank. 

The  General  Effect  of  Denudation. — The  total  effect  of  all  the 
subaerial  processes  of  denudation  is  to  lower  or  degrade  the  general 
level  of  the  dry  land.  It  is  obvious  that  if  denudation  continued 
long  enough,  the  land  would  be  reduced  to  a  plain  not  rising  much 
above  sea-level. 


FIG.  10. — Showing  winding  course  of  stream. 

We  have  already  seen  that  rivers  are  fed  with  detrital  matter  at 
their  sources,  a  large  proportion  of  which  in  a  finely  divided  form 
is  transported  to  the  sea.  But  a  river  possesses  main  tributaries, 
and  the  tributaries  have  their  branches.  These  branches  are  in 
their  turn  fed  by  streamlets  composed  of  innumerable  trickling 
rills.  A  river  system  with  its  numerous  primary,  secondary,  and 
tertiary  branches  covers  the  land  with  a  network  of  water-courses, 
each  of  which  carries  its  quota  of  denuded  material  into  the  trunk 
river,  whence  it  is  carried  to  the  sea. 

It  is  therefore  obvious  that  in  all  regions  drained  by  rivers  every 
portion  of  the  surface  is  continually  under  the  influence  of  the 
ever  active  agents  of  denudation,  and  must,  therefore,  in  process  of 
time,  be  reduced  or  degraded  in  level. 

The  rate  of  denudation  will  be  greatest  in  the  highlands,  less  in 
the  foothills,  and  least  in  the  downs  and  plains  bordering  the  sea. 


THE  WORK  OF  STREAMS  AND  RIVERS.         41 

It  is  greatest  in  the  highlands  because  frosts  are  harder  and  the 
rainfall  more  copious  than  in  the  lowlands.  The  slopes  also  being 
steeper,  the  broken  and  disintegrated  rocks  receive  more  assistance 
from  gravity  in  their  downward  course.  Moreover,  the  gradient 
of  the  stream-beds  is  steeper  and  the  transporting  power  of  the 
current  proportionately  greater  than  in  the  low  country.  The 
degradation  of  the  highlands  may  be  regarded  as  a  species  of  active 
warfare  in  which  the  rocks  are  shattered,  broken,  and  ground  into 
gravel,  sand,  and  silt ;  that  of  the  lowlands  as  a  silent  wasting  of 
the  whole  surface  by  the  slow  and  almost  imperceptible  removal 


FIG.  11. — Taieri  River,  N.Z.,  with  its  network  of  tributaries 
on  the  north  side. 

of  the  soil  by  rain,  partly  in  solution,  but  mainly  in  the  form  of 
mud  or  silt. 

While  discussing  the  effects  of  the  denudation  accomplished  by 
running  water,  it  is  as  well  to  bear  in  mind  that  the  elementary 
function  of  rain  is  not  to  disintegrate  and  denude  the  land,  but  to 
find  its  way  back  to  the  ocean  from  which  it  came.  The  breaking 
up  and  eroding  of  the  land  over  which  the  water  flows  are  merely 
happenings  incidental  to  the  haste  with  which  the  return  journey 
is  made  to  the  parent  source. 

Development  of  River  Erosion. — When  a  stream  commences  the 
dissection  and  denudation  of,  let  us  say,  a  plain  of  deposition 
gradually  rising  from  the  sea,  it  starts  life,  so  to  speak,  as  a  tiny 
rivulet.  As  time  goes  on  this  infant  stream  extends  its  opera- 
tions. It  grows  longer,  and  by  draining  a  larger  area  gets 


42  A    TEXT-BOOK    OF    GEOLOGY. 

larger  and  stronger.  With,  increasing  age  it  is  joined  by  lateral 
streams  or  tributaries  which,  like  the  parent  stream,  also  extend 
their  courses,  and  in  time  are  supplemented  by  the  development 
of  branches  that  also  possess  other  branches  in  the  form  of  rills 
and  trickling  streamlets. 

Where  the  slopes  are  steep,  erosion  will  occur  ;  and  near  the  sea, 
where  the  gradients  are  gentle,  deposition  of  the  water-borne 
sediments  will  take  place,  so  that  eventually  the  whole  course  of 
the  stream  will  be  reduced  to  a  uniform  gradient.  When  this 
condition  is  reached  the  stream  is  said  to  have  found  its  grade-level. 
If  the  volume  of  the  water  and  the  rate  of  erosion  were  uniform 
throughout  the  whole  course  of  the  stream,  the  profile  of  the  river- 
bed would  be  a  straight  line.  But  the  volume  of  all  streams,  except 
those  flowing  across  an  arid  desert,  increases  from  the  source  down- 
wards, and  there  is  a  corresponding  increase  in  the  rate  of  erosion 
until  the  point  is  reached  where  the  gradient  begins  to  flatten. 
Below  this  point  the  erosion  gradually  decreases  until  it  eventually 
vanishes  at  sea-level. 

The  natural  tendency  of  the  differential  stream  erosion  and 
sedimentation  is  to  produce  what  is  called  a  curve  of  erosion  with 
the  concave  side  upwards.  This  curve  is  obtained  by  the  stream 
planing  off  the  projections  and  filling  in  the  hollows. 

A  river  reaches  its  greatest  erosive  activity  at  maturity.  But 
its  activity  possesses  within  it  the  germ  of  its  own  decay,  for 
the  more  the  land  is  degraded  the  flatter  become  the  slopes  of 
the  hills  and  the  gradients  of  the  streams.  Thus,  as  the  land 
gets  lower  and  lower,  the  eroding  power  of  the  river  and  its 
tributaries  gets  less  and  less,  until  a  stage  is  reached  when  the 
river  becomes  a  mere  transporting  agent  of  mud  and  silt.  At 
last  even  this  action  ceases  and  the  cycle  of  fluviatile  erosion  is 
complete.  The  decadence  arising  from  extreme  old  age  can  only 
be  arrested  by  an  increased  supply  of  water,  or  by  an  uplift  of 
the  land  which  will  once  more  provide  gradients  that  will  again 
revive  the  erosive  power  of  the  running  water. 

If  the  uplift  of  the  land  begins  at  a  late  stage  in  the  cycle  of 
erosion,  say  at  the  time  when  the  river  is  approaching  the  exhaus- 
tion of  its  denuding  power  owing  to  the  land  having  been  worn 
down  to  a  nearly  level  plain,  a  second  cycle  of  erosion  will  begin  on 
the  old  plain  ;  and  if  no  change  of  conditions  takes  place,  the  first- 
formed  plain  will  be  dissected  and  denuded  into  a  second  and  lower 
plain  of  denudation. 

Effects  of  Uplift  and  Subsidence. — It  is  obvious  that  the  pro- 
gressive growth  and  decay  of  river-erosion,  ending  in  the  formation 
of  a  plain  of  denudation,  can  only  take  place  if  the  land  remains 
throughout  in  a  state  of  rest,  neither  rising  nor  subsiding. 


THE    WORK    OF    STREAMS    AND    RIVERS.  43 

The  effect  of  uplift  occurring  at  any  time  before  maturity  has 
been  reached  will  be  to  increase  the  erosive  activity  of  the  river, 
while  occurring  after  maturity  it  will  cause  rejuvenation. 

On  the  other  hand,  a  subsidence  of  the  land  by  lowering  the 
gradients  will  accelerate  the  decadence  of  the  drainage  system, 
and  if  continued  it  will  finally  lead  to  extinction  by  destroying  the 
erosive  and  transporting  power  of  the  river  and  its  affluents. 

Development  of  a  River  System. — The  primary  requirement  in 
the  development  of  a  river  system  is  progressive  continental  uplift. 
Moreover,  the  nature  of  the  uplift  is  of  material  consequence  in 
determining  the  topographical  effects  that  may  be  produced  by 
subaerial  erosion  acting  on  the  surface  of  the  rising  land. 

When  the  uplift  is  uniform,  the  ultimate  effect  may  be  the 
formation  of  a  deeply  dissected  plateau  of  the  Colorado  type  ;  but 
if  the  uplift  is  differential,  the  upward  movement  being  faster  along 
the  axial  divide  of  the  ancient  land  than  it  is  along  the  sea-coast, 
the  result  will  be  the  development  of  foothills  characterised  by 
long  dip-slopes  and  corresponding  escarpments,  the  long  slopes 
being  presented  to  the  sea. 

If  the  rate  of  uplift  is  slower  than  the  normal  rate  of  the  marine 
erosion,  the  uprising  sea-floor  with  its  sheet  of  sediments  will  be 
worn  down  to  a  gently  sloping  plain  of  marine  erosion  that  will 
never  rise  above  sea-level ;  but  if  it  is  faster  and  continuous  over 
a  long  period,  there  will  be  developed  a  system  of  topographical 
features  the  form  of  which  will  be  mainly  dependent  on  the  nature 
of  the  uplift,  the  character  and  inclination  of  the  newly  uplifted 
strata,  and  the  climatic  conditions. 

Let  us  consider  the  case  of  a  uniform  uplift  in  an  arid  region. 
If  this  region  is  backed  by  a  prominent  chain  of  mountains  possess- 
ing a  copious  rainfall,  there  will  be  formed  an  arid  plateau  composed 
of  horizontal  strata  deeply  dissected  by  the  rivers  draining  the 
neighbouring  highlands.  By  such  uniform  uplift  we  may  obtain 
a  replica  of  the  Colorado  plateau  with  its  profound  canons  excavated 
by  the  rivers  that  drain  the  western  slopes  of  the  Eocky  Mountains. 

The  same  uniform  uplift  in  a  temperate  region  where  the  annual 
rainfall  exceeds  35  or  40  inches  will  produce  a  maritime  plain  or 
plateau  traversed  by  trunk  rivers  draining  the  ancient  highlands, 
and  scored  by  innumerable  tributary  streams,  by  which  the  surface 
is  carved  into  a  maze  of  narrow  ridges  and  flat-topped  hills.  Of 
such  origin  is  the  Wanganui  maritime  plain  in  New  Zealand,  the 
surface  of  which  is  sculptured  into  a  terrain  of  undulating  hills  and 
flat-topped  ridges,  the  survivals  of  the  original  plain.  In  a  distance 
of  fifty  miles,  the  younger  Tertiary  strata,  which  compose  this 
plain,  rise  gently  from  sea-level  to  a  height  of  2000  feet  as  a 
consequence  of  the  crustal  arching  of  the  central  volcanic  region, 


44  A   TEXT-BOOK    OF   GEOLOGY. 

the  southern  limits  of  which  are  dominated  by  Mount  Ruapehu, 
a  gigantic  volcano  girdled  on  three  sides  by  a  ring  of  limestone 
escarpment. 

Relatively  rapid  uplift  in  a  moist,  temperate  latitude  accom- 
panied by  axial  arching,  whereby  the  strata  are  tilted  at  angles 
above  10°  or  15°,  produces  a  series  of  more  or  less  parallel  foothill 
ridges  characterised,  as  already  indicated,  by  long  dip-slopes  and 
corresponding  escarpments,  which  are  especially  well-developed 
where  the  uplifted  rocks  consist  of  alternating  bands  of  hard  and 
soft  material.  Many  fine  examples  of  this  type  of  topographical 
feature  are  found  in  the  maritime  regions  of  most  continents  where 
uplift  has  taken  place  in  late  Tertiary  times. 

The  succession  of  long  dip-slopes  and  steep  escarpments  that 
lie  between  the  sea  and  the  Ruahine  Chain,  in  the  province  of 
Hawkes  Bay,  New  Zealand,  is  a  beautiful  and  picturesque  example 
of  the  work  performed  by  running  water  during  the  development  of 
a  river  system  resulting  from  differential  uplift. 

The  Actual  Development. — As  a  starting-point  let  us  assume  an 
old  land-surface  forming  highlands,  and  drained  by  a  river  running 
into  the  sea  approximately  at  right  angles  to  the  general  trend  of 
the  coast-line  as  shown  in  A,  fig.  HA. 

In  the  second  phase,  as  the  result  of  differential  uplift,  the  sea- 
floor  with  its  pile  of  sediments  gradually  rises  until  it  forms  a  strip 
of  new  land  running  parallel  with  the  old  strand.  The  ancient 
river,  that  existed  before  the  uplift  began,  still  finds  its  way  to  the 
sea  ;  for,  as  the  uplift  progressed,  it  encountered  little  difficulty  in 
cutting  its  channel  across  the  slowly  rising  truncated  edges  of  the 
sediments. 

The  course  of  the  river  is  transverse  to  the  strike  of  the  uprising 
beds  as  shown  in  B,  fig.  HA,  and  hence  is  called  a  transverse  river. 

As  the  uplift  progresses,  the  transverse  river  increases  in  length, 
and  its  channel  becomes  deeper  and  deeper. 

The  newly  raised  maritime  strip  of  land,  as  shown  in  the  second 
phase,  now  forms  the  foothills  of  the  ancient  highlands.  The  rain- 
fall on  the  foothills  creates  small  lateral  tributaries  that  run  more 
or  less  parallel  with  the  strike,  their  course  following  the  zones  of 
softer  rock.  And  because  these  streams  run  approximately  along 
the  strike  of  the  uplifted  strata  they  have  been  called  longitudinal 
streams. 

So  long  as  the  uplift  continues,  the  transverse  river  and  its 
longitudinal  tributaries  become  longer,  and  their  channels  deeper 
and  broader.  (C,  fig.  HA.) 

Where  the  transverse  river  cuts  through  bands  of  limestone, 
conglomerate,  or  other  hard  rock,  the  profile  of  its  channel  is  more  or 
less  V-shaped.  In  the  softer  zones  the  valley  is  usually  broad  and 


THE    WORK    OF    STREAMS   AND    RIVERS. 


45 


bounded  by  gentle  slopes.  Thus,  when  we  trace  such  a  river  from 
its  source  to  the  sea,  we  find  that  it  passes  alternately  through  a 
succession  of  deep  gorges  and  open  valleys.  As  a  good  example 
we  have  the  great  snow-fed  Clutha  River  in  New  Zealand,  which 
rushes  through  the  picturesque  Kawarau  Gorge  to  the  Cromwell 
Basin,  then  through  the  profound  Dunstan  Gorge  to  the  Manu- 
herikia  Basin,  and  finally  through  the  narrow  Roxburgh  Gorge  to 
the  Roxburgh  Flats. 


New  land 


New  land  surface. 


FIG.  HA. — Section  on  line  Aj-Bj,  showing  development  of  river  system 
arising  from  differential  uplift. 

(a)  Hard  stratum.         (6)  Soft  stratum.         (c)  Ancient  rock. 

(A)  First  phase.         (B)  Second  phase.         (C)  Third  phase. 

(D)  Section  along  line  A^B!. 

The  type  of  river  system  which  comprises  a  main  transverse 
river  with  numerous  longitudinal  tributaries  is  most  often  met  with 
in  maritime  regions  occupied  by  Cainozoic  or  younger  Mesozoic 
formations  that  were  laid  down  marginal  to  the  ancient  strands. 

Where  a  plain  of  marine  sedimentation  emerges  from  the  sea  as 
an  anticlinal  ridge  or  dome,  a  number  of  more  or  less  parallel 
transverse  rivers  will  be  developed  on  each  side  of  the  axis  of  eleva- 
tion. This  structure  is  well  seen  in  the  North  of  England,  where 
the  eastern  slopes  of  the  Pennine  Chain  are  drained  by  a  number 
of  large  rivers  that  rise  in  the  central  divide.  On  the  west  side  of  the 


46  A    TEXT-BOOK    OF    GEOLOGY. 

chain  the  symmetry  has  been  almost  obliterated. by  the  uplift  of  the 
Lake  District  and  the  greater  steepness  of  the  Pennine  Chain  itself. 

In  the  terminology  adopted  by  some  geographers,  the  transverse 
rivers  are  called  consequent,  because  they  are  the  result  of  uplift ; 
and  the  longitudinal  streams,  subsequent,  since  their  formation  is 
always  subsequent  to  that  of  the  consequent  rivers.  It  is  almost 
a  truism  to  say  that  all  trunk  rivers  are  a  consequence  of  uplift, 
and  hence  consequent,  and  the  tributaries  subsequent.  The  notable 
exceptions  are  the  streams  and  rivers  that  drain  the  slopes  of 
volcanoes,  which  owe  their  origin  not  to  crustal  uplift  or  arching, 
but  to  the  piling  up  of  lava  streams  and  ashes. 

Striking  examples  of  consequent  rivers  of  this  type  are  found 
draining  the  slopes  of  Mount  Egmont,  a  beautifully  symmetrical 
volcanic  cone  which  rises  abruptly  from  the  sea  in  the  south-west 
angle  of  the  North  Island  of  New  Zealand.  The  densely  wooded 
slopes  of  this  cone  are  drained  by  numerous  large  torrential  streams 
which  radiate  outward  from  the  cone  like  the  spokes  from  the  hub 
of  a  wheel. 

Base-Level  of  Erosion. — Theoretically  the  sea  is  the  ultimate 
base-level  of  all  streams  and  rivers,  and  is  the  level  to  which  the 
dry  land  should,  in  process  of  time,  be  reduced,  provided  no 
change  of  level  relatively  to  the  sea  took  place  during  the  cycle  of 
denudation. 

When  a  river  has  denuded  its  watershed  to  an  area  of  such  low 
relief  that  it  has  lost  its  eroding  and  transporting  power,  it  is  said 
to  have  reached  its  base-level,  and  the  land  surface  so  planed  down 
is  termed  its  plain  of  erosion.  Such  a  plain  is  a  plain  of  fluviatile 
erosion. 

When  elevations  of  harder  or  more  resistant  rock  stand  above 
the  general  level  of  the  surface,  such  a  plain  of  erosion  is  termed 
a  peneplain. 

Many  ancient  peneplains  have  been  elevated  by  faulting,  or  by 
slow  crustal  movement,  until  they  have  attained  such  a  height 
above  the  sea  as  to  form  plateaux. 

Peneplain  of  Arid  Erosion. — By  long-continued  exposure  to  the 
disintegrating  action  of  rain,  frost,  wind,  and  changes  of  tempera- 
ture, the  arid  interior  of  continental  areas  has  in  some  regions 
become  worn  down  to  a  nearly  level  surface,  or  a  level  surface 
dotted  here  and  there  with  hummocks  and  elevations  of  hard  rock 
that  have  been  able  to  resist  the  attacks  of  subaerial  denudation 
longer  than  the  surrounding  country.  Peneplains  of  arid  erosion 
frequently  occur  at  a  considerable  elevation  above  the  sea.  Among 
familiar  examples  we  have  the  veldt  or  plateaux  lands  of  the  Trans- 
vaal and  the  great  interior  plateau  of  Australia,  on  a  portion  of 
which  is  situated  the  goldnelds  of  Yilgarn  and  Kalgoorlie. 


THE    WORK    OF    STREAMS    AND    RIVERS.  47 

River-Piracy.— Streams  have  not  always  held  the  course  in  which 
they  now  flow.  If  a  stream  cuts  back  its  course  and  deepens  its 
bed  more  rapidly  than  a  stream  in  a  neighbouring  basin,  it  may 
work  its  way  across  the  intervening  divide  and  rob  the  head 
waters  of  that  stream,  always  provided  its  bed  is  deeper  than  the 
floor  of  the  valley  that  has  been  invaded. 

If  the  valley  occupied  by  stream  a  b  c  (fig.  12)  is  deepened  more 
rapidly  than  the  valley  drained  by  stream  gfde,  a  tributary  of  the 
former  b  c  may  cut  its  course  back  to  d,  and  thereby  steal  the  head- 


Fia.  12. — Showing  progress  of  stream-piracy.  • 

waters  d  e  of  stream  gfd  e,  which  is  then  said  to  be  beheaded.  The 
invading  stream  is  known  as  a  pirate. 

The  beheaded  stream  is  diminished  in  volume  by  the  amount 
of  water  contributed  by  d  e,  while  the  volume  of  the  pirate  stream 
is  correspondingly  increased. 

Stream  a  b  c  with  its  larger  volume  of  flow  now  acquires  a  greater 
erosive  power,  and  continues  to  deepen  its  channel  faster  than  the 
beheaded  stream  fg.  The  result  of  this  is  that  the  divide  at 
the  head  of  fg  is  slowly  shifted  down  the  valley  towards/,  so  that 
the  drainage  of  the  portion  of  the  valley  lying  between  d  and /is 
in  time  reversed,  as  shown  in  fig.  13. 

Protecting  Effect  of  Basalt  Flow. — In  late  Tertiary,  that  is  in 
quite  recent  geological  time,  many  of  the  valleys  of  the  State  of 
Victoria  in  Australia  were  invaded  by  floods  of  basaltic  lava 


48 


A   TEXT-BOOK    OF   GEOLOGY. 


that  filled  up  the  river-courses  and  in  places  even  overflowed  the 
valley  walls. 

Since  the  emission  of  the  basalts,  the  country  has  been  dissected 


FIG.  13. — Showing  stream-piracy  accomplished. 

and  denuded  by  streams,  and  sculptured  into  the  existing  ridges 
and  valleys.  The  basalt-flows,  being  harder  than  the  older  slates 
and  sandstones,  have  resisted  the  wear  and  tear  of  denudation, 


FIG.  14. — Section  of  Mount  Greenock,  showing  protective  effects 
of  basalt-cap.     (After  A.  Brough  Smyth.) 

(a)  Wash-dirt  in  new  river-course.  (6)  and  (d)  Existing  water-courses, 

(c)  Wash-dirt  underlying  basalt  and  marking  site  of  ancient  river. 

with  the  result  that  the  old  valley  walls  have  been  worn  down  into 
new  valleys,  while  the  basalts  remain  as  flat-topped  ridges  as  shown 
in  fig.  14. 

Rate  of  Denudation. — This  relates  to  the  lowering  of  the  whole 


THE    WORK    OF    STREAMS    AND    RIVERS.  49 

surface  of  the  land  as  effected  by  the  united  action  of  all  the  agents 
of  denudation.  The  data  required  for  the  computation  are  (a)  the 
mean  annual  rainfall  as  determined  by  observations  extending 
over  a  number  of  years ;  (6)  the  area  of  the  watershed ;  (c)  the 
annual  discharge  ;  and  (d)  the  quantity  of  suspended  matter  carried 
to  the  sea  as  found  by  numerous  experimental  tests. 

The  margin  of  possible  error  that  may  be  introduced  into  com- 
putations of  this  kind  is  always  very  great,  mainly  on  account  of 
the  difficulty  and  expense  involved  in  the  obtaining  of  trustworthy 
mean  values  for  the  rainfall,  run-off,  and  quantity  of  transported 
matter.  Even  with  the  best  data  obtainable  the  results  cannot  be 
regarded  as  other  than  wide  approximations.  When  the  computa- 
tion is  based  on  a  few  isolated  observations,  the  results  are  likely 
to  be  quite  misleading  and  altogether  erroneous. 

The  rate  of  degradation  of  two  adjacent  watersheds  enjoying 
the  same  rainfall  may  be  quite  different.  Thus,  in  the  area  occupied 
by  the  hardest  and  most  resistant  rocks  the  rate  will  be  slowest. 
Moreover,  the  mean  altitude  above  the  sea,  steepness  of  contour, 
and  climate  must  be  included  among  the  many  conditions  that  may 
tend  to  modify  the  waste  of  the  land. 

The  Mississippi  has  been  estimated  to  lower  its  basin  1  foot  in 
5400  years,  and  the  Danube  1  foot  in  3500  years ;  while  the  whole 
area  of  England  is  reduced  by  subaerial  mechanical  denudation 
1  foot  in  about  3000  years. 

All  streams  and  rivers  carry  to  the  sea  a  considerable  annual 
load  of  mineral  matter  in  solution,  and  although,  perhaps,  a  large 
proportion  of  this  is  contributed  by  underground  waters  issuing 
at  the  surface  as  springs,  a  certain  but  indeterminate  portion  of 
it  must  represent  matter  dissolved  on  the  surface  by  moist  air 
and  rain.  The  English  rivers,  it  has  been  computed,  lower  their 
basins  1  foot  in  about  13,200  years  by  solution  alone,  but  it  should 
be  noted  that  estimates  of  this  kind  when  based  on  the  total  annual 
quantity  of  dissolved  matter  carried  to  the  sea  are  liable  to  be 
misleading,  as  there  seems  at  present  to  be  no  means  of  ascertaining 
what  proportion  of  the  dissolved  matter  is  due  to  underground 
dissolution  and  what  to  superficial. 

CONSTRUCTIVE  WORK  OF  RIVERS. 

Hitherto  we  have  regarded  streams  and  rivers  as  merely  agents 
of  erosion ;  but  they  are  not  always  destructive.  In  some  circum- 
stances they  may  also  be  constructive.  As  a  matter  of  every- 
day observation  we  know  that  streams  and  rivers  gradually  fill 
up  the  basins  of  the  lakes  into  which  they  drain  with  piles  of 
fluviatile  drift,  sand,  and  mud.  This  infilling  of  lake-basins  is 

4 


50  A   TEXT-BOOK    OF   GEOLOGY. 

relatively  rapid  in  the  case  of  valley-lakes  fed  by  torrential  alpine 
rivers. 

The  heavier  and  coarser  gravels  are  shot  into  the  head  of  the 
lake,  where  they  fall  to  the  bottom  almost  at  once,  forming  sheet 
after  sheet  of  the  inclined  beds  that  always  mark  the  arrangement 
of  fluviatile  drifts  discharged  into  still  water. 

The  finer  sands  and  silts  are  spread  as  a  sheet  over  the  floor  of 
the  lake,  and  as  the  infilling  progresses,  this  sheet  is  covered  over 
with  the  inclined  coarser  drifts  as  shown  in  fig.  15. 

When  the  lake  is  completely  filled  up  a  flood-plain  is  formed,  on 
the  surface  of  which  the  river  now  flows  towards  the  sea. 

As  the  barrier  at  the  lower  end  of  the  basin  becomes  worn  down, 
the  river  with  the  greater  slope  thereby  obtained  once  more 
becomes  destructive.  It  now  begins  to  cut  away  and  remove  the 
material  it  previously  laid  down,  and  in  this  way  the  dissection  of 
the  flood-plain  is  effected.  It  is  seldom  that  the  whole  of  the 
gravel  infilling  of  the  basin  is  completely  removed  during  this 


FIG.  15. — Showing  filling  of  lake-basin  by  river  detritus. 
(a)  Sand  and  silt.  (6)  Coarse  drift. 

period  of  destruction.  Commonly  we  find  that  benches  of  gravel 
have  escaped  destruction  in  various  places  around  the  margin 
of  the  lake-basin,  not  only  at  the  original  level  of  the  flood-plain, 
but  also  at  the  different  levels  at  which  the  river  temporarily 
established  itself  during  its  cycle  of  erosion.  These  gravel  benches 
or  terraces  are  a  striking  feature  of  alpine  valleys  in  many  lands. 

Many  of  the  great  alpine  lakes  of  New  Zealand  have  been  com- 
pletely filled  up  with  fluviatile  drifts  ;  and  all  the  existing  lakes 
are  being  rapidly  reclaimed  by  the  piles  of  detritus  unceasingly 
shot  into  them  by  the  torrential  rivers  that  drain  the  neighbouring 
alpine  chains. 

The  finer  sediments  discharged  into  a  lake  are  sorted  and  spread 
out  over  the  floor  in  a  succession  of  parallel  layers  or  beds  that  in 
a  general  way  conform  to  the  contour  of  the  bottom.  Such  deposits 
are  coarsest  near  the  edge  of  the  lake  and  finest  near  the  middle 
and  towards  the  lower  end.  The  remains  of  freshwater  fish,  mussels, 
and  other  molluscs,  of  land  animals,  of  tree-trunks,  twigs,  and  leaves 
are  frequently  found  in  consolidated  lacustrine  sediments. 

Besides  filling  up  lake-basins,  rivers  frequently  discharge  enor- 
mous masses  of  detritus  on  to  the  sea  littoral,  whereby  in  time 


THE    WORK    OF    STREAMS    AND    RIVERS. 


51 


wide  belts  of  land  are  reclaimed  from  the  sea.  Much  of  the  land 
thus  reclaimed  is  of  a  deltaic  character.  The  celebrated  Canter- 
bury Plains  in  New  Zealand  are  composed  of  gravel,  sand,  and 
mud  shot  into  the  sea  by  the  large  torrential  rivers  that  drain  the 
alpine  chain.  They  are  over  100  miles  long,  and  vary  from  20  to 
50  miles  wide.  The  low-lying  deltaic  plains  reclaimed  from  the 
sea  by  the  Yang-tse-Kiang,  Ganges,  Congo,  Nile,  Mississippi,  and 
Amazon  amount  to  many  thousands  of  square  miles. 

River-Fans. — Many  lateral  mountain  streams  at  the  point  where 
they  emerge  from  their  narrow  defile  gradually  pile  up  their  load 
of  sand,  gravel,  and  rocky  detritus  in  the  form  of  a  wide-spreading 


y  - —  *^*5— — P  '-c*.^* 

FIG.  16.— Fan  at  Tigar,  Ladakh.     (After  Drew.) 


fan,  which  may  in  time  encroach  so  far  over  the  floor  of  the  main 
valley  as  to  push  the  trunk  river  against  the  opposite  wall..  Good 
examples  of  river-fans  may  be  seen  in  most  mountain  valleys  where 
the  rainfall  is  abundant  and  the  denudation  rapid. 

Detrital  fans  of  great  extent  are  frequently  piled  up  on  the  sea- 
coast  by  mountain  streams,  and  a  number  of  such  confluent  fans 
may  form  wide  coastal  plains  like  the  Canterbury  Plains  in  New 
Zealand. 

SUMMARY. 

(1)  A  portion  of  the  rain  that  falls  on  the  surface  of  the  land 
soaks  into  the  rocks  and  soil,  but  another  and  larger  portion  flows 
over  the  surface,  at  first  forming  streamlets  that  eventually  unite 
and  form  brooks.  The  confluent  brooks  as  they  descend  the  slopes 


52  A   TEXT-BOOK    OF   GEOLOGY. 

form  large  streams  and  rivers.     The  ultimate  destination  of  most 
rivers  is  the  sea  or  a  lake,  while  a  few  die  out  in  sandy  deserts. 

(2)  The  streams  and  rivers  in  their  haste  to  reach  their  base- 
levels,  which  is  the  sea  or  some  large  lake,  wear  away  all  obstructions 
that  lie  in  their  path  ;    and  sweep  before  them  all  loose  particles 
and  rocks  that  tend  to   obstruct  their  downward  course.     The 
steeper  their  slope  the  greater  is  their  velocity  of  flow  ;    and  the 
greater  their  velocity  and  volume  the  greater  is  their  eroding  and 
transporting  power. 

(3)  Pure  water  is  a  nearly  perfect  lubricant ;    consequently  it 
possesses  little  or  no  abrasive  power.     But  the  loose  particles  of 
rock  and  boulders  transported  by  the  flowing  waters  rasp  and 
abrade  the  obstructing  rocks  which  are  thus  worn  away.     The 
transported   particles   do   not   escape   damage   in   this   continual 
warfare.     Like  the  rocks  which  they  abrade  they  also  are  abraded 
and  thereby  reduced  in  size.     Moreover,  the  mutual  wear  and  tear 
of  the  heavier  material  as  it  rolls  over  and  over  along  the  floor  of 
the  channel  reduces  the  size  of  the  fragments.     In  these  ways, 
that  is  by  the  attrition  of  the  obstructions  and  the  mutual  chafing 
and  grinding  on  the  river-floor,  we  find  that  angular  blocks  are 
rounded,  boulders  are  reduced  to  the  size  of  pebbles,  pebbles  to 
sand,  and,  finally,  sand  to  silt  and  mud. 

(4)  The  heavier  particles  are  rolled  along  the  floor  or  bed  of  the 
stream,  while  the  lighter  are  carried  in  suspension.     Hence  the 
heaviest  material  travels  the  shortest  distance,  and  the  finest  the 
furthest. 

(5)  Running  water  in  the  form  of  streams  and  rivers  therefore 
acts  as  (a)  an  agent  of  erosion,  and  (6)  as  an  agent  of  transport. 

(6)  Soft  rocks  are  eroded  more  rapidly  than  the  hard  and  more 
resistant. 

(7)  When  a  plateaux  is  occupied  by  a  formation  crowned  with  a 
hard  stratum,  the  wearing  away  of  the  softer  underlying  layers  of 
rock  enables  a  stream  or  river  draining  the  plateaux  to  excavate 
a  gorge. 

At  the  point  where  the  stream  plunges  into  the  gorge  there  is 
generally  a  waterfall  or  series  of  cascades.  The  rate  at  which 
the  recession  of  the  waterfall  takes  place  depends  on  the  rate 
at  which  the  hard  protecting  cornice  is  undercut  and  worn 
away. 

(8)  When  flowing  across  alluvial  plains  streams  and  rivers  possess 
an  inveterate  tendency  to  deviate  from  the  straight  course.     They 
generally  meander  across  the  plain  in  a  winding  course  consisting 
of  many  loops  and  bends.     The  winding  of  streams  is  due  to  the 
slow  rate  of  flow  which  enables  obstructions,  even  those  of  the 
feeblest  kind,  to  divert  the  stream  from  its  course. 


THE    WORK    OF    STREAMS    AND    RIVERS.  53 

(9)  The  greatest  erosive  effect  of  a  stream  is  on  the  concave  bank, 
which  is  commonly  the  steeper  for  this  reason. 

(10)  The  total  effect  of  all  the  subaerial  processes  of  denudation 
is  to  reduce  the  general  level  of  the  land.     If  denudation  were 
continued  long  enough,   without   compensating  uplift,   the   land 
would  be  in  time  reduced  to  a  condition  of  low  relief  not  much 
above  sea-level. 

(11)  A  river  with  the  aid  of  its  affluents  tends  to  reduce  the  level 
of  the  land  within  its  drainage  area.     Throughout  the  infantile 
and  youthful  stages  its  denuding  effect  is  ever  increasing,  and  this 
is  continued  up  to  maturity.     Through  continued  denudation  in 
the  uplands  and  constructive  work  in  the  lowlands,  the  gradients 
become  less  and  less  until  a   time  is  reached  when  the  sluggish 
waters  no  longer  possess  any  transporting  power.     This  period  of 
enfeebled  denudation  is  termed  the  decadent  stage.     When  denuda- 
tion practically  ceases,  the  land  having  been  reduced  to  a  plain 
or  peneplain  of  low  relief,  the  river  is  said   to  have  reached  its 
base-level. 

If  an  uplift  of  the  land  now  sets  in,  the  decadent  river  system 
would  be  rejuvenated,  and  the  more  rapid  the  uplift  the  greater 
will  be  the  denuding  activity.  In  this  way  the  peneplain  previously 
formed  will  become  dissected,  and  if  denudation  continues  long 
enough,  a  second  peneplain  lying  at  a  lower  level  will  be  carved  out 
of  the  first. 

(12)  Peneplains  may  be  also  formed  in  elevated  arid  regions  by 
long-continued   exposure  to  the  action  of  rain,  frost,  wind,  and 
changing  temperature.     The  elevated   plateaux   of  South  Africa 
and  Australia  were  probably  formed  in  this  way. 

(13)  One  of  the  local  effects  of  rapid  river-erosion  is  stream- 
piracy.     When  a  stream  cuts  back  its  course  more  rapidly  than  a 
neighbouring  stream,  it  may  work  its  way  across  the  divide  and 
annex  the  head  waters  of  the  other  stream. 

(14)  Sheets  of  basalt  frequently  afford  effective  protection  to 
softer  underlying  rocks. 

(15)  The  rate  at  which  the  whole  surface  of  the  land  within  a 
given  watershed  is  worn  away  by  the  united  processes  of  denuda- 
tion is  extremely  slow.     In  England  it  amounts  to  about  1  foot 
in  3000  years. 

(16)  Although  mainly  destructive,  rivers  are  also  constructive. 
For  example,  they  fill  up  lake-basins,  and  reclaim  large  maritime 
belts  of  land  from  the  sea.     The  Canterbury  Plains  in  New  Zealand 
are  a  striking  example  of  sea  reclamation  by  fluviatile  drifts. 


CHAPTER   V. 
SNOW   AND   GLACIERS. 

THE  present  glaciation  of  the  polar  regions  and  of  some  alpine 
chains  is  a  survival  in  a  diminished  form  of  the  glaciation  of 
the  Great  Ice  Age,  which  reached  its  maximum  severity  in  the 
Pleistocene,  the  period  which  immediately  preceded  the  time 
in  which  we  now  live. 

In  the  Great  Ice  Age  the  greater  portion  of  North  America  and 
Northern  Europe  was  covered  with  an  invading  sheet  of  polar  ice.  At 
the  same  time  large  portions  of  South  America,  Australia,  Tasmania, 
and  New  Zealand  were  covered  with  huge  glaciers  and  ice-sheets. 

The  best  evidences  of  ice-erosion  are  found  in  the  regions  that 
were  overrun  by  ice  in  the  Great  Ice  Age. 

Distribution  of  Glaciers  and  Snowfields. — Permanent  snowfields 
and  glaciers  exist  in  the  polar  regions  of  both  hemispheres,  and 
elsewhere  among  the  higher  mountain-chains  where  the  annual 
mean  temperature  is  below  the  freezing-point. 

Snowfields  of  less  permanency  are  found  in  more  temperate 
latitudes,  and  on  the  lower  slopes  of  high  ranges.  They  mostly 
disappear  with  the  advent  of  spring  and  summer.  The  line  above 
which  the  snow  remains  unmelted  throughout  the  year  is  termed 
the  snoivline.  At  the  poles  the  snowline  comes  down  to  the  sea. 
In  the  lower  latitudes  it  gradually  rises  till  it  attains  its  greatest 
altitude  in  the  tropics.  On  the  northern  slopes  of  the  Himalayas 
it  is  19,000  feet  above  the  sea,  and  in  the  Andes  18,000  feet. 

The  Action  of  Snow. — Snow  as  a  geological  agent  is  both  (1) 
protective  and  (2)  destructive. 

Protective  Effect. — As  a  winter  covering  on  the  foothills  and 
flat  slopes,  snow  protects  the  ground  and  vegetation  from  the 
action  of  frost  and  rain. 

Destructive  Effect. — When  snow  accumulates  on  steep  slopes 
it  slides  down,  and  in  doing  so  dislodges  obstructing  masses  of  rock 
and  furrows  the  soil,  pushing  all  loose  material  before  it.  A  good 
deal  of  broken  rock  and  soil  is  also  picked  up  by  the  frozen  snow, 
by  which  it  is  carried  from  a  higher  to  a  lower  level.  In  this  way 

54 


SNOW   AND    GLACIERS.  55 

screes  of  angular  shingle,  frequently  of  considerable  magnitude,  are 
piled  up  at  the  lower  limits  of  melting  snowfields. 

Where  snowfields  descend  to  the  edge  of  a  precipice  or  accumu- 
late on  steep  mountain  slopes,  large  masses  become  detached  in 
spring  and  summer  and  rush  down  as  avalanches  that  sweep  trees, 
soil,  rock,  and  all  movable  obstructions  before  them. 

Avalanches  are  frequently  compelled  by  the  contour  of  the 
ground,  down  which  they  bound  with  crashing  leaps,  to  follow  the 
same  route  year  after  year.  In  such  places  they  are  found  to  have 
carved  out  for  themselves  deep  gulches  in  the  solid  rock.  Such 
gulches  resemble  gigantic  chutes  and  are.  known  as  avalanche 
slides  (fig.  17).  Their  sides  are  frequently  walled  in  with  banks  of 
rocky  debris  torn  from  the  floor  by  the  masses  of  semi-frozen  snow 
as  they  thunder  down  to  the  valley  below. 

Streams  may  be  blocked  or  partially  dammed  by  masses  of  snow 


FIG.  17. — Showing  cross-section  of  avalanche  slide  on  slopes  of 

Ben  Ohau  Range,  N.Z. 
(a)  Bed  rock  of  slate  and  sandstone.  (6)  Piled-up  debris. 

that  have  fallen  from  the  heights  above.  In  1870  a  large  avalanche 
that  fell  from  Mount  Aspiring  in  the  Alps  of  New  Zealand  blocked 
up  the  bed  of  the  Upper  Matukituki  River  for  several  months,  and 
even  after  the  barrier  was  breached  masses  of  ice  remained  in  the 
valley  for  two  whole  years. 

When  the  winter  snows  melt  rapidly  in  spring  and  summer,  as 
they  frequently  do  in  temperate  climates  under  the  influence  of 
warm  rains,  they  may  cause  a  sudden  inundation  of  the  snow-fed 
rivers,  the  erosive  and  transporting  power  of  which  is  thereby 
enormously  increased  for  a  time.  Again,  in  arid  regions  bounded 
by  snow-clad  mountains,  many  of  the  streams  and  rivers,  as  in 
Central  Otago,  New  Zealand,  are  entirely  dependent  for  their 
summer  flow  on  the  supply  of  water  derived  from  the  melting 
snows  and  icefields  at  their  sources. 

GLACIERS  AND  ICE-SHEETS. 

The  Motion  of  Glaciers. — Glaciers  are  composed  of  compressed 
snow,  to  which  the  term  neve  is  usually  applied.  They  are 


56 


A    TEXT-BOOK    OF    GEOLOGY. 


nothing  more  than  rivers  of  ice  fed  by  the  snowfields  lying  on  the 
summits  and  slopes  of  the  adjacent  mountains. 

Ice  possesses  all  the  properties  of  a  viscous  body  ;  hence,  when 
it  accumulates  on  the  floor  of  a  sloping  valley,  it  descends  or  flows 
under  the  influence  of  gravity.  Where  the  valley  is  wide  it  spreads 
itself  out  like  a  river,  and  where  it  is  narrow  it  gathers  itself 
together  just  as  a  river  does  in  flowing  through  a  gorge.  It  flo\vs 
like  pitch  placed  on  a  sloping  plane,  and  accommodates  itself  to 
all  the  inequalities  of  its  bed. 

The  rate  of  flow  of  water  and  ice  is  alike  mainly  dependent  on 
the  amount  of  precipitation  and  the  gradient  of  their  bed.  Thus, 
while  the  flow  of  rivers  may  vary  from  1  to  15  miles  an  hour 
according  to  the  steepness  of  descent,  that  of  glaciers  is  found  to 
vary  from  1  foot  or  less  to  70  feet  a  day. 


T 

FIG.  18. — Plan  of  glacier  showing  differential  surface  flow.  Extent 
of  flow  in  middle,  6  e ;  and  at  the  sides  for  the  same  unit  of 
time,  a  d  and  c  /. 

On  account  of  its  comparative  rigidity  and  the  enormous  pressure 
exerted  by  the  moving  mass  of  ice  behind,  a  glacier  will  surmount 
and  ride  over  all  hummocks  and  projecting  spurs  that  lie  in  its 
path.  A  glacier  is  thus  able  to  pluck  blocks  of  rock  from  its  bed 
and  leave  them,  when  it  retreats,  perched  high  up  on  the  valley 
slopes.  For  example,  the  ancient  Wakatipu  glacier  in  New 
Zealand  tore  off  large  masses  of  Tertiary  limestone  at  the  edge  of 
the  lake  of  that  name,  carried  them  nearly  twenty  miles  and  left 
them  stranded  on  the  schist  slopes  of  Ben  Lomond,  nearly  2000 
feet  above  the  parent  rock.1 

The  flow  of  a  glacier,  like  that  of  running  water,  is  greatest  at  the 
upper  surface  near  the  middle  of  the  stream  of  ice,  and  least  at  the 
bottom  and  sides  where  the  friction  and  drag  are  greatest. 

A  striking  result  of  this  inequality  of  flow  is  the  formation  of 
crevasses  which  commonly  run  transversely  across  the  longest  axis 

1  Geology  of  Queenstown  Subdivision,  N.  Zi.  Oeo.  Survey  Bulletin,  No.  7, 
p.  28. 


SNOW    AND    GLACIERS.  57 

of  the  glacier.  The  differential  flow  is  also  responsible  for  the 
banded  structure  of  glacier-ice  termed  neve-stratification,  and  for 
the  semi-bedded  arrangement  of  the  rocky  debris  frequently  seen 
among  the  material  piled  at  the  terminal  face. 

What  is  meant  by  the  differential  surface  flow  will  be  easily 
understood  by  referring  to  fig.  18.  Let  us  suppose  that  a,  6,  and  c 
represent  three  blocks  of  stone  or  marks  placed  in  a  line  across  the 
glacier.  It  will  be  found  that  in  the  time  it  has  taken  blocks  a  and 
c  to  reach  points  d  and  /  respectively,  block  b  will  have  travelled 
to  point  e.  Blocks  a  and  c  have  moved  slower  than  e  because 
the  flow  of  the  ice  which  carried  them  has  been  retarded  by  the  drag 
or  friction  of  the  rocky  walls  of  the  valley. 

Referring  to  fig.  19,  we  find  that  while  block  6  on  the  surface  of 
the  ice  has  travelled  from  6  to  e,  block  p  on  the  bottom  has  in  the 


jpIQ  19  — Longitudinal  section  of  middle  of  glacier  showing  differ- 
ential flow  as  between  surface  and  bottom.  Extent  of  flow  at 
surface,  6  e ;  and  at  bottom,  p  s 

same  time  travelled  only  from  p  to  s,  the  slower  rate  of  travel  being 
due  to  the  friction  of  the  bed-rock. 

It  is  obvious  that  the  greater  the  drag  or  frictional  resistance 
the  greater  will  be  the  difference  of  flow  as  between  the  centre  and 
the  bottom  and  sides  of  the  stream  of  ice. 

Effect  of  Precipitation  and  Temperature.-  -The  size  of  a  glacier 
depends  on  the  amount  of  precipitation  and  the  temperature. 
With  an  increase  of  precipitation,  or  a  decrease  of  temperature, 
the  glacier  will  advance  ;  and,  conversely,  with  a  decrease  of  pre- 
cipitation, or  an  increase  of  temperature,  the  glacier  will  retreat. 
When  the  precipitation  and  the  temperature  act  in  the  same 
direction  there  will  be  an  acceleration  of  the  advance  or  retreat. 

A  good  example  of  the  effect  of  varying  precipitation  is  seen 
among  the  glaciers  that  descend  from  the  New  Zealand  Alps. 
On  the  west  side  of  the  chain,  where  the  precipitation  is  excessive 
and  the  slopes  steep,  the  Fox  glacier,  9- 75  miles  long,  and  the 
Franz  Josef  glacier,  8'5  miles  long,  descend  within  670  feet  and 
690  feet  of  the  sea  respectively,  in  43°  and  44°  of  south  latitude 


58  A    TEXT-BOOK    OF    GEOLOGY. 

corresponding  to  the  latitude  of  Boston  in  North  America  and 
Marseilles  in  South  France.  On  the  east  side  of  the  alpine  divide, 
where  the  precipitation  is  about  half  of  that  on  the  west  side,  and 
the  gradient  of  the  valley-floors  flatter,  none  of  the  glaciers  descend 
below  2350  feet  above  the  sea.  The  terminal  face  of  the  Tasman 
glacier,  19  miles  long,  is  2354  feet  above  the  sea,  but  the  thickness 
of  the  ice  below  that  level  is  unknown. 

Greater  Summer  Rate  of  Flow.-  The  flow  of  glaciers  and  ice- 
sheets  is  faster  during  the  day  than  at  night,  and  during  the 
summer  than  in  winter.  The  reason  for  this  increase  is  still 
obscure,  but  it  probably  arises  from  the  greater  temperature 
during  the  day  and  in  summer  causing  expansion  of  the  surface 
layers  of  ice  as  compared  with  what  they  are  at  temperatures 
below  32°  F. 

The  lineal  coefficient  of  steel  is  0-0000063  and  of  ice  0-0000528. 
Hence  with  a  rise  of  temperature  of  10°  F.,  a  strip  of  steel  as  long 
as  the  Tasman  glacier,  say  10,000  feet,  would  expand  6- 3  feet, 
while  a  strip  of  ice  the  same  length  would  expand  52-8  feet.  Ice 
maintained  at  a  temperature  below  32°  F.  must  expand  under 
the  influence  of  the  sun's  heat.  Being  free  at  the  terminal  end, 
the  effect  of  expansion  on  glacier-ice  would  be  to  augment  the 
normal  flow  due  to  gravity.  The  pressure  of  the  valley-walls 
would  prevent  lateral  expansion,  and  this  may  explain  the  arching 
of  some  10  feet  which  is  reported  to  take  place  on  the  surface  of 
the  Tasman  glacier  during  the  summer  months. 

Glacier  Tongues. — Prolongations  or  tongues  of  ice  that  extend 
beyond  the  limits  of  the  main  body  are  of  frequent  occurrence 
along  the  margin  of  continental  and  Piedmont  ice-sheets.  Many 
good  examples  of  these  may  be  seen  on  the  sea-front  of  the  great 
glaciers  that  descend  to  the  coast-line  of  Greenland,  fed  by  the 
inland  ice-sheet.  But  the  most  notable  are  found  in  the  Antarctic 
region.  One  that  has  become  well  known  in  connection  with 
Antarctic  exploration  is  Glacier  Tongue,  near  Hut  Point,  in 
M'Murdo  Sound,  South  Victoria  Land.  It-is  a  narrow,  elongated, 
somewhat  tabular  mass  of  ice  that  stretches  5  miles  into  the 
sea.  Where  it  rests  against  the  land  it  is  about  a  mile  wide,  and 
at  its  sea  end  about  half  a  mile.  Its  height  above  the  sea  varies 
from  20  to  100  feet.  The  great  depth  of  water  obtained  by  sound- 
ings off  the  sea-end  led  Professor  David  to  conclude  that  a  con- 
siderable portion  of  the  tongue  must  be  afloat.1  Some  distance 
further  north,  the  Nordenskyj old  and  Drygalski  ice-tongues  extend 
over  the  sea  20  and  30  miles  respectively. 

Mountain  glaciers  that  lie  in  basins  guarded  by  projecting  spurs 
or  buttresses  of  hard  rock  frequently  terminate  in  narrow  pro- 
1  The  Heart  of  the  Antarctic,  E.  H.  Shackleton,  ii.  p.  284,  1909. 


SNOW    AND    GLACIERS.  59 

longations  or  snouts  of  ice  that  may  extend  far  beyond  the  portals 
of  the  ravine. 

Distribution  of  Glaciers. — Glaciers  occur  at  sea-level  in  the  polar 
regions,  but  passing  towards  the  equator  they  are  found  at  gradually 
increasing  elevations. 

A  great  ice-sheet  covers  the  whole  of  Greenland  except  a  narrow 
fringe  around  the  coast.  Long  tongues  of  ice  descend  to  the  sea 
in  the  valleys  and  sounds. 

The  icefields  of  the  Antarctic  are  even  more  extensive  than  those 
of  the  Arctic.  They  everywhere  descend  to  sea-level,  and  even 
extend  over  the  surface  of  the  sea  for  many  hundreds  of  miles. 
The  ice  is  so  thick  and  spreads  over  the  sea  so  far  that  the  limits 
of  the  dry  land  cannot  be  ascertained.  The  Great  Ice  Barrier 
that  fringes  South  Victoria  Land  is  believed  to  have  extended  at 
one  time  far  north  of  its  present  limits. 

Valley-glaciers  of  great  size  exist  at  the  present  day  in  Alaska, 
Scandinavia,  Alps,  Himalayas,  and  New  Zealand. 

Among  the  most  notable  glaciers  in  the  globe  we  have  the 
following  : — 

In  Alaska  the  Malaspina  glacier,  30  miles  long,  descending 
from  Mt.  St  Elias,  with  sea-front  over  50  miles  long. 

In  Greenland  the  Humboldt  glacier  with  sea-face,  45  miles 
long,  presenting  ice-cliffs  from  300  to  500  feet  high. 

In  the  Swiss  Alps  the  Aletsch  glacier,  nearly  10  miles  long, 
or,  with  snowfields,  15  miles  ;  mean  breadth,  1 J  mile  : 
the  Mer-de-  Glace  descending  from  Mont  Blanc,  9  miles 
long. 

In  the  Himalayan  Mts.,  India,  Biafo  glacier,  36  miles  long. 

In  New  Zealand  the  Tasman  glacier,  18  miles,  or,  with  snow- 
fields,  21  miles  ;  mean  breadth,  1J  mile. 

In  South  Victoria  Land  the  Beardmore  glacier,  of  unknown 
length,  deploys  on  to  the  Great  Ice  Barrier  ;  the  Farrar 
glacier. 

The  glaciers  of  the  Arctic,  Antarctic,  and  Northern  India,  in 
the  Karakoram  Mts.  are  grouped  as  glaciers  of  the  first  order  ;  and 
those  of  New  Zealand  and  Southern  Europe  of  the  second  order. 

Valley  glaciers  are  fed  by  summit-glaciers  and  snowfields  that 
sometimes  descend  gentle  slopes  to  the  main  glacier,  and  sometimes 
where  the  slope  is  steep  tumble  down  in  a  cascade  of  broken  blocks 
of  ice,  forming  what  is  known  as  an  ice-cascade. 

Glaciers  that  push  their  way  to  the  sea  break  up  at  their  terminal 
end  into  masses  that  float  away  as  icebergs. 

When  a  number  of  glaciers  deploy  from  the  mountains  and 
unite,  they  form  what  is  termed  a  piedmont  glacier  or  ice-sheet. 


60  A    TEXT-BOOK    OF    GEOLOGY. 

Of  such  a  nature  is  the  great  Beardmore  glacier  in  South  Victoria 
Land  in  the  Antarctic  and  the  Malaspina  glacier  in  Alaska. 

Confluent  glaciers  of  this  kind  covered  a  large  portion  of  Northern 
Europe  and  America  in  the  Pleistocene  or  Great  Ice  Age. 

The  Surface  Features  of  Glaciers. — The  surface  of  glaciers  is 
seldom  smooth.  More  often  it  is  rough,  broken,  and  hummocky, 
and  covered  more  or  less  with  rocky  debris.  Moreover,  glaciers 
that  occupy  valleys  with  steep  gradients  are  generally  crevassed 
in  all  directions  by  the  unequal  tensions  set  up  in  the  body  of  the 
ice  by  the  differential  rate  of  flow. 

Ablation  of  Glaciers.— The  surface  and  terminal  end  of  a  glacier 
are  subject  to  the  heat  of  the  summer  sun.  The  daily  rise  of 
temperature  causes  the  melting  of  the  surface  ice.  This  surface 
melting  or  ablation,  as  it  is  called,  may  amount  to  many  feet  in  the 
course  of  a  single  year.  Desor  has  estimated  the  mean  ablation  of 
the  Swiss  glaciers  at  10  feet  a  year.  The  surface  measured  rate 
of  melting  of  Alaskan  glaciers  varies  in  summer  from  1  to  7 
inches  a  day,  all  on  retreating  glaciers. 

The  effect  of  continued  ablation  on  the  upper  surface  is  well  seen 
in  the  formation  of  what  are  known  as  ice-tables.  These  are  ice- 
pillars  capped  with  a  flat  slab  of  stone.  The  stone  protects  the 
ice  below  it  from  the  direct  rays  of  the  sun,  with  the  result  that, 
while  the  surrounding  ice  is  melted  away,  a  pillar  of  ice  remains, 
growing  taller  and  taller  until  it  eventually  becomes  too  slender 
to  support  its  protecting  cap. 

The  portions  of  a  glacier  covered  with  morainic  debris  are 
generally  higher  than  the  portions  free  from  debris,  the  former 
being  protected  from  ablation  by  their  load  of  rocky  material. 

Ablation  at  the  terminal  face  causes  an  apparent  recession  of 
the  glacier.  When  the  rates  of  melting  and  flow  are  equal  the 
glacier  remains  stationary.  But  when  the  rate  of  melting  is  less 
than  the  rate  of  flow,  the  terminal  end  advances,  and  when  more, 
it  recedes. 

Glacier-River. — Every  valley-glacier  is  drained  by  a  river  which 
generally  issues  from  an  ice-tunnel  at  the  terminal  end  of  the 
glacier.  Except  in  the  polar  regions  this  river  flows  summer  and 
winter,  but  the  winter  flow  is  always  much  less  than  the  summer. 
Its  waters  are  at  all  times  charged  with  a  large  amount  of 
suspended  silt. 

The  outflowing  water  is  partly  derived  from  springs  issuing  from 
the  rocks  within  the  drainage  area  of  the  glacier  and  its  snowfields, 
but  mainly  from  the  melting  of  the  glacier-ice. 

During  the  summer  months  the  surface  of  the  ice  is  melted,  the 
ice-water  finding  its  way  into  every  crack  and  fissure  in  the  neve. 
Much  of  this  water  sinks  to  the  bottom  of  the  glacier,  whence  a 


SNOW    AND    GLACIERS.  61 

portion  of  it  soaks  into  the  ground,  while  the  remainder  gravitates 
as  small  englacial  streams  towards  the  river  draining  the  glacier- 
bed. 

The  internal  heat  of  the  Earth  is  conducted  to  the  surface  in  all 
parts  of  the  globe.  This  heat  comes  in  contact  with  the  base  of 
the  glacier  and  melts  the  ice  at  an  estimated  average  rate  of  about 
one-fourth  of  an  inch  a  year. 

The  pressure  exerted  by  a  moving  body,  such  as  a  mass  of  ice, 
represents  the  expenditure  of  mechanical  energy  which  is  not  lost, 
but  transformed  into  heat.  When  the  f oot-lbs.  of  energy  are  known, 
the  equivalent  calorific  value  can  easily  be  determined.  The 
pressure  lowers  the  melting-point  of  ice,  so  that  in  the  case  of  thick 
sheets  the  melting-point  will  be  sensibly  less  than  32°  F. 

It  has  been  proved  experimentally  that  every  atmosphere  of 
pressure  (14-7  Ibs.  per  square  inch)  lowers  the  melting-point 
0-0133°  F.,  which  means  that  a  pressure  of  1103  Ibs.  per  square 
inch  will  lower  the  melting-point  1°  F.  Taking  the  specific  gravity 
of  ice  at  0-918,  we  find  that  to  obtain  a  pressure  of  1103  Ibs.  per 
square  inch  we  require  a  sheet  of  ice  2775  feet  thick.1 

In  other  words,  the  melting-point  at  the  base  of  a  glacier  2775 
feet  thick  will  be  1°  F.  less  than  32°  F=31°  F.  And  since  the 
pressure  is  proportional  to  the  depth,  it  follows  that  for  a  thickness 
of  5550  feet  the  melting-point  will  be  2°  less  =30°  F.  Therefore, 
as  a  near  approximation,  we  may  say  that  for  every  mile  thick  of 
ice  the  melting-point  is  lowered  2°  F. 

Agassiz  proved  by  numerous  experiments  in  a  hole  sunk  to  the 
depth  of  200  feet  in  solid  glacier-ice  that  the  temperature  at  that 
depth  was  only  31-24°  F.  when  the  surface  temperature  was  at 
freezing-point. 

Hence  it  is  assumed  that  in  all  thick  glaciers  the  temperature 
of  the  base  of  the  ice  is  constantly  maintained  at  melting- 
point. 

Retreat  of  Glaciers. — In  certain  circumstances  the  rate  of  retreat 
of  a  glacier  may  be  not  less  rapid  than  the  rate  of  advance.  The 
Barry  glacier  in  Harriman  Fiord,  Alaska,  retreated  3|  miles 
between  1899  and  1910 ;  and  approximately  600  feet  of  this 
retreat  took  place  in  the  year  1909-1910.  Most  existing  glaciers 
in  both  hemispheres  are  shrinking  in  size. 

The  glaciers  in  Jakutat  Bay,  Alaska,  show  clear  evidence  of 

0-0133 
1  — j—- x  14-7  =  1103  Ibs.  per  sq.  in.  ;    S.G.  of  ice  =  0-918;    weight  of  a 

cubic   foot   of   water  =  62-32   Ibs.;    therefore    pressure   of   1   foot   of    water 

62-32 
=  0-433   Ib.    per   sq.   in.,  as  ~TTr  =0-433.     Therefore  pressure  of  1  foot  of 

ice =0-433x0-918  =0-397  Ib.  per  sq.  in.  And  1103  Ibs. -f-0-397  =  2775  feet 
of  ice  for  1°  F. 


62  A   TEXT-BOOK   OF   GEOLOGY. 

three  periods  of  temporary  advance  during  the  general  recession 
now  in  progress.  The  last  advance,  in  1906,  was  short  and  spas- 
modic, and  has  been  not  unreasonably  attributed  by  Tarr  and 
Martin1  to  the  unusual  supply  of  snow  and  ice  shaken  down  from 
the  mountains  by  the  great  Jakutat  earthquakes  in  September 
1899.  On  account  of  the  slow  rate  of  flow,  the  terminal  end  of 
the  glaciers  did  not  respond  to  the  new  impulse  until  six  years 
had  elapsed,  and  naturally  the  shortest  glaciers  were  the  first  to 
be  affected. 

GEOLOGICAL  WORK  OP  GLACIERS. 

Glaciers  as  agents  of  denudation  perform  a  twofold  office. 
They  (1)  transport  material  from  a  higher  to  a  lower  level ;  and 
(2)  they  degrade  the  land  by  eroding  the  bottom  and  sides  of 
their  bed. 

Glaciers  as  Transport  Agents.— A  valley-glacier  may  transport 
material  (a)  on  its  surface  ;  (b)  scattered  throughout  the  body  of 
the  ice  ;  or  (c)  dragged  along  the  floor. 

The  surface  load  may  find  its  way  on  to  the  glacier  in  various 
ways.  Among  the  higher  mountain-chains  where  glaciers  exist 
the  frosts  are  very  severe.  The  winter  frosts  break  up  and  shatter 
the  rocks  forming  the  valley-walls.  The  fragments  and  masses 
thus  broken  may  form  talus  deposits  that  slowly  slide  down  on  to 
the  edge  of  the  glacier ;  or,  when  assisted  by  gravity  on  steep 
slopes,  they  may  fall  on  to  the  ice  as  soon  as  they  are  detached  ; 
or  they  may  be  carried  down  by  avalanches  ;  or  transported 
from  the  heights  above  by  the  snowfields  that  feed  the  glacier 
at  its  sources. 

The  rocky  load  that  lies  on  the  surface  of  a  glacier,  or  accumulates 
at  the  end,  may  be  lateral,  medial,  or  terminal,  according  to  the 
position  it  occupies. 

The  debris  that  falls  on  to  the  sides  of  the  glacier  forms  marginal 
belts  or  lateral  moraines. 

When  two  glaciers  from  adjacent  valleys  unite,  their  inner 
lateral  moraines  come  together  and  form  what  is  called  a  medial 
moraine  (figs.  20,  21). 

When  more  than  two  glaciers  unite,  the  surface  of  the  trunk 
glacier  may  carry  several  belts  of  medial  moraine,  although  the 
position  of  these  will  not  be  quite  medial. 

At  the  place  where  the  glacier  ends,  that  is  the  terminal  face, 
the  surface  debris  is  tipped  over  and  piled  up  in  a  pell-mell  fashion. 
Where  it  falls  into  the  glacier-river  it  is  washed  away  and  soon 
becomes  rounded  and  water-worn. 

1  R.  S.  Tarr  and  Lawrence  Martin,  "  The  Earthquakes  of  Jakutat  Bay, 
Alaska,  in  Sept.  1899,"  Prof.  Paper  69,  U.S.  Oeo.  Survey,  1912. 


SNOW   AND    GLACIERS. 


63 


A  large  proportion  of  the  rocky  surface  load  falls  into  the  crevasses 
and  thus  becomes  engulfed  in  the  body  of  the  ice.  This,  with 
much  of  the  debris  carried  down  by  the  tributary  snowfields, 
forms  the  inter  glacial  load  of  the  glacier. 

The  material  broken  up  by  the  pressure  and  grinding  action  of 


FIG.  20. — Medial  moraines,  Mer-de-Glace. 


the  moving  ice,  or  plucked  from  the  bed  and  carried  forward  along 
the  floor  of  the  valley  at  the  base  of  the  glacier,  is  termed  sub- 
glacial. 

The  inter  glacial  and  a  portion  of  the  sub  glacial  debris  is  carried 
down  to  the  terminal  face,  where  it  is  piled  up  with  the  rocky 
material  tipped  from  the  surface  moraines.  A  large  portion  of  the 
subglacial  debris  is  washed  away  by  the  glacier-river. 


64 


A    TEXT-BOOK    OF    GEOLOGY. 


The  material  composing  the  surface  moraines  consists  mainly  of 
angular  blocks  of  all  sizes,  ranging  up  to  masses  200  tons  or  more 
in  weight,  mixed  with  small  angular  fragments  and  clay.  A  few 
of  the  blocks  show  striated  surfaces  produced  by  the  stones  rubbing 
against  one  another,  or  by  rubbing  against  the  valley-walls  when 
frozen  in  the  moving  ice. 

The  interglacial  debris  is  frequently  arranged  in  layers  lying 
parallel  to  the  neve  foliation.  This  foliation  is  frequently  very 
minute,  and  is  always  very  striking  where  layers  of  clean  ice 
alternate  with  layers  of  earthy  matter.  These  dirt  layers  are 
frequently  inclined  at  steep  angles,  the  inclination  being  generally 
towards  the  head  of  the  glacier.  In  some  glaciers,  particularly 


FIG.  21. — Showing  lateral  and  medial  moraines, 
(a)  Lateral  moraines.  (6)  Medial  moraine. 


near  the  terminal  face,  the  layers  are  sharply  folded  and  contorted, 
due  to  the  differential  flow  of  the  upper  and  lower  layers  of  the  ice. 

Many  glaciers  that  are  quite  free  from  morainic  matter  in  the 
upper  portion  of  their  course  are  covered  with  rocky  debris  at 
their  lower  end.  This  material,  which  sometimes  appears  in  patches 
and  sometimes  as  a  continuous  sheet  across  the  whole  width  of 
the  ice,  is  subglacial  and  englacial  debris  that  has  found  its  way  to 
the  surface  partly  owing  to  the  culminative  effects  of  long-con- 
tinued surface  ablation,  and  partly  owing  to  the  upward  flow  of 
the  lower  layers  of  ice  due  to  pressure  and  the  obstructing  apron 
of  debris  in  front. 

There  is  abundant  evidence  in  Alaska  and  Spitsbergen  that 
glaciers  which  have  crossed  an  arm  of  the  sea  have  picked  up 
marine  material  from  the  sea-floor  and  transported  it  over  the 
land  lying  in  the  path  of  their  advance.  It  was  doubtless  in  this 


SNOW   AND    GLACIERS.  65 

way  that  the  shelly  glacial  drifts  of  North- Western  Europe  were 
spread  by  the  ice-sheets  of  the  Glacial  Period  over  the  land  sur- 
rounding the  sea-basins. 

Glacier-Drifts  (fig.  22).— At  the  terminal  end  of  valley-glaciers 
we  thus  find  two  classes  of  matter,  namely,  the  angular  rubble 
transported  by  the  ice,  and  the  more  or  less  water-worn  drifts 
transported  by  the  glacier-river.  Both  classes  are  mixed  at 
certain  points,  and  they  form  what  are  termed  fluvio-glacial 
drifts. 

Ground-Moraines. — The  broken-up  rock  and  clays  that  accumu- 
late under  a  glacier  or  sheet  of  ice,  as  well  as  all  the  drift  that  is 
deposited  beneath  the  advancing  ice,  constitutes  what  is  termed 
ground-moraine,  boulder-clay,  or  till. 

The  thickness  of  the  ground-moraine  is  notably  irregular.  It 
may  vary  in  a  hundred  yards  from  a  few  feet  to  many  hundreds  of 


FIG.  22. — Section  of  glacier-valley. 

(a)  Glacier  moraines.  (c)  Basement  rock. 

(6)  Glacier  gravels.  (d)  Glacier-river. 

feet.  The  distribution  is  equally  variable,  but  for  the  most  part 
subglacial  deposits  of  this  kind  are  principally  developed  in  the 
lower  end  of  glacial  valleys  and  in  depressions  among  the  foothills. 
Subglacial  drifts  are  sometimes  present  on  ridges  and  absent  in 
the  neighbouring  low  ground  and  valleys. 

When  the  boulder-clay  occurs  in  long  ridges,  as  it  frequently 
does  in  the  lower  foothills,  it  forms  what  in  Scotland  are  called 
drums  or  drumlins,  that  run  in  the  general  direction  of  the  rock- 
striation  or  ice-movement. 

The  till  of  Scotland  varies  from  0  to  160  feet  thick,  and  that  of 
North  America  from  0  to  500  feet.  In  Germany  the  Pleistocene 
glacial  drift  varies  from  0  to  670  feet  thick.  In  Greenland  there 
are  enormous  accumulations  of  ground-moraine  on  the  edge  of  the 
inland  ice  at  Austmannatjern,  where  there  are  no  nunataks,  and  not 
a  vestige  of  surface  moraine  visible. 

Nunataks  are  peaks  of  rock  projecting  above  the  level  of  an  ice- 
sheet  or  ice-plateau,  and  where  they  are  absent  it  is  obvious  that 
no  fragments  of  rock  can  be  shed  on  to  the  surface  of  the  ice. 

5 


66  A   TEXT-BOOK   OF    GEOLOGY. 

Glaciers  as  Agents  of  Erosion. — When  a  thin  sheet  of  snow  lying 
on  a  steep  mountain  slope  moves  downhill,  it  heaps  the  loose 
shingle  and  soil  on  which  it  rests  into  small  furrows  that  are  not 
unlike  those  made  by  a  harrow  on  cultivated  land.  This  action 
cannot  be  described  as  erosion  since  the  material  is  already  loose 
and  is  merely  displaced  by  the  sliding  snow.  Even  a  thick  snow- 
field  will  glide  downhill  without  eroding  its  bed,  except  where  a 
boulder  is  frozen  into  its  base.  In  this  case  the  boulder  will  furrow 
the  loose  material  through  which  it  moves,  and  scratch  projecting 
rocks  that  lie  in  its  path. 

Large  glaciers  are  capable  of  wearing  away  the  surface  of  the 
rocks  forming  their  bed  by  the  pressure  of  their  mass  alone  which 
amounts  to  25-5  tons  per  square  foot  for  every  thousand  feet 
of  ice.  When  the  pressure  exerted  by  the  ice  exceeds  the  ultimate 
strength  of  the  rock,  the  crumbling  and  erosion  of  the  rock-surface 
must  be  the  inevitable  result.  It  is  probable,  however,  that  most 
of  the  erosion  is  effected  by  the  fragments  of  rock  frozen  into  the 
base  of  the  ice.  As  the  glacier  moves  onward,  these  fragments, 
being  held  in  the  firm  grip  of  the  ice,  plough  into  the  softer  rocks, 
while  they  scratch  and  abrade  the  harder  like  a  gigantic  rasp. 
In  this  way  a  glacier  deepens  and  widens  the  valley  in  which  it 
flows. 

The  maximum  thickness  of  the  Greenland  ice  is  estimated  to  be 
not  less  than  5000  feet,  and  that  of  the  Antarctic  probably  much 
more.  In  the  Pleistocene  the  ice  is  believed  to  have  attained  a 
thickness  of  5000  feet  in  Scotland,  6000  feet  in  the  Alps  and 
Scandinavia,  7000  feet  in  New  Zealand,  and  8000  feet  in  North 
America.  The  erosive  power  of  such  masses  of  moving  ice  must 
have  been  enormous  (Plate  IX.). 

It  is,  of  course,  impossible  to  watch  the  progress  of  the  erosive 
work  being  carried  on  by  existing  glaciers  ;  but  if  we  examine  a 
valley  that  has  been  at  one  time  occupied  by  a  glacier,  we  find  that 
all  the  irregularities  have  been  worn  down,  and  that  the  bottom 
and  sides  are  smooth  or  gently  undulating.  Hummocks  of  rock 
that  lie  in  the  valley-floor  are  found  to  be  scored  and  furrowed, 
while  projecting  spurs  are  truncated.  Where  the  glaciation  has 
been  prolonged  and  severe,  rock-basins  are  found  in  the  floor  of  the 
valley,  and  in  many  cases  the  neighbouring  mountain  slopes  are 
excavated  into  benches  or  platforms. 

No  other  natural  agent  than  ice  is  known  that  could  effect  these 
changes  ;  and  when  we  find  at  the  lower  end  of  the  valley  huge 
piles  of  ancient  morainic  matter,  then  are  we  sure  that  moving  ice 
was  responsible  for  the  work. 

Country  that  has  been  at  one  time  overrun  by  ice  always  presents 
smooth  rounded  contours  and  flowing  outlines,  except  among  the 


SNOW    AND    GLACIERS.  67 

higher  mountains  where  the  recent  work  of  frost  has  shattered  and 
broken  up  the  ice-shorn  surfaces.  The  most  striking  effects  of 
glacial  erosion  are  generally  found  in  the  valleys  and  foothills  of 
recently  glaciated  countries. 

All  geologists  are  agreed  as  to  the  ability  of  glaciers  to  wear 
away  and  plane  the  surfaces  over  which  they  flow  ;  but  all  are  not 
agreed  as  to  the  maximum  amount  of  erosion  they  are  capable  of 
performing. 

Many  maintain  that  the  erosive  effects  of  glaciers  are  small  and 
can  never  amount  to  more  than  the  smoothing,  scratching,  arid 
superficial  planing  of  the  rocks  over  which  the  ice  flows.  Glaciers, 
they  contend,  are  incapable  of  excavating  to  any  extent,  but  always 
occupy  pre-existing  valleys.  Others  are  prepared  to  maintain  that 
glaciers  are  not  only  able  to  excavate  valleys  but  even  to  over- 
deepen  them  in  certain  circumstances. 


FIG.  23. — Section  of  lower  end  of  Lake  Wakatipu  rock-basin,  N.Z. 
(a)  Altered  greywacke.  (6)  Kingston  ancient  moraine. 

The  truth  probably  lies  between  these  extreme  views.  Recent 
investigation  would  tend  to  show  that  the  present  drainage  systems 
in  glaciated  regions  had  been  already  determined  at  the  advent 
of  the  Great  Ice  Age,  and  that  the  glaciers  merely  took  possession 
of  valleys  already  in  existence.  These  valleys  they  widened  and 
deepened,  and  in  favourable  situations  overdeepened,  so  as  to 
form  the  rock-basins  of  many  of  the  mountain  valley-lakes  of  the 
present  day. 

The  enormous  amount  of  rock-flour  in  the  form  of  suspended 
matter  annually  transported  from  below  a  glacier  by  the  glacier- 
river  is  satisfactory  evidence  of  the  wear  and  tear  that  is  constantly 
going  on  under  the  moving  river  of  ice. 

Glacial  Striae. — Many  of  the  blocks  of  stone  in  a  ground-moraine 
are  scratched  and  grooved  with  parallel  striae  (fig.  24).  Some 
boulders  are  striated  with  two  systems  of  striae,  crossing  each  other 
at  a  more  or  less  acute  angle.  This  may  mean  that  the  first 
position  of  the  block  became  changed  in  respect  of  the  line  of  move- 


68  A   TEXT-BOOK   OF    GEOLOGY. 

ment  of  the  ice,  thus  permitting  a  second  set  of  striae  to  be  scored 
on  the  face  of  the  stone. 

Striated  stones,  although  most  common  in  boulder-clays,  are 
frequently  found  in  the  lateral  moraines  of  existing  glaciers. 

The  strise  are  best  preserved  on  granite,  diorite,  quartzite,  grey- 
wacke,  and  all  hard  sandstones.  Where  the  glacier  flowed  over  such 
soft  rocks  as  shale,  chalky  clays,  marls,  phyllite,  slate,  or  mica-schist, 
striated  stones  are  seldom  or  never  met  with  in  the  subglacial 
debris.  The  strise  formed  on  basalts  and  all  basic  igneous  rocks 
are  soon  effaced  by  weathering,  as  the  felspar  constituents  of  these 
rocks  are  acted  on  by  atmospheric  carbonic  acid  and  moisture 
with  comparative  rapidity.  Where  the  striated  basaltic  boulder 
has  been  embedded  in  impervious  clay,  the  striae  may  remain  fresh 
and  sharp  for  a  considerable  time. 

Roches  Moutonnees  (fig.  25). — These  are  rounded,  hummocky, 
or  whale-backed  bosses  or  ridges  of  hard  rock  that  have  been  worn 


FIG.  25. — Showing  roches  moutonnees. 

down  by  an  overriding  stream  of  ice.  They  generally  occur  on  the 
floor  of  ancient  glacial  valleys  and  on  valley  slopes,  but  some 
beautiful  examples  are  found  at  high  altitudes,  as,  for  example, 
near  the  summit  of  Mount  Kosa  in  New  Zealand,  at  a  height  of 
5500  feet  above  the  sea  and  3000  feet  above  the  Hooker  Glacier. 

Erratics. — These  are  blocks  of  rock  that  have  been  transported 
by  ice  some  distance  from  the  parent  rock.  In  some  cases  they 
have  been  carried  from  one  watershed  to  another,  and  even  from 
one  country  to  another.  The  Scandinavian  ice-sheet  which  flowed 
down  the  North  Sea  carried  many  foreign  rocks  from  the  frozen 
north  and  left  them  stranded  on  the  shores  and  inland  parts  of 
England. 

Perched  Blocks. — Masses  of  rock  that  have  been  left  stranded  by 
the  retreating  ice  on  the  summit  of  ridges  or  on  the  flanks  of  moun- 
tains are  termed  perched  blocks.  Some  perched  blocks  have  been 
transported  many  miles  from  their  parent  rock  ;  and  in  many 
cases  they  have  been  left  by  the  melting  ice  in  prominent  or  pre- 
carious positions,  hence  the  name. 

Perched  blocks  (fig.  26)  are  angular  or  partially  rounded 
according  to  the  amount  of  wear  and  tear  they  have  suffered 


[To  face  page  68. 


FIG.  24. — Showing  ice-striated  stone. 


SNOW    AND    GLACIERS. 


69 


during  their  journey.  When  carried  on  the  surface  or  in  the 
body  of  the  ice,  they  are  angular,  but  when  they  were  frozen  into 
the  base  of  the  glacier  and  dragged  along  the  rocky  floor,  they 
were  generally  smoothed,  striated,  and  sometimes  polished  on  the 
lower  side. 

One  of  the  most  noted  perched  blocks  in  Europe  is  the  Pierre  a 
Bot,  a  huge  block  of  granite  from  the  Mont  Blanc  range,  stranded 
about  two  miles  from  Neufchatel.  It  is  estimated  to  weigh  about 
3000  tons. 

Glacial  Benches. — These  may  be,  according  to  their  origin,  (a) 
detrital  or  (6)  rock-cut. 


FIG.  26.— Perched  block,  Arran. 

Detrital  Benches  (fig.  27)  are  formed  in  parallel  lines  along  the 
slopes  of  ancient  glacial  valleys.  Two,  three,  or  as  many  as  twenty 
or  more  of  these  may  rise  on  the  mountain  side  one  above  another 
like  a  flight  of  gigantic  steps.  They  are  not  horizontal,  but  slope 
at  a  low  angle  towards  the  lower  end  of  the  valley. 

These  detrital  terraces  are  ancient  lateral  moraines  that  accumu- 
lated along  the  edge  of  the  glacier.  When  the  glacier  shrunk  in 
depth,  the  rocky  belt  of  detritus  was  dropped  on  the  mountain 
slope,  and  piled  up  in  the  form  of  a  rubble  platform  or  bench. 
At  its  next  resting-place  another  belt  of  debris  accumulated  on  the 
edge  of  the  ice  again  to  be  deposited  on  the  flank  of  the  range  as 
the  melting  ice  shrank  in  depth,  thus  forming  a  second  bench  ; 
and  so  on,  other  benches  being  formed  in  the  same  way  so  long  as 
the  glacier,  by  fits  and  starts,  continued  to  shrink  in  its  bed. 

Glacial  Rock- Terraces  have  been  excavated  out  of  the  mountain 


70 


A    TEXT-BOOK    OF    GEOLOGY. 


slope  forming  the  wall  of  the  ancient  glacial  valley.  Two,  three, 
or  many  of  these  benches  may  occur  one  above  the  other.  They 
are  not  so  continuous  as  detrital  benches  nor  is  their  inclination 
so  uniform.  They  are  more  or  less  undulating  in  longitudinal 
section,  and  they  vary  considerably  in  width,  this  variation  being 


S.W 


FIG.  27. — Showing  rubble  terraces  now  being  formed  on  the 
edge  of  the  Hooker  Glacier,  New  Zealand. 

(a)  Glacier-ice.  (d)  Ancient  lateral  moraines  forming 

(b)  Glacier-river.  rubble  terraces. 

(c)  Lateral  moraines.  (e)  Greywacke  and  slaty  shales. 

due  to  the  irregularities  of  the  original  slope  in  which  they  were 
excavated. 

Kock-cut  terraces  are  only  found  in  regions  that  have  been 
subject  to  intense  glacial  erosion.  A  striking  example  of  this  kind 
of  ice-erosion  is  seen  on  the  slopes  of  Ben  More  in  New  Zealand, 


FIG.  28. — Showing  truncation  and  planing  of  projecting  spur. 

(a)  Floor  of  glacial  valley.  (6)  Truncated  end  of  spur, 

(c)  Crest  of  spur  planed  into  a  platform. 

where  more  than  thirty  benches  have  been  carved  in  the  mountain 
slope  between  Lake  Luna  and  the  summit  of  the  mountain  in  a 
height  of  about  3000  feet. 

Spurs  that  projected  into  glacial  valleys  are  generally  found  to 
have  been  truncated,  and  where  the  ice  flowed  over  them  their 
crest  is  in  most  cases  planed  down  into  a  platform  as  shown  in 
fig.  28. 

Crag  and  Tail.— Ice-worn  ridges  usually  present  a  long  continuous 


To  face  page  70.] 


[PLATE  X. 


A.    SHOWING  DISSECTION  OF  ALLUVIAL  PLAIN. 


B.   HANGING  VALLEY.     (After  Tarr,  U.S.  Geol.  Survey.) 


SNOW    AND    GLACIERS.  71 

slope  in  the  direction  from  which  the  ice  travelled,  and  a  steep  slope 
on  the  lee  or  further  end  where  frequently  a  certain  amount  of 
rocky  debris  collected.  This  form  of  ice-erosion  is  known  as  the 
crag  and  tail  and  is  well  seen  in  fig.  25. 

Fluvio-Glacial  Work  of  Glaciers. — In  another  place  we  have  seen 
that  every  valley- glacier  is  drained  by  a  river  which  issues  from 
an  ice-tunnel  at  the  end  of  the  glacier.  Glacier-rivers  always  carry 
a  certain  amount  of  suspended  matter  in  the  form  of  silt  or  rock- 
flour  ;  and  in  the  case  of  many  ice-fed  streams  the  amount  of  fine 
silt  thus  carried  in  suspension  is  so  large  that  their  waters  possess 
a  milky-white  colour. 


FIG.  29. — Valley-train  below  Hidden  Glacier,  Alaska. 
(After  Gilbert,  U.S.  Geo.  Survey.) 

Besides  rock-flour  the  river  also  transports  sand  and  gravel 
derived  from  the  bottom  of  the  glacier.  The  coarser  material, 
frequently  mingled  with  angular  morainic  debris,  is  dropped  first, 
further  on  the  finer  gravels,  and  lastly  the  sand  and  silt.  In 
this  way  glacier-streams  frequently  build  up  alluvial  plains  called 
glacier  valley -trains. 

A  valley-train  (fig.  29)  is  thus  a  continuous  sheet  of  glacial  drift, 
graduating  from  the  purely  morainic  drift  at  the  head  to  the 
purely  fluviatile  deposit  at  the  end.  The  material  is  always  more 
or  less  stratified  throughout,  and  in  all  of  it,  both  coarse  and  fine, 
the  angular  blocks  as  well  as  the  water-worn  gravels  have  a  common 
glacial  origin. 

Where  the  glacial  river  discharges  its  load  into  a  lake  or  a  bay, 


72  A    TEXT-BOOK    OF    GEOLOGY. 

a  delta  is  formed.  In  this  way  many  valley-lakes  have  been  filled, 
or  partially  filled,  and  large  areas  reclaimed  from  the  sea. 

When  the  outlet  of  a  glacier-river  has  become  blocked  with  some 
obstruction,  such  as  an  ice-fall  or  an  accumulation  of  morainic 
debris,  the  flow  of  the  river  is  checked,  with  the  result  that  the 
transported  load  of  sand  and  gravel  can  no  longer  be  carried  forward 
to  the  valley-train,  but  is  deposited  in  the  ice-tunnels  of  the  sub- 
glacial  streams.  When  the  glacier  retreats,  these  deposits  of  sand 
and  gravel  remain  in  the  form  of  ridges  that  occupy  the  sites  of  the 
subglacial  streams,  their  form,  length,  and  height  being  determined 
by  the  form  and  size  of  the  ice-tunnels  which  they  filled. 

Deposits  of  this  kind  frequently  run  parallel  with  the  valley- 
walls.  They  are  common  in  all  recently  glaciated  regions.  In  the 
Central  Plain  of  Ireland  they  are  called  es~kers,  and  in  Scotland 
kames.  The  sand  and  gravel  of  eskers  are  generally  sorted  into 
layers  of  coarse  and  fine  material,  and  in  this  respect  they  cannot 
be  distinguished  from  ordinary  river-drifts. 

At  the  time  the  confluent  glaciers  deployed  from  the  alpine  valleys 
and  still  occupied  the  low  country  and  plains,  numerous  streams 
would  doubtless  issue  from  the  melting  front  of  the  ice. 

The  ice-sheet  would  override  the  land,  and  its  flow  would  be 
towards  the  sea  independently  of  the  minor  irregularities  of  the 
contours.  Hence  the  escaping  waters  would  at  first  flow  with 
little  relation  to  the  existing  drainage  lines.  Streams  would  be 
discharged  over  hills  and  ridges  as  well  as  over  the  plains,  walls  of 
ice  forming  the  enclosing  barriers  of  the  channels.  Wherever  they 
went  these  streams  would  leave  a  trail  of  well-worn  glacial  drift 
in  the  form  of  sand  and  gravel,  with  perhaps  here  and  there  a 
sporadic  mass  of  rock  dropped  from  the  base  of  the  melting  ice. 

It  was  probably  in  this  way  that  the  sheets  of  plateau-gravels  of 
South  Germany,  Alaska,  and  New  Zealand  were  formed.  These 
high-level  gravels  are  spread  over  plateaux,  hills,  ridges,  and 
mountain  slopes  frequently  without  any  relation  to  the  present 
topography.  In  many  places  they  fill  up  inland  alpine  valleys. 

According  to  their  situation  they  may  merge  into  or  overlie 
boulder-cla-y,  morainic  debris,  marine  sands,  and  raised-beach 
gravels. 

These  drifts  are  most  striking  when  they  form  mounds  which 
run  across  valleys  and  plains,  or  even  over  watersheds.  Such 
mounds  when  found  on  plains  or  along  hillsides  constitute  the 
eskers  described  above. 

Topographical  Effects  of  Glaciers. — A  glacier  or  ice-sheet  modifies 
the  topography  of  the  land  over  which  it  moves  by  its  destructive 
effect  as  an  agent  of  erosion,  but  its  constructive  effect  as  an  agent 
of  transport  is  not  insignificant. 


SNOW   AND    GLACIERS.  73 

A  valley- glacier  in  an  alpine  region  deepens  and  widens  the  valley 
which  it  occupies.  The  extent  to  which  the  glacier  is  able  to 
modify  the  original  topographical  features  is  dependent  on  the 
hardness  of  the  country  rock,  the  depth  of  the  ice,  the  rate  of  flow, 
and  the  duration  of  the  glaciation. 

The  cross-section  of  valleys  that  owe  their  existence  to  stream 
erosion  is  usually  V-shaped  ;  and  the  sides  are  generally  rough 
and  irregular.  Glacier  erosion  usually  changes  the  V-shaped  form 
into  a  U-shaped  one. 

Where  the  valley  traverses  granite,  gneiss,  or  other  hard  rock, 
its  sides  are  frequently  carved  into  approximately  vertical  walls  ; 
but  in  softer  rocks  the  form  assumed  is  commonly  that  of  a  U  with 
its  sides  spread  out  in  a  gentle  catenary  curve. 

Where  the  trunk  glacier  has  deepened  its  bed  more  rapidly  than 
the  lateral  tributary  ice-streams,  the  valley-floor  of  the  tributary 
streams  is  found  to  occupy  a  higher  level  than  that  of  the  trunk 
valley.  Such  hanging  valleys,  as  they  are  called,  are  common  in 
all  recently  glaciated  regions.  Many  fine  examples  of  hanging 
valleys  are  seen  in  the  fiordland  of  Otago  in  New  Zealand,  in  North 
America,  and  elsewhere  (Plate  X.). 

The  descent  from  the  end  of  the  hanging  valley  is  frequently 
quite  abrupt,  with  the  result  that  the  drainage  of  the  lateral  valley 
is  compelled  to  find  its  way  to  the  main  valley  in  the  form  of  a 
waterfall  or  cascade. 

•  In  fiordlands  and  in  recently  glaciated  mountains  composed  of 
hard  crystalline  rocks,  the  cross-section  of  the  valleys  frequently 
shows  two  phases  of  formation — namely,  a  wide  upper  flat-bottomed 
valley  and  a  lower  narrow  U-shaped  valley  excavated  in  the  floor 
of  the  upper  one.  The  portions  of  the  floor  of  the  upper  valley 
that  have  escaped  denudation  form  the  terraces  or  mountain 
meadows  known  in  Norway  as  albs.  Small  rock-cut  basins  or 
tarns  are  common  features  of  these  high  mountain  meadows. 

We  have  already  seen  that  the  effect  of  moving  ice  is  to  wear 
away  all  the  corners  and  minor  irregularities  of  the  landscape. 
The  result  of  this  planing  down  of  the  surface  features  is  that  a 
region  recently  overrun  by  an  ice-sheet  usually  presents  smooth 
flowing  contours  and  a  monotonous  sameness  of  outline,  dome- 
shaped  hills,  whale-backed  ridges,  and  mammillated  slopes  every- 
where meeting  the  eye. 

The  slopes  of  glaciated  valleys  are  generally  even,  being  free 
from  projecting  spurs  and  ridges  ;  but  where  spurs  do  project 
into  the  valley  they  are  usually  truncated,  and  their  crests  planed 
down  into  terrace-like  platforms. 

All  the  recently  glaciated  valleys  in  New  Zealand  and  many  in 
Switzerland  and  Alaska  have  been  overdeepened  by  the  ice.  The 


74 


A    TEXT-BOOK    OF    GEOLOGY. 


depressions  thus  formed  are  now  lake-basins.  The  size  and  depth 
of  many  of  these  basins  has  been  increased  by  barriers  of  morainic 
debris  deposited  at  their  lower  end  (fig.  23). 

The  head  of  glaciated  valleys  is  frequently  found  to  open  out 
into  a  circular  basin  known  as  a  cirque,  on  the  floor  of  which 
there  may  be  one  or  more  shallow  corrie-lakes  (fig.  30)  or  lagoons 


Alb 


FIG.  29A. — Showing  albs  or  mountain  meadows, 

occupying  rock-cut  basins.  According  to  Professor  Bonney's  view, 
cirques  occupy  pre-glacial  hollows  cut  by  convergent  streams 
with  walls  modified  by  ice  erosion.  Richter.  on  the  other  hand, 
maintains  that  they  are  the  result  of  frost-shattering  above  the 
level  of  the  ice.  The  frost  dislodges  masses  of  rock,  which  fall  on 


FIG.  30. — Showing  cross-section  of  two  cirques  on  opposite  sides  of  a  range. 

(a)  Corrie-lakes. 

to  the  ice  and  are  carried  away,  whereby  the  formation  of  a  talus 
is  prevented.  The  ice  protects  the  floor  of  the  cirque,  which  is 
thus  relatively  flat,  while  the  absence  of  a  protecting  apron  or 
talus  permits  the  frost  to  sap  and  wear  away  the  walls  until  they 
become  steep  or  even  vertical. 

Where  two  glacial  valleys  on  opposite  sides  of  a  mountain-chain 
head  together,  as  they  so  frequently  do  where  the  original  valleys 
follow  some  powerful  fault  or  line  of  structural  weakness,  the  cutting 
back  of  the  cirques  may  result  in  the  removal  of  the  dividing  ridge, 


SNOW   AND    GLACIERS.  75 

and  in  its  place  we  find  a  plateau  or  flat  saddle  occupied  by  swamps 
or  shallow  lagoons. 

Where  one  cirque  lies  at  a  lower  level  than  the  other,  the  lower 
by  its  recession  is  able  to  pirate  the  drainage  of  the  higher,  which 
is  thus  reversed  or  carried  to  the  other  watershed.  Examples  of 
reversed  drainage  are  not  uncommon  in  alpine  New  Zealand  and 
North  America.  Much  of  the  stream-pirating  and  beheading 
described  in  a  preceding  page  has  been  doubtless  accelerated,  if 
not  initiated,  by  ice-erosion. 

Sharp-peaked  mountains  with  excessively  steep  walls  are  fre- 
quently found  in  glaciated  regions  on  the  margins  of  ice-caps. 
Many  good  examples  of  these  remarkable  peaks,  which  are  called 
tinds  t  have  been  developed  on  the  margin  of  the  Norwegian  plateau 
glaciers.  Mitre  Peak  in  the  fiordland  of  New  Zealand  is  a  typical 
example  of  a  tind.  Tinds  have  been  obviously  exposed  to  intense 
ice-erosion,  and  the  conical  shape  which  they  frequently  assume 
may  not  improbably  be  due  to  the  shattering  and  exfoliation  of 
successive  layers  of  rock  by  frost  action  since  the  retreat  of  the 
ice-sheet. 

The  terminal  debris  of  glaciers  is  frequently  piled  up  in  the  form 
of  hills  and  mounds  disposed  in  crescent  shape  on  the  plains  where 
the  glacier  deployed  from  its  alpine  valley,  and  although  these 
morainic  hills  seldom  exceed  a  height  of  300  feet,  they  are  nearly 
always  a  striking  feature  in  the  landscape,  as  they  present  a  curious 
assemblage  of  hills  and  undrained  hollows  of  the  knob-and-basin 
type. 

Crescent-shaped  morainic  mounds,  one  behind  another,  may  be 
traced  up  an  alpine  valley,  each  mound  marking  a  place  where  the 
glacier  rested  for  a  time  during  its  final  retreat.  The  highest 
mounds  being  the  last  formed  are  always  the  freshest. 

The  valley-glaciers  of  Alaska,  Alps,  Pyrenees,  Himalayas,  and 
New  Zealand  are  but  the  remnants  or  stumps  of  great  ice-streams 
that,  during  the  Great  Ice  Age,  descended  from  the  mountains 
and  spread  over  the  neighbouring  foothills  and  plains. 

The  freshness  of  some  of  the  glaciated  slopes  and  esker-mounds 
of  sand  and  gravel  in  Scotland,  Alaska,  and  New  Zealand  is  often 
very  striking.  In  many  places  the  contours  look  so  smooth 
and  fresh  that  one  is  tempted  to  think  they  were  moulded  but 
yesterday,  or  that  the  forces  of  denudation  must  have  been  held 
in  abeyance  since  the  close  of  the  Glacial  Period.  This  freshness 
is  all  the  more  remarkable  when  we  observe  that  contiguous  areas 
occupied  by  hard  rock  have  been  deeply  dissected  by  streams. 
This  differential  erosion  is  a  common  feature  of  all  glaciated  regions. 

The  preservation  of  the  glaciated  forms  and  mounds  of  morainic 
debris  is  believed  to  have  been  due  to  the  protection  afforded  by 


76  A    TEXT-BOOK    OF    GEOLOGY. 

permanent  snowfields.  It  is  contended  that  during  and  for  some 
time  after  the  recession  of  the  ice,  the  refrigeration  would  still  be 
sufficient  to  allow  the  formation  of  sheets  of  permanent  or  nearly 
permanent  snow  that  would  protect  the  ground  on  which  they  lay 
from  subaerial  waste  or  denudation,  stream  erosion  being  confined 
to  the  defined  water-courses  and  lines  of  drainage. 

The  evidences  of  recent  glaciation  in  the  form  of  boulder  clays, 
moraines,  and  erratics  are  not  always  conspicuous  in  regions  that 
have  been  subjected  to  intense  ice  erosion.  In  Alaska  and  Antarc- 
tica during  the  maximum  glaciation  the  great  work  of  the  ice  was 
erosion  with  deposition  of  detritus  off-shore ;  and,  as  in  all  intensely 
glaciated  regions,  the  glacial  deposits  above  sea-level  are  thin  and 
scattered,  and  were  mostly  deposited  during  the  ice  retreat. 

Glacier  Lakes. — Ice-dammed  lakes -exist  on  the  margin  of  the 
Frederikshaab  ice-apron  on  the  fringe  of  the  Greenland  ice-cap. 
One  of  these,  the  Tasersuak,  12  miles  long  and  over  2  wide, 
stands  at  a  height  of  940  feet  above  the  sea  and  is  blocked  by  ice 
at  both  ends.  It  drains  through  a  canal  to  a  smaller  lake  at  a 
height  of  640  feet. 

A  glacier  descending  a  steep  tributary  valley  may  by  a  sudden 
advance  impound  the  drainage  of  the  main  valley  and  form  a  lake. 
Such  a  lake  is  necessarily  short-lived,  since  the  ice-barrier  is  soon 
destroyed.  A  remarkable  case  is  that  of  the  tributary  glacier 
which  blocked  the  Suru  Valley  in  the  Himalayas,  in  1896,  and  held 
up  the  drainage  until  a  lake  over  20  miles  long  was  formed.  When 
the  ice-barrier  was  broken  through,  the  valley  below  was  devastated 
for  a  distance  of  40  miles. 

More  important  is  the  case  where  the  drainage  of  a  tributary 
valley  is  held  up  by  the  glacier  occupying  the  main  valley.  If  the 
ice-barrier  stands  above  the  level  of  the  pass  or  col  at  the  head  of 
the  tributary  valley,  the  drainage  of  the  lake  may  be  reversed,  and 
the  height  of  the  col  will  represent  the  highest  level  to  which  the 
lake  can  rise.  A  typical  example  of  a  lake  of  this  class  is  the  well- 
known  Marjelen  See  at  the  elbow  of  the  great  Aletsch  glacier  in 
the  Alps.  This  glacier-lake  is  impounded  in  a  tributary  valley, 
and  at  one  time  drained  over  a  low  col  into  the  adjoining  valley 
occupied  by  the  Viesch  glacier.  Such  a  lake,  it  is  thought  by 
some  writers,  might  in  time  give  rise  to  a  detrital  beach  at  the 
level  of  the  col. 

Ice-dammed  lakes  possess  a  peculiar  interest  in  that  they  are 
believed  by  some  writers  to  offer  a  satisfactory  explanation  of  the 
origin  of  certain  step-like  detrital  terraces  that  are  a  conspicuous 
picture  in  many  recently  glaciated  regions.  It  was  first  suggested 
by  Agassiz,  and  afterwards  urged  by  Jamieson,1  that  the  famous 
1  Quart.  Jour.  Geo.  Soc.,  vol.  xix.  pp.  235-259,  1863. 


SNOW   AND    GLACIERS.  77 

Parallel  Roads  of  Glenroy,  in  Argyllshire,  are  the  beaches  of  fresh- 
water lakes  that  seem  to  have  arisen  from  glaciers  damming  the 
mouths  of  the  valleys  and  reversing  the  drainage.  According  to 
this  view,  each  of  the  three  terraces  marks  a  temporary  level  of 
the  ancient  Glenroy  lake.  The  terraces  are  perfectly  horizontal, 
contour  around  the  valley- walls,  and  occur  at  a  height  of  847  feet, 
1059  feet,  and  1140  feet  above  sea-level  respectively. 

The  existence  of  ice-dammed  lakes  has  been  clearly  demonstrated 
in  New  Zealand,  North  England,  Scotland,  North  America  (e.g. 
the  glacial  lake  Agassiz),  and  other  intensely  glaciated  regions  ; 
but  there  is  grave  doubt  as  to  the  ability  of  such  glacier-lakes  to 
explain  the  genesis  of  many  of  the  remarkable  tiers  of  glacial 
terraces  that  are  such  a  prominent  feature  in  the  topography  of 
Alpine  New  Zealand.  Take  a  typical  case.  On  the  east  side  of 
the  coastal  range  the  walls  of  the  main  valley  leading  up  to  Burke's 
Pass  are  terraced  nearly  up  to  the  crest,  the  remains  of  about  forty 
benches  being  clearly  discernible.1  The  valley  opens  on  to  the 
foothills  at  the  back  of  the  Canterbury  Plains,  and  it  is  almost 
inconceivable  that  there  ever  existed  in  these  low  foothills  a  mass 
of  ice  of  sufficient  magnitude  to  form  a  barrier  across  the  main 
valley  and  impound  a  glacier-lake  at  a  height  of  4000  feet  above 
the  sea.  It  seems  easier  to  suppose  that  the  terraces  represent 
the  lines  of  frost-shattered  debris  that  collected  on  the  edge  of  the 
plateau  ice  in  the  form  of  lateral  talus-like  aprons.  Beautiful 
terraces  of  this  kind  have  been  recently  formed  by  the  Hooker 
glacier  near  Mount  Cook.  They  contour  round  the  valley-walls 
at  different  levels,  and  consist  of  angular  rock-debris  mingled  with 
waterworn  sands  and  gravel  brought  down  by  the  small  rivulets 
that  drain  the  adjoining  slopes.  The  Hooker  glacier  has  been 
subject  to  alternating  periods  of  rapid  shrinkage  and  comparative 
rest.  The  lateral  fringing  terraces  were  obviously  formed  during 
the  intervals  of  rest.  A  well-marked  terrace  has  already  accumu- 
lated at  the  present  surface-level  of  the  glacier.  This  glacier,  it 
should  be  noted,  is  little  more  than  the  shrunken  skeleton  of  the 
great  ice-river  that  at  one  time  filled  the  valley  ;  and  it  illustrates, 
in  a  striking  manner,  the  fact  that  a  considerable  retreat  of  the 
terminal  face  of  a  glacier  is  always  accompanied  by  a  corresponding 
shrinkage  in  depth.  In  other  words,  terminal  ablation  and  surface 
ablation  are  contemporaneous  and  co-relative. 

SUMMARY. 

(1)  Snow  (a)  protects  the  land  on  which  it  rests  from  the  in- 
fluence of  frost,  rain,  and  other  subaerial  agents  of  denudation; 

1  J.  Park,  The  Geology  of  New  Zealand,  p.  237,  London,  1910. 


78  A    TEXT-BOOK    OF    GEOLOGY. 

and  (b)  it  has  a  destructive  effect  when  it  falls  or  slides  down 

steep  slopes  by  carrying  loose  rocks  from  a  higher  to  a  lower 

level. 

y^(2)  Glaciers  are  found  in  the  polar  regions  and  in  the  higher 

alpine  regions  of  temperate  and  warm  latitudes.     They  flow  like 

pitch  or  asphalt. 

(3)  Glaciers  and  ice-sheets  are  both  destructive  and  constructive. 
They  wear  away  the  rocks  over  which  they  flow  by  their  sheer 
weight,  while  the  boulders  frozen  into  their  base  plough  into  the 
rocks,  which  are  thus  scored  and  furrowed  and  in  time  deeply 
eroded  or  removed. 

(4)  Glaciers  that  are  overlooked  by  mountains  always  carry  a 
rocky  load  of  debris  partly  on  their  surface,  partly  interglacial,  and 
partly  sub  glacial. 

(5)  The  debris  on  the  surface  of  a  glacier  is  arranged  in  belts 
running  parallel  with  the  sides,  forming  lateral  moraines. 

(6)  Where  the  glaciers  unite,  their  adjacent  lateral  moraines 
form  a  medial  moraine. 

(7)  The  transported  load  when  piled  at  the  end  of  the  glacier 
constitutes  what  is  called  a  terminal  moraine. 

(8)  The  broken-up  rock  and  clay  that  remains  on  the  floor  of  a 
glacial  valley  after  the  ice  has  disappeared  is  termed  a  ground  or 
bottom  moraine,  boulder-clay,  or  till. 

(9)  When  the  boulder-clay  is  piled  up  in  ridges,  often  running 
parallel  with  the  hillsides,  it  forms  what  are  known  as  drums  or 
drumlins. 

(10)  Many  of  the  boulders  found  in  ground-moraines  are  scored 
and  striated,  as  also  are  hard  bosses  of  rock  that  were  overridden 
by  the  ice.     Such  ice-shorn  bosses  are  called  roches  moutonnees. 

(11)  Large  blocks  of  rock  left  by  the  melting  ice  in  conspicuous 
places  are  termed  perched  blocks. 

(12)  Glaciers  are  drained  by  rivers  which  issue  from  ice-tunnels 
at  the  terminal  end. 

(13)  Glacier-rivers  carry  a  load  of  gravel,  sand,  and  silt,  which 
is  spread  out  as  a  valley -train  or  deposited  in  a  lake  or  sea. 

(14)  The  contours  of  recently  glaciated  regions  are  smooth  and 
undulating,  all  the  irregularities  and  corners  having  been  worn 
down.     The  V-shaped  form  of  stream-valleys  has  been   changed 
to  a  U-shaped  form. 


CHAPTER  VI. 
THE   GEOLOGICAL  WORK  OF  THE   SEA. 

THE  sea  covers  about  three-fourths  of  the  surface  of  the  globe. 
It  is  the  destination  to  which  most  streams  and  rivers  hasten,  and 
the  repository  into  which  they  discharge  the  detrital  load  borne 
by  their  waters.  The  sea  ramifies  everywhere  throughout  the  globe, 
and  therefore  exercises  an  equalising  influence  on  climate.  It 
is  the  ultimate  source  of  all  streams,  which,  without  it,  have  no 
separate  existence.  It  must  therefore  rank  as  the  greatest  of  all 
geological  agents. 

Composition  and  Volume. — A  thousand  parts  of  sea-water 
contain  34-40  parts  of  mineral  matter  in  solution,  of  which  common 
salt  (sodium  chloride)  comprises  about  78  per  cent.,  magnesium 
chloride  nearly  11  per  cent.,  magnesium  sulphate  4-7  per  cent., 
sulphate  of  lime  and  potassium  together  6  per  cent. 

Sonstadt  has  shown  that  sea-water  contains  over  half  a  grain 
of  gold  per  ton  ;  and  nearly  all  the  common  metals  and  many  of 
the  rarer  have  been  detected  in  it.  Oxygen,  nitrogen,  and  carbonic 
acid  are  also  present  in  considerable  quantity,  the  amount  of 
carbonic  acid  being  estimated  to  be  eighteen  times  as  great  as  in 
the  atmosphere. 

Murray  has  estimated  that  the  mean  depth  of  the  ocean  is  12,456 
feet,  and  that  the  total  amount  of  sea-water  is  fifteen  times  the 
volume  of  the  dry  land  above  the  sea.  The  mineral  matter  in 
solution  would,  he  estimates,  if  precipitated  cover  the  floor  of  the 
ocean  to  a  depth  of  about  175  feet. 

The  total  river  discharge  into  the  sea  is  estimated  at  6524  cubic 
miles  per  year,  carrying  half  a  cubic  mile  of  mineral  matter.  At 
this  rate  it  would  take  the  streams  9,000,000  years  to  add  to  the 
sea  an  amount  of  mineral  matter  equal  to  what  it  now  contains, 
figures  which  contain  a  useful  suggestion  as  to  the  age  of  the 
ocean. 

WORK  OF  THE  SEA. 

The  sea  as  a  geological  agent  (a)  erodes  and  wears  away  the  dry 
land  ;  (b)  sorts  and  spreads  out  the  material  poured  into  it  by 

79 


80 


A   TEXT-BOOK   OF   GEOLOGY. 


streams  and  rivers,  as  well  as  the  products  derived  from  its  own 
erosive  work  ;  and  (c)  by  its  currents  transports  material  from  one 
place  to  another.  In  other  words,  the  sea  is  (1)  destructive  and  (2) 
constructive. 

THE   SEA  AS  A  DESTRUCTIVE   AGENT. 

Erosive  Effects  of  the  Sea. — This  is  (a)  chemical  and  (b)  mechanical. 

Chemical  Effects. — The  extent  of  the  chemical  effects  of  the 
sea  have  not  yet  been  investigated  to  any  extent.  It  is,  however, 
well  known  that  solutions  of  salt  (sodium  chloride)  exercise  a 
corrosive  effect  on  many  rock-forming  minerals.  Besides,  sea- 


FIG.  31. — Coastal  erosion. 

A,  Showing  form  of  shore  before  erosion. 

B,  Showing  sea-cliff  after  erosion. 

water,  as  stated  above,  contains  a  considerable  amount  of  free 
carbonic  acid,  the  corrosive  and  disintegrating  effect  of  which 
on  the  felspar  minerals  of  rocks  or  limestones  and  calcareous 
aggregates  of  all  kinds  cannot  be  less  than  that  of  the  atmospheric 
carbonic  acid  on  the  same  kind  of  rock  when  forming  dry  land. 
The  chemical  erosion  of  the  sea,  although  no  more  measurable 
to  the  eye  than  the  gradual  and  silent  waste  of  an  undulating 
upland,  must  be  considerable  in  the  course  of  the  centuries, 
and  by  its  softening  and  disintegrating  action  cannot  fail  to  be  a 
powerful  ally  to  the  more  active  and  apparent  forces  of  mechanical 
erosion. 

Everyone  must  have  observed  how  prone  to  decomposition  and 
surface  weathering  are  the  rocks  exposed  in  cliffs  facing  the  sea. 
This  may  be  in  part  due  to  the  briny  spray  which  is  carried  over 


THE    GEOLOGICAL   WORK   OF   THE    SEA.  81 

the  land  by  the  wind,  and  in  part  the  work  of  the  powerful  and 
active  oxidiser  ozone  which  is  more  abundant  on  the  seashore 
than  elsewhere. 

Mechanical  Effects. — The  most  obvious  effect  of  the  sea  is 
seen  in  the  cutting  back  of  the  land  so  as  to  form  steep  faces 
and  cliffs. 

The  rate  of  cutting  back  or  recession  of  the  land  will  depend  on 
the  hardness  of  the  rock,  its  composition,  presence  or  absence  of 
joints,  stratification  or  cleavage  planes.  Where  the  rock  consists 
of  alternating  hard  and  soft  beds,  the  soft  beds  will  be  cut  back 
more  rapidly  than  the  hard,  thus  leaving  an  overhanging  cornice 
of  hard  rock.  Even  the  hardest  rocks  possess  relatively  little 
transverse  strength ;  consequently  the  overhanging  ledge  soon 
breaks  off  owing  to  the  stress  of  its  own  weight. 


FIG.  32. — Showing  marine  erosion  of  sloping  bench  in 
Auckland  Harbour,  N.Z. 

A,  High-water  mark.  B,  Low-water  mark. 

The  masses  of  broken  rock  fall  to  the  foot  of  the  cliff,  where  they 
act  as  a  protecting  apron  by  breaking  up  and  in  part  destroying 
the  erosive  effect  of  the  advancing  waves.  But  in  time  the  fallen 
blocks  become  pounded  up  and  removed,  and  once  more  the  active 
undermining  of  the  sea-cliffs  begins. 

The  manner  in  which  marine  erosion  is  effected  varies  with  the 
mood  of  the  sea.  In  its  normal  mood,  which  is  the  tranquil  or 
semi-tranquil,  the  sea  by  the  constant  rise  and  fall  of  the  tide 
alternately  covers  and  uncovers  a  marginal  strip  of  land  that  in 
time  becomes  worn  down  into  a  bench  that  slopes  from  low-water  to 
high-water  mark.  Where  the  rocks  are  soft,  the  bench  may  be  worn 
into  a  flat  platform  lying  a  foot  or  two  above  low-water  mark 
(fig.  33). 

During,  and  for  some  time  after  a  heavy  gale,  the  sea  flings 
itself  furiously  against  the  shore,  and  in  this  mood  it  is  very 
destructive.  Pinnacles  of  rock  that  have  been  undermined  or 
loosened  by  the  thundering  blows  of  previous  storms  are  toppled 

6 


82 


A   TEXT-BOOK   OF   GEOLOGY. 


over,  while  the  overhanging  portions  of  cliffs  are  torn  off  and  the 
debris  spread  along  the  strand,  where  it  is  slowly  broken  up  and 
rounded  by  the  unceasing  wave-action  of  the  advancing  and 
retreating  tides. 

Masses  of  rock  that  in  normal  times  lie  undisturbed  at  the  foot 
of  the  cliffs,  during  great  storms  are  picked  up  by  the  advancing 
waves  and  hurled  against  one  another  with  terrific  force.  Or 


FIG.  33. — Showing  flat  bench  excavated  in  chalky  marls, 
Amuri  Bluff,  N.Z. 

A,  High -water  mark.  B,  Low -water  mark. 

where  the  shore  is  unprotected  by  an  apron  of  broken  rock, 
the  masses  are  employed  by  the  waves  as  battering-rams  for 
bombarding  the  foot  of  the  cliffs,  which  thus  in  time  become 
shattered  and  finally  undermined.  When  the  undermined  over- 
hanging portion  of  the  cliff  breaks  off  under  the  force  of 
gravity,  the  tumbled  rock  provides  fresh  ammunition  for  another 
bombardment. 


d 

FIG.  34. — Showing  coastal  erosion. 

Wave  and  Tidal  Effects.— -The  constant  movement  of  the  sea 
gradually  eats  away  the  edge  of  the  land,  and,  obviously,  if  the 
process  were  continued  long  enough,  first  the  islands  and  then  the 
continents  would  be  shorn  down  to  an  almost  even  plain  not  much 
below  sea-level.  As  a  matter  of  fact,  many  flat  reefs  that  are  just 
awash  at  low-water  are  all  that  now  remain  of  what  were  at  one 
time  islands  standing  near  the  mainland.  But  the  power  of  the 
waves  does  not  end  here.  The  angular  blocks  that  are  broken 


THE    GEOLOGICAL    WORK    OF    THE    SEA.  83 

off  the  cliffs  are  acted  on  by  the  tidal  movements  of  the  sea  as  well 
as  by  the  larger  waves  of  fierce  storms,  and  in  time  become  worn 
down  and  rounded.  The  sharp  angular  blocks  lie  along  the  foot 
of  the  cliffs.  In  the  tide- way  the  blocks  are  somewhat  rounded 
and  smaller.  Further  out  the  blocks  get  smaller  and  smaller,  and 
more  and  more  rounded,  until  they  eventually  pass  into  shingle 
or  beach-gravels.  Still  further  out  the  shingle  becomes  smaller 
and  smaller,  and  finally  graduates  into  sand. 

The  push  and  drag  of  the  tides  rolls  the  shingle  over  and  over, 
and  by  this  everlasting  attrition  and  grinding,  the  pebbles  become 
more  and  more  rounded,  and  consequently  smaller  and  smaller, 
until  eventually  they  are  reduced  to  sand.  In  the  same  way  the 
swish  of  the  waves  causes  a  similar  abrasion  and  grinding  of  the 
sands,  which  in  time  are  reduced  to  a  fine  silt  which  is  caught  up 
by  currents  and  spread  far  and  near  over  the  floor  of  the  sea.  The 


Funnel-shaped  Blow  hole 


FIG.  35. — Showing  funnel-shaped  blow-hole,  Puketeraki,  N.Z. 

place  of  the  ground-up  shingle  and  sand  is  constantly  replenished 
by  fresh  material  broken  from  the  cliffs,  which  are  continually 
crumbling  away  under  the  combined  attack  of  subaerial  and 
marine  erosion. 

Erosive  Effect  of  Compressed  Air. — When  the  advancing  waves 
fling  themselves  against  a  fissured  cliff,  or  one  containing  fissure- 
like  cavities  such  as  are  frequently  formed  along  joint  planes  or 
the  stratification  lines  of  inclined  strata  of  different  degrees  of  hard- 
ness, the  contained  air  is  compressed  by  a  pressure  equal  to  the 
force  exerted  by  the  falling  wave.  When  the  wave  suddenly 
retreats,  the  air  expands  with  shattering  force,  and  in  this  way  the 
cracks  and  fissures  are  enlarged  and  fresh  ones  opened. 

In  some  places  the  fissures  communicate  with  the  upper,  surface 
of  the  cliff,  forming  what  are  known  as  blow-holes,  from  which  air 
and  spray  or  even  a  column  of  water  may  be  projected  with  great 
force  when  the  waves  dash  on  the  cliffs  below.  As  time  goes  on, 
the  blow-hole  becomes  larger  and  larger,  until  at  last  a  cavern  with 


84  A    TEXT-BOOK    OF    GEOLOGY. 

a  wide  funnel-shaped  opening  on  the  top  of  the  cliff  is  formed  as 
shown  in  fig.  35. 

Erosive  Effects  of  Floating  Ice. — In  the  high  latitudes  of  both 
hemispheres  where  the  seas,  lakes,  and  rivers  become  frozen  over 
in  winter,  the  effects  of  ice-erosion  are  sometimes  very  striking. 

In  spring  when  the  river-ice  breaks  up,  it  is  frequently  piled  up 
in  narrow  gorges  until  a  block  takes  place.  When  the  impounded 
waters  eventually  break  away,  the  sharp-edged  sheets  of  ice  scrape 
and  fret  against  the  banks,  which  in  time  become  undermined  and 
ultimately  break  away  in  long  strips.  Where  the  course  of  the  river 
runs  through  alluvial  flats  the  ice  is  particularly  destructive. 

When  the  ice  breaks  up  in  lakes,  the  floating  masses  move 
towards  the  outlet,  where  they  sometimes  accumulate  until  a 
"  jam  "  takes  place.  The  force  exerted  by  ice  piled  up  in  this 
way  is  enormous.  Logs  of  wood  entangled  in  the  "  jam  "  are 
frequently  broken  into  splinters,  while  all  the  rocks  within  the 
reach  of  the  ice  are  scored  and  carved  into  ledges,  or  shattered 
and  tumbled  over.  This  erosive  effect  is  seen  in  most  Arctic 
gulfs  and  narrow  seas. 

Floating  bergs  moving  past  headlands  and  along  contracted 
passages  chafe  and  grind  against  the  shores,  whereby  they  may 
in  time  excavate  narrow  benches  in  the  solid  rock.  Some  of  the 
raised  benches  that  contour  around  the  fiords  of  Norway  are 
believed  by  some  writers  to  have  been  formed  in  this  way. 

Topography  of  Marine  Erosion. — In  sheltered  bays  the  shore  is 
generally  bounded  by  low  undulating  downs  fringed  with  a  strip 
of  sandy  beach.  Typical  examples  of  this  are  seen  on  the  shores 
of  the  Wash,  in  England,  and  Golden  Bay,  in  New  Zealand.  The 
headlands  enclosing  or  sheltering  such  bays  frequently  present 
steep  cliff-faces  to  the  sea,  the  formation  of  which  in  these  exposed 
situations  is  commonly  the  result  of  the  more  rapid  erosion  of  the 
land  by  the  prevailing  sea-currents. 

Where  the  bay  is  land-locked,  the  cliffs  may  extend  some  distance 
inside  the  harbour,  their  excavation  being  the  work  of  the  incom 
ingand  outgoing  tides  which  travel  with  great  velocity  in  narrow 
passages,    and    are    hence    endowed    with   great   erosive   power 
(Plate  XL). 

In  many  places  where  the  coast-line  is  deeply  indented  with 
numerous  ramifying  bays  and  fiords,  the  land  everywhere  rises 
steeply  from  the  water's  edge.  In  these  cases  the  neighbouring 
land  is  generally  high  and  mountainous.  The  fiords  are  deep 
valleys  that  have  become  invaded  by  the  sea  through  the  sub- 
mergence of  the  land.  They  are  what  are  known  as  drowned 
valleys,  and  the  steepness  of  their  shores  is  mainly  the  work  of  sub- 
aerial  agencies  of  erosion.  The  newer  cliffs  of  marine  erosion  that 


THE    GEOLOGICAL   WORK    OF   THE    SEA.  85 

are  sometimes  seen  in  process  of  formation  in  these  land-locked 
fiords  can  easily  be  distinguished  from  the  old  valley  contours. 
Fine  examples  of  this  class  of  coastal  topography  are  seen  on 
the  coasts  of  Chile,  Alaska,  and  New  Zealand,  but  their  con- 
figuration has  no  relation  to  marine  erosion. 

On  exposed  coast-lines  the  shore  is  generally  bounded  by  steep 
cliffs  that  may  vary  from  a  few  feet  to  many  hundreds  of  feet  high. 
These  cliffs  may  present  many  diversities  of  form  according  to  the 
character  of  the  country  rocks.  The  hardness  of  the  rocks,  the 
presence  of  bedding  and  joint  planes,  the  direction  of  the  dip  and 
strike  of  stratified  rocks,  all  tend  to  retard  or  accelerate  the  progress 
of  marine  erosion.  Generally  speaking,  the  softer  coherent  rocks 
present  steep  cliffs  with  even  slopes  ;  while  sandstones  and  all  the 
harder  rocks,  such  as  basalt  and  granite,  give  vertical  and  fantastic 
forms  that  are  frequently  undermined,  tunnelled,  and  arched. 
Where  the  rock  is  fissured  or  intersected  by  joints,  narrow  tunnel- 
like  caves  will  be  formed,  along  which  the  waves  will  rush  with 


FIG.  36. — Showing  effect  of  coastal  recession  on  river 
grading,  S.  Canterbury,  N.Z. 

great  force.  Where  the  rock  is  intersected  by  two  sets  of  joints,  the 
recession  of  the  cliffs  may  result  in  blocks  or  stacks  of  rock  being 
left  standing  as  outlines  or  pinnacle-shaped  islands. 

In  all  cases  the  softer  rocks  will  be  worn  away  more  rapidly  than 
the  harder  ;  therefore,  while  the  former  will  be  eaten  back  into 
bays  and  inlets,  the  latter  will  form  the  headlands. 

Effects  of  Coastal  Recession  on  River  Grading. — The  cutting  back 
of  the  coast-line  shortens  the  length  of  the  streams  and  rivers  that 
drain  into  the  sea.  The  obvious  effect  of  this  recession  and  shorten- 
ing is  to  give  the  rivers  a  greater  velocity  on  account  of  the  steeper 
gradient.  The  greater  velocity  enables  a  stream  or  river  to  regrade 
and  cut  down  its  bed. 

The  effects  of  coastal  recession  are  always  most  marked  where  the 
river  flows  over  an  alluvial  plain  before  entering  the  sea.  The 
river  in  this  situation  is  enabled,  in  the  process  of  cutting  down 
its  bed  to  its  base-level — the  sea,  to  excavate  terraces  in  the  lower 
part  of  its  course.  The  effect  of  coastal  recession  as  regards 
terrace  formation  is  therefore  the  same  as  an  elevation  of  the 
land. 


86  A    TEXT-BOOK    OF    GEOLOGY. 

The  Canterbury  Plains  in  New  Zealand  are  composed  of  gravel- 
drift  carried  down  from  the  alpine  ranges  by  a  number  of  large 
rivers.  They  extend  along  the  coast  for  over  a  hundred  miles  ;  and 
north  of  Timaru,  where  the  coast  is  swept  by  a  strong  northerly 
current,  they  have  been  cut  back  until  they  present  sea-cliffs, 
varying  from  10  to  50  feet  high,  the  highest  cliffs  being  found  where 
the  recession  is  greatest.  The  old  plane,  along  which  the  rivers 
flowed  before  the  cutting  back  of  the  coast-line,  is  indicated  by  the 
line  a  b  c  (fig.  36),  the  former  point  of  discharge  being  at  c.  By  the 
wearing  away  of  the  land  the  point  of  discharge  is  now  at  d,  and 
b  d  shows  the  height  of  the  present  sea-cliff.  The  present  plane 
of  flow  is  along  g  d.  During  the  process  of  cutting  down  their  beds, 
the  rivers  have  excavated  a  series  of  terraces  as  indicated  by  the 


FIG.  37. — Showing  formation  of  plain  of  marine  denudation. 

a,  6,  c,  d,  e,  and  /  are  successive  slices  shorn  off  the  edge  of  the  land  forming 
the  marine  plain  p  p. 

broken  line  e  /.  The  old  flood-level  a  b  now  forms  the  highest 
terrace. 

Plain  of  Marine  Denudation.— In  a  preceding  chapter  we  found 
that  the  general  effect  of  all  the  processes  of  subaerial  denudation 
was  to  wear  down  the  dry  land  to  a  base-level  or  peneplain  ;  and 
similarly  we  find  that  the  final  effect  of  all  the  processes  of  marine 
erosion  is  the  slicing  away  of  the  edge  of  the  land  to  a  horizontal 
or  gently  sloping  platform  to  which  the  name  plain  of  marine 
denudation  is  applied.  The  existence  of  this  marine  shelf  is  proved 
by  the  soundings  around  the  coast-line.  It  is  found  that  nearly 
all  the  continents  and  larger  islands  are  surrounded  by  a  marine 
shelf. 

The  shelf  generally  slopes  gently  seawards,  and  when  its  edge 
is  reached  there  is  a  sudden  drop  in  the  floor  of  the  sea.  The 
manner  in  which  this  marine  shelf  is  carved  out  of  the  land  will 
be  easily  understood  by  a  reference  to  fig.  37. 

Where  the  land  fronting  the  sea  is  high,  the  shelf  is  generally 


THE    GEOLOGICAL   WORK    OF   THE    SEA.  87 

narrow,  and  where  low  it  is  relatively  wide.  But  the  total  amount 
of  erosion  is  probably  about  the  same  everywhere,  for  the  greater 
height  and  less  width  in  one  place  will  balance  the  less  height 
and  greater  width  at  another. 

Where  the  land  has  been  recently  elevated,  the  dissected  remains 
of  ancient  plains  of  marine  denudation  can  still  be  clearly  traced 
in  regions  lying  near  the  sea. 

Rate  of  Marine  Erosion. — The  rate  of  marine  erosion  is  dependent 
(a)  on  the  resistance  offered  by  the  rocks,  (b)  on  the  erosive  power 
of  the  waves,  and  (c)  on  the  transporting  power  of  oceanic  currents. 

Soft  rock  or  incoherent  gravel  and  sand  will  be  worn  away  more 
rapidly  than  hard  rock.  Moreover,  the  rate  of  erosion  will  be  more 
rapid  on  a  bleak  coast-line  exposed  to  the  force  of  storms  sweeping 
over  a  wide  expanse  of  sea  than  in  sheltered  bays  and  land-locked 
harbours,  or  on  the  lee-side  of  a  cape  or  peninsula. 

When  the  wind  blows  for  some  time  over  a  broad  expanse  of 


FIG.  38. — Showing  section  of  marine  shelf  around  Otago,  N.Z.,  to 
100-fathom  line. 

(a)  Edge  of  marine  shelf. 

ocean,  the  water  is  piled  up  against  the  shore,  with  the  result  that 
the  waves  are  able  to  reach  far  above  the  point  reached  by  normal 
tides.  When  the  heaping  up  of  the  water  happens  to  coincide 
with  high-water,  the  erosion  effected  by  the  pounding  of  the 
abnormal  waves  may,  in  a  few  hours,  exceed  the  normal  tidal 
erosion  of  half  a  year  or  more. 

The  cliffs  excavated  in  compact  rock  are  generally  steep  and 
often  overhanging.  Unconsolidated  material,  on  the  other  hand, 
naturally  assumes  the  angle  of  rest,  and  hence  commonly  presents 
gentle  slopes  to  the  sea. 

A  compact  homogeneous  rock,  like  granite,  when  unfissured 
frequently  wears  away  into  dome-shaped  forms  that  have  a  curious 
resemblance  to  the  rounded  roches  moutonnees  of  glacial  erosion. 
The  smooth  dome-shaped  islands  and  headlands  on  the  coast  of 
Western  Australia,  where  there  is  no  reason  to  suspect  recent 
glaciation,  are  notable  examples  of  this  form  of  erosion  which  is 
the  united  work  of  subaerial  and  marine  denudation. 

When  stratified  rocks  dip  towards  the  sea,  landslips  are  of  frequent 


88  A    TEXT-BOOK    OF    GEOLOGY. 

occurrence,  as  a  small  amount  of  excavation  at  the  foot  of  the  cliff 
is  generally  sufficient  to  destroy  the  supporting  toe  of  the  block. 

The  undermining  and  breaking  up  of  the  hard  rock  provides 
boulders  and  shingle,  which  the  waves  use  with  destructive  effect 
in  carrying  on  the  assault  on  the  land  fringing  the  shore.  On  the 
other  hand,  soft  rocks  like  chalk,  marls,  and  shaly  clays  provide  no 
materials  for  the  making  of  shingle,  except  they  happen  to  contain 
nodules  of  flint,  as  chalk  and  chalky  clays  frequently  do. 

Where  the  rocks  consist  of  alternating  soft  and  hard  bands,  the 
soft  bands  are  worn  away  more  rapidly  than  the  hard.  The  result 
of  this  is  that  the  hard  bands  are  in  time  left  unsupported,  and  soon 
crumble  away  under  the  vigorous  pounding  of  the  waves. 

Coral  reefs,  submerged  shoals,  sand-pits,  and  outlying  islands 
frequently  protect  the  mainland  from  the  direct  influence  of 
sea-currents,  or  the  violence  of  storms.  For  example,  the  Great 
Barrier  Reef  of  Australia,  which  runs  parallel  with  the  coast  of 
Queensland  for  over  a  thousand  miles,  affords  the  most  effective 
shelter  against  all  easterly  gales  sweeping  across  the  Pacific. 

The  actual  rate  of  marine  erosion  is  so  slow  that  there  is  little 
authentic  record  of  coastal  changes  within  historic  times.  Even 
where  changes  have  taken  place  it  is  not  always  clear  that  the 
encroachment  is  the  direct  result  of  marine  erosion.  Much  of  the 
encroachment  of  the  sea  in  the  eastern  Mediterranean  and  on  the 
east  coast  of  Australia  is  obviously  the  result  of  general  subsidence 
of  the  land.  In  other  places  where  low-lying  lands  have  become 
submerged  in  recent  times,  the  inroad  of  the  sea,  if  not  the  result 
of  subsidence,  may  be  due  to  coastal  sag,  resulting  from  the  accumu- 
lation of  a  great  thickness  of  detritus  on  the  sea-floor.  Coastal  sag 
is  always  most  conspicuous  where  the  detritus  rests  on  a  soft  sea- 
bottom. 

The  Sea  as  a  Constructive  Agent. 

Effects  of  Sea-Currents. — The  great  oceanic  currents  do  not 
reach  the  bottom,  and  therefore  possess  little  or  no  power  either 
to  transport  detritus  or  to  erode  the  floor  of  the  sea  ;  but  many 
coast-lines  are  swept  by  currents  that  hug  the  shore  and  run  in  one 
direction  during  the  whole  or  greater  part  of  the  year.  These 
littoral  currents  may  be  termed  rivers  of  sea-water,  and,  like  fresh- 
water rivers,  they  are  important  agents  of  transport  and  erosion. 

The  gravel,  sand,  and  silt  discharged  by  a  stream  or  river  into 
the  head  of  a  sheltered  bay,  inlet,  or  land-locked  harbour  is  spread 
put  on  the  bottom,  where  it  gradually  accumulates,  until  in  time 
it  fills  up  extensive  areas  which  are  thus  reclaimed  from  the  sea. 
It  is  in  this  way  that  the  alluvial  flats  and  deltas  that  are  found 
fringing  the  head  of  so  many  bays  and  inlets  have  been  formed. 


THE    GEOLOGICAL    WORK    OF   THE    SEA. 


89 


But  where  the  stream  or  river  discharges  its  load  into  the  open 
sea,  the  discharged  detritus  is  picked  up  by  the  coastal  currents 
and  spread  over  the  sea-floor,  or  piled  up  on  distant  strands,  perhaps 
scores,  or  even  hundreds,  of  miles  from  the  river  mouth.  In  this 
way  vast  quantities  of  detritus  are  daily  moved  from  one  place 
to  another. 

Only  the  fine  silt  and  mud  is  carried  in  suspension.  The  bulk 
of  the  material  in  the  form  of  sand  or  shingle  is  trailed  or  rolled 
along  the  sea-floor,  and  in  consequence  exercises  a  powerful  erosive 
effect  on  all  submerged  reefs  and  ledges,  on  outlying  islands,  and 
projecting  headlands.  The  travelling  sands  and  shingle  possess 
the  same  rasping  and  eroding  effect  as  the  moving  sands  and  gravel 
on  the  floor  and  sides  of  a  river-channel. 


FIG.  39. — Showing  piling  up  of  sea-borne  sand  on  sheltered  side  of 
headland  at  St  Clair,  Otago,  N.Z. 

Sea-borne  sands  generally  accumulate  on  the  sheltered  side  of 
headlands  or  in  bays,  where  they  form  sand-banks  that  are  some- 
times awash  or  bare  at  low-water.  In  many  places  the  wind  piles 
up  the  sands  thus  placed  within  its  reach  into  dunes  and  ridges 
running  parallel  with  the  beach. 

The  coastal  currents  of  Otago  in  New  Zealand  travel  northward 
all  the  year  round.  They  strike  Black  Head  at  St  Clair,  and  are 
diverted  seawards  for  some  distance  ;  but  during  south-east  gales 
they  are  deflected  inshore,  with  the  result  that  many  millions  of 
tons  of  sea-borne  sand  are  sometimes  thrown  up  on  the  beach  in 
the  course  of  a  few  hours,  frequently  covering  up  the  protecting 
groins  as  shown  in  fig.  39. 

Where  two  coastal  currents  travelling  in  different  but  converging 
directions  meet  one  another,  their  impact  causes  their  rate  of  flow 
to  be  diminished  or  altogether  destroyed  along  the  line  of  contact, 


90 


A    TEXT-BOOK    OF    GEOLOGY. 


with  the  result  that  the  sands  they  carry  are  allowed  to  settle  and 
accumulate  along  that  line,  until  submerged  sand-banks  and  long 
sand-spits  are  formed.  A  notable  example  of  this  class  of  con- 
structive work  is  found  at  the  extreme  north-east  corner  of  the 
South  Island  of  New  Zealand.  Here  a  spit  of  sand,  20  miles  long, 


FIG.  40. — Showing  formation  of  sand-spit  by  two  converging 
sea-currents  at  Cape  Farewell,  N.Z. 

has  been  formed  by  the  converging  Farewell  and  Golden  Bay 
currents  as  shown  in  fig.  40. 

Littoral  Shingle  Deposits. — These  may  be  divided  into  three 
classes,  according  to  their  form  and  origin,  namely  (1)  the  fringing 
beach,  (2)  the  shingle-spit,  and  (3)  the  shingle-flat. 


FIG.  40A. — Boulder  Bank,  Nelson,  N.Z. 

The  fringing  beach  is  the  simplest  and  most  common  type.  It 
consists  of  a  strip  of  shingle  along  the  strand,  formed  by  the  coastal 
currents  directed  at  right  angles  against  the  shore. 

The  shingle-spit  is  a  deposit  of  shingle  beginning  at  the  point 
where  the  coast-line  suddenly  changes  its  direction  and  turns 
inwards,  while  the  current  running  along  it  still  pursues  its  course 


THE    GEOLOGICAL   WORK    OF   THE    SEA. 


91 


past  the  point  of  deflection.  The  drifting  shingle  accumulates 
along  the  line  of  currents,  and  in  time  forms  a  bank  or  causeway 
that  may  be  many  miles  long.  The  bank  frequently  curves  inwards 
at  its  growing  end.  Good  examples  of  shingle-spits  are  the  Chesil 
Bank,  which  runs  parallel  with  the  coast  of  Dorsetshire  for  15 
miles ;  and  the  Boulder  Bank,  at  Nelson,  New  Zealand,  a  gigantic 
causeway,  12  miles  long,  which  encloses  a  deep,  well-sheltered 
harbour.  When  the  bank  grows  till  it  again  touches  the  land  it 
forms  a  bar. 

The  shingle-flat  is  formed  when  the  coastal  current,  due  to  some 
local  cause,  follows  the  inward  trend  of  the  deflected  coast-line,  the 


FIG.  40B. — Showing  formation  of  marine  shingle -flat, 
(a,  a)  Old  shore-line.         (6)  Shingle-flat. 


shingle  being  deposited  as  a  succession  of  parallel  banks.  In  this 
way  large  areas  may  be  reclaimed  from  the  sea. 

Sorting  and  Spreading  Action  of  the  Sea. — The  gravel,  sand,  and 
silt  discharged  into  the  sea  by  streams  and  rivers  are  sorted  by 
the  laving  action  of  the  waves  into  three  main  grades  of  different 
sizes.  The  shingle  is  spread  along  the  shore  in  the  shallow  water, 
the  sands  are  distributed  over  the  sea-floor  for  many  hundred 
yards  on  the  seaward  side  of  the  shingle,  while  the  silts  and  muds 
are  transported  still  further  seaward. 

We  have  thus  three  zones  of  deposition  running  approximately 
parallel  with  one  another  and  with  the  shore.  There  is  seldom  a 
sharp  line  of  demarcation  between  the  different  zones.  More  often 
the  one  graduates  insensibly  into  the  next ;  but  the  extremes  are 
always  clearly  denned.  Thus  the  clean  gravel  is  easily  distin- 
guished from  the  sand,  and  the  sand  from  the  silt  and  mud. 


92 


A   TEXT-BOOK    OF    GEOLOGY 


Fia.  41. — Plan  showing  lenticular  distribution  of  gravel,  sand,  and  silt 
on  coast-line. 

(a)  Gravel.  (6)  Sand.  (c)  Silt  and  mud. 

A  B,  Line  of  section  at  right  angles  to  shore-line. 

Beginning  with  the  shore-line  deposits  we  have  thus  :— 

(1)  The  shingle  and  gravel  zone. 

(2)  The  sand  zone. 

(3)  The  silt  and  mud  zone. 

Where   the   deposition  takes   place   without   disturbance   from 


THE    GEOLOGICAL   WORK    OF   THE    SEA. 


93 


currents,  we  can  generally  distinguish  six  grades  of  material  that 
insensibly  pass  into  one  another,  namely  : — 

(1)  Shingle. 

(2)  Gravel. 

(3)  Sandy  gravel. 

(4)  Coarse  sand. 

(5)  Fine  sand. 

(6)  Silt  and  mud. 

Lenticular  Form  of  Marine  Deposits. — The  different  zones  of 
material  starting  from  the  point  of  discharge  are  nearly  always 
found  to  be  more  or  less  lenticular  in  form.  It  thus  happens  that 
in  passing  along  the  coast-line  we  encounter  the  same  succession  of 
gravel,  sand,  and  silt  as  we  do  by  following  a  line  running  at  right 


Scale  of  English  Miles 
0     10   20  30   40   50 


FIG.  42. — Plan  showing  delta  of  the  Nile  in  Egypt. 

angles  to  the  shore-line.  Thus,  as  shown  in  fig.  41,  we  find  that 
the  beds  a,  6,  c,  found  along  section  line  A  B,  are  the  same  as  beds 
a,  b,  c  that  successively  abut  against  the  shore-line  in  passing 
northward  from  the  point  of  discharge. 

Formation  of  Deltas. — Streams  and  rivers  that  are  sluggish  in 
their  rate  of  flow  are  only  able  to  transport  sand  and  silt  to  the  sea. 
In  the  absence  of  coastal  currents  the  sands  and  silts  are  deposited 
at  the  mouth  of  the  river,  where  they  accumulate  until  they  form 
obstructing  banks  through  which  the  river  flows  in  numerous 
intricate  shallow  channels.  By  the  piling-up  action  of  the  wind 
at  low- water,  and  by  the  deposit  of  sand  and  silt  during  times  of 
flood,  many  of  the  sand-banks  rise  above  ordinary  water-level. 
When  this  happens  vegetation  soon  establishes  itself,  and  a  delta 
is  thus  formed. 

Estuarine  Deposition. — In  marine  deposition,  as  we  have  seen, 


94  A    TEXT-BOOK    OF    GEOLOGY. 

the  coarsest  material  is  laid  down  nearest  the  shore-line,  and  the 
finest  furthest  seaward ;  each  zone  of  graded  material  being  spread 
over  a  lenticular  or  meniscus-shaped  area.  In  estuarine  deposition 
this  arrangement  is  reversed,  the  coarsest  material  being  deposited 
at  the  entrance  and  the  finest  at  the  head  of  the  estuary. 

At  the  entrance  the  tide  generally  rushes  in  with  great  velocity. 
When  once  inside  the  harbour  the  tide  as  it  advances  spreads  over 
an  increasing  area,  with  the  result  that  its  rate  of  flow  shows  a 
corresponding  decrease. 

The  result  of  this  gradual  slackening  of  the  current  is  that  the 
coarsest  material  is  deposited  at  the  entrance,  and  the  finest  at  the 
extreme  limits  reached  by  the  tide. 

In  places  where  the  coastal  currents  transport  gravel,  sand,  and 
mud,  the  incoming  tide  deposits  first  gravel,  then  sand,  and  lastly 
mud  ;  the  first  at  the  entrance  and  the  last  in  the  upper  reaches  of 
the  estuary.  Where  the  tide  carries  only  sand  and  mud,  as  is  so 
frequently  the  case,  the  sand  is  deposited  at  the  entrance  and  the 
muds  inside.  Hence  we  find  that  the  entrance  of  some  estuaries 
is  protected  by  a  bank  of  gravel,  and  of  others  by  shoals  and  bars 
of  sand.  In  many  cases  the  sands  deposited  inside  the  estuary 
are  piled  by  the  wind  into  dunes  and  ridges  running  parallel  with 
the  coast-line. 

The  area  covered  by  the  gravels  is  commonly  narrow.  On  the 
other  hand,  the  sands  are  deposited  over  a  wider  belt,  but  even  this 
is  relatively  small  in  extent  compared  with  the  mud-covered  area. 
The  limited  distribution  of  the  coarser  material  is  due  to  the 
rapidity  with  which  the  current  of  the  inflowing  waters  diminishes 
when  once  the  waters  begin  to  spread  over  the  banks  and  shoals 
of  the  estuary. 

In  some  of  the  great  estuaries  and  tidal  harbours  of  northern 
and  south-east  Australia  the  mud  covers  hundreds  of  square  miles, 
while  the  sands  near  their  entrance  occupy  but  a  relatively  narrow 
belt.  In  these  shallow  harbours  the  tidal  waters  come  in  laden 
with  mud  and  retire  laden  with  mud,  but  each  tide  as  it  slowly 
creeps  over  the  mud-banks  deposits  a  thin  coating  of  sediment 
which  imperceptibly  but  steadily  raises  the  mud-covered  area  until 
it  is  high  enough  to  enable  a  semi-aquatic  vegetation  to  establish 
itself  on  its  surface.  In  this  way  these  tidal  inlets  are  being  gradu- 
ally filled  up  and  reclaimed  from  the  sea. 

Nearly  all  estuaries  receive  the  drainage  of  one  or  more  streams 
or  rivers.  Some  of  the  larger  streams  discharge  a  load  of  gravel 
and  sand  into  the  estuary,  where  it  is  spread  out  and  mingled  with 
the  fine  harbour  muds.  In  this  way  we  frequently  get  the  curious 
spectacle  of  gravels  and  sands  mingled  with  almost  impalpable 
mud,  or  intercalated  with  layers  of  mud. 


THE    GEOLOGICAL    WORK    OF    THE    SEA.  95 

Marine  Organic  Deposits. — The  floor  of  the  deeper  or  abysmal 
portions  of  the  sea  has  been  shown  by  soundings  to  consist  of 
a  fine  calcareous  ooze  mainly  composed  of  the  tiny  shells  of 
Foraminifera,  etc. 

On  their  seaward  limits,  the  fine  mechanical  sediments  reach- 
ing out  from  the  land  mingle  with  this  ooze,  forming  deposits 
which  become,  when  hardened,  what  are  termed  chalky  clays  or 
chalky  marls.  The  ooze  itself,  when  free  from  foreign  sediment, 
forms,  when  consolidated,  a  limestone  resembling  chalk.  These 
organic  oozes  will  be  described  more  fully  in  the  succeeding 
pages. 

Time-Plane  of  Deposition. — Going  seawards,  the  coastal  gravels 
graduate  into  sands,  the  sands  into  muds,  and  the  muds  into 
calcareous  ooze.  The  gravels,  sands,  muds,  and  ooze  were  deposited 
at  the  same  time  and  lie  on  the  same  plane,  and  form  what  is  known 
in  geology  as  a  time-plane  of  deposition. 

Faunal  Differences  in  Same  Plane.— Each  grade  of  material 
will  be  distinguished  by  the  forms  of  life  that  prevailed  in  the 
zone  in  which  it  was  laid  down.  That  is,  the  gravels  will  contain 
the  broken  and  rolled  remains  of  such  littoral  shells  as  oysters, 
mussels,  and  cockles  ;  the  sands,  the  remains  of  Pinna,  Tellina, 
and  other  fragile  shells  ;  the  muds,  minute  molluscs,  and  foramin- 
ifera ;  while  the  ooze  will  consist  mainly  of  Pteropods  and  various 
Foraminifera,  among  which  the  genus  Globigerina  will  be  the 
commonest. 

A  change  in  the  character  of  the  deposits  is  usually  followed  by 
a  change  in  the  fauna  ;  but  with  a  recurrence  of  the  same  sediments 
there  will  frequently  be  a  reappearance  of  the  displaced  fauna  ; 
and  if,  in  some  places,  the  lithological  character  of  the  deposit 
remains  unchanged,  some  species  may  persist  in  that  place  into  a 
higher  horizon  than  is  usual  elsewhere. 

Besides  the  faunal  differences  due  to  the  various  character  of 
the  sediments,  it  is  found  that  certain  organisms  inhabit  shallow, 
and  others  deep  water.  Hence  it  must  be  remembered  that  faunal 
differences  in  the  same  plane  may  arise  as  much  from  influence  of 
station  as  from  differences  in  the  texture  of  the  sediments. 

The  depth  of  the  sea  normally  increases  with  the  distance  from 
the  land,  but  great  depths  may  be  obtained  in  certain  conditions 
quite  close  to  the  land,  as  in  the  fiords  of  Norway  and  New  Zealand, 
where  a  depth  of  100  fathoms  may  frequently  be  found  a  few  yards 
from  the  shore.  In  such  situations  we  are  liable  to  find  a  curious 
commingling  of  littoral  and  deep-water  species. 

Some  organisms  find  a  congenial  habitat  in  muddy  waters,  others 
in  clear  ;  some  flourish  only  on  rocks  and  reefs,  or  on  mud  banks, 
exposed  between  the  upper  and  lower  tide-marks  ;  and  while  some 


96  A    TEXT-BOOK    OF    GEOLOGY. 

prefer  still,  clear  waters,  others  can  only  exist  in  situations  exposed 
to  the  break  of  the  ocean-waves. 

Differences  of  latitude,  with  the  attendant  differences  of 
temperature,  exercise  a  powerful  influence  on  the  character  of  the 
marine  fauna.  In  New  Zealand,  which  runs  through  700  miles  of 
latitude,  the  differences  which  distinguish  the  molluscous  fauna  of 
Southland  and  Cook  Strait,  and  of  Cook  Strait  and  North  Auckland, 
are  almost  startling.  And  perhaps  no  less  potent  than  latitude  is 
the  influence  of  oceanic  currents,  as  witness  the  widely  different 
faunas  of  Labrador  and  Ireland  arising  from  the  Gulf  Stream. 

We  have  no  reason  to  believe  that  faunal  differences  in  the 
same  geographical  plane  were  relatively  less  conspicuous  in  past 
geological  ages  than  they  are  to-day  ;  hence,  when  carrying  on 
palseontological  research,  we  must  ever  remember  that  the  most 
diverse  faunas  may  be  co-existent  on  the  same  geographical  plane. 
It  is  this  fact  which  often  tends  to  render  the  correlation  of  distant 
formations  of  doubtful  value. 

Classification  of  Marine  Deposits. — Marine  deposits,  according 
to  their  distance  from  the  land,  may,  for  convenience  of  descriptive 
purposes,  be  divided  into  four  natural  zones  as  under  : — 

(1)  Littoral1    Zone,    including    pebbly,    sandy,    and    coralline 

deposits. 

(2)  Thalassic2  Zone,  including  fine   sediments,   such  as   muds 

and  silts. 

(3)  Pelagic  3  Zone,  including  calcareous  accumulations  that  form 

limestones. 

(4)  Abysmal*  Zone,  including  Red  Clays  of  volcanic  and  cosmic 

origin. 

Varying  Thickness  of  Marine  and  Estuarine  Deposits.— From  what 
has  been  said  in  the  preceding  pages  it  will  be  obvious  that  all  the 
detritus  discharged  into  the  sea,  as  well  as  the  material  derived 
from  the  erosion  of  the  land  by  marine  agencies,  is  sorted  and 
spread  out  as  a  sheet  on  the  floor  of  the  sea.  The  thickness  of 
this  sheet  is  greatest  along  the  shore-line,  and  least  towards  the 
deeper  sea.  Thus,  if  a  thick  bed  of  coastal  gravel  is  traced  seaward, 
it  will  be  found  to  taper  rapidly  until  it  dwindles  down  to  a  thin 
layer  that  eventually  emerges  into  the  sandy  zone,  which  in  its 
turn  thins  out  until  it  passes  into  silt  and  then  mud. 

The  different  layers  of  estuarine  sediments  are  also  wedge-shaped 

1  Lat.  litus  =  seashore. 

2  Gr.  thalassa  =  sea,,  i.e.  shallow  sea. 

3  Gr.  pelagos  =  sea,  i.e.  deep  sea. 

4  Gr.  abussos  =  bottomless. 


THE    GEOLOGICAL   WORK    OF    THE    SEA.  97 

in  cross-section,  being  thickest  at  the  lower  end  near  the  entrance, 
and  least  at  the  upper  end  of  the  estuary. 

It  should,  however,  be  remembered  that  the  distribution  of  both 
purely  marine  and  estuarine  sediments  is  liable  to  considerable 
variation  through  the  disturbing  influence  of  coastal  currents  in 
respect  of  the  first,  and  of  large  streams  or  rivers  in  respect  of  the 
second.  Disturbance  from  the  sea-currents  is,  perhaps,  of  com- 
moner occurrence  than  disturbance  from  large  rivers,  as  in  the  case 
of  large  estuaries  or  mediterranean  seas,  the  effects  of  the  inflowing 
streams  will  be  mainly  local,  or  confined  to  narrow  limits  on  the 
margin  of  the  greater  sheet  of  fine  sediments.  Wash-outs  that 
take  place  during  abnormal  floods  are  often  filled  in  with  coarse, 
gravelly  sands,  or,  at  any  rate,  with  material  differing  from  that 
deposited  in  normal  conditions. 

The  Sea  as  a  Source  of  Life. — It  is  almost  certain  that  the  first 
forms  of  plant  and  animal  life  were  aquatic  or  marine  ;  and  it  was 
probably  in  the  sea  that  the  first  steps  in  the  evolution  of  the  more 
highly  organised  forms  took  place.  The  sea,  ever  since  the  beginning 
of  geological  time,  has  been  the  universal  cradle  and  preserver  of 
life.  Earthquakes  and  volcanic  eruptions  might  devastate  the 
dry  land,  but  in  the  sea,  life  always  found  a  safe  asylum. 

Around  the  shores  of  islands  and  continents  in  the  comparatively 
shallow  water,  the  sea  is  very  prolific  in  molluscous  life.  The 
molluscs  manufacture  their  shells  from  carbonate  of  lime  secreted 
from  the  sea-water,  and  where  they  grow  in  colonies,  their  shells 
frequently  accumulate  until  they  form  shell-banks  of  great  extent. 
Many  shelly  limestones  that  now  form  hard  rocks  are  composed 
of  shells  that  grew  on  shell-banks  on  the  floor  of  the  sea. 

The  coral  polyp  in  the  warmer  seas  of  the  tropics  builds  up  reefs 
of  coral  that  in  time  become  converted  into  solid  limestones. 

The  Sea  as  a  Highway. — :The  sea  stretches  over  the  whole  globe 
and  therefore  affords  an  easy  means  of  migration  for  all  kinds  of 
marine  life.  The  sea-currents  also  carry  seeds  and  seed-spores 
from  place  to  place,  and  thus  enables  vegetation  to  spread  to  new 
islands  or  to  islands  that  have  been  devastated  by  volcanic 
eruptions.  The  comparative  rapidity  with  which  sea-borne  plants 
may  reclothe  an  isolated  land  is  well  illustrated  at  Krakatoa.  In 
1883  that  island  was  overwhelmed  with  volcanic  ejecta  which 
destroyed  all  the  plant  and  animal  life.  In  less  than  twenty 
years  the  island  was  reclothed  with  a  dense  jungle  from  seeds 
carried  to  its  shores  by  sea-currents.  What  we  now  see  taking 
place  in  the  dispersion  of  plants  doubtless  took  place  through  all 
the  past  geological  ages. 

Fossils. — Sediments  laid  down  on  the  floor  of  the  open  sea  contain 
the  imbedded  remains  of  marine  plants  and  shells,  the  bones  and 

7 


98  A    TEXT-BOOK    OF    GEOLOGY. 

teeth  of  fishes  and  other  creatures  that  lived  in  the  sea.  From  a 
study  of  these  fossil  remains  we  are  able  to  construct  a  picture  of 
the  depth  of  the  sea  and  climatic  conditions  prevailing  at  the  time 
the  sediments  were  laid  down. 

Sediments  laid  down  in  estuaries,  tidal  harbours,  and  deltas  are 
found  to  contain  the  bones  and  teeth  of  land  animals  whose  bodies 
were  washed  into  the  sea,  the  shells  of  land  and  freshwater  molluscs, 
the  trunks  of  trees,  as  well  as  seeds,  nuts,  twigs,  and  leaves.  With 
these  are  mingled  the  remains  of  animal  life  that  frequent  the 
brackish  waters  of  deltas  and  estuaries,  together  with  those  of 
marine  organisms  washed  up  by  sea-currents  and  tides. 

The  mingling  of  land,  fresh  water,  brackish  water,  and  marine 
forms  is  characteristic  of  deposits  that  were  laid  down  in  estuaries 
and  mediterranean  seas. 

Variations  of  Sea-Level. — Up  till  the  beginning  of  this  century 
it  was  the  general  belief  that  the  level  of  the  sea  was  invariable, 
and  any  departure  from  this  view  was  looked  upon  as  a  geological 
heresy.  All  transgressions  of  the  sea  on  the  dry  land  were  regarded 
as  an  evidence  of  actual  subsidence  of  the  land  ;  and  all  recessions 
of  the  sea  as  proofs  of  uplift.  Uplift  and  subsidence  of  the  land 
have  taken  place  in  all  geological  ages,  both  local  and  continental, 
of  small  amount  and  of  great  magnitude,  as  the  result  of  crustal 
folding  or  of  volcanic  or  earthquake  disturbance.  It  is  obvious 
that  no  movement  of  the  crust,  whether  it  affects  the  sea-floor 
or  dry  land,  can  take  place  without  a  corresponding  displacement 
of  the  sea-level.  When  a  portion  of  the  ocean-floor  sinks,  the  sea 
recedes  from  the  dry  land,  and  the  effect  is  the  same  as  an  actual 
uplift  of  the  land.  Conversely,  when  a  segment  of  the  ocean- 
floor  rises,  the  sea-level  is  correspondingly  raised,  and  we  get  a 
transgression  of  the  sea  producing  an  effect  similar  to  an  actual 
sinking  of  the  land. 

But  the  sea  covers  such  a  large  portion  of  the  surface  of  the  globe 
that  any  changes  of  its  level  produced  by  the  rising  or  sinking  of 
crustal  segments  must  be  relatively  small  compared  with  the  local 
effects.  If  the  continent  of  Australia,  due  to  crustal  collapse,  were 
to  sink  500  feet,  a  large  portion  of  its  surface  would  be  invaded  by 
the  sea,  but  the  displacement  caused  by  the  submergence  would 
raise  the  general  sea-level  datum  less  than  10  feet.  Obviously 
the  great  transgressions  of  the  sea  recorded  in  geological  history 
were  the  result  of  land  movement  rather  than  changes  of  sea-level. 

DEPOSITION  DURING  UPLIFT  AND  SUBSIDENCE. 

The  sediments  laid  down  on  the  floor  of  the  sea  and  in  great 
estuaries  are  the  materials  of  which  sedimentary  rocks  are  formed. 


THE    GEOLOGICAL    WORK    OF   THE    SEA.  99 

When,  therefore,  we  are  able  by  actual  observation  to  see  how  such 
deposits  are  laid  down  at  the  present  day,  we  are  confronted  with 
fewer  difficulties  in  our  study  of  the  rocks  formed  under  similar 
conditions  in  past  geological  ages.  In  other  words,  the  better  we 
understand  the  first  principles  governing  the  deposition  of  sedi- 
ments in  lake,  estuary,  and  sea,  the  better  will  we  be  able  to  grapple 
with  the  problems  presented  by  the  varying  texture,  distribution, 
and  fossil  contents  of  sedimentary  rocks.  The  present  conditions 
afford  the  key  to  the  past ;  hence  we  must  study  present  conditions 
in  order  to  understand  the  past. 

Effect  of  Deposition  on  Rising  Sea-Floor. — When  a  general  uplift 
of  the  land  takes  place,  the  shore-line  advances  on  the  sea,  with  the 
result  that  the  sediments  are,  as  previously  described,  carried 
further  and  further  seaward,  thereby  causing  seaward  overlap 
whereby  gravels  may  be  deposited  on  sand,  sand  on  mud,  and  mud 
on  the  abysmal  calcareous  ooze. 

If  the  uplift  is  rapid  and  persists  for  a  considerable  time,  the 
overlap  will  be  more  and  more  marked,  and  thus  it  may  happen 
that  pebble  and  sand  beds  may  be  eventually  deposited  over  the 
calcareous  ooze,  the  abysmal  zone  being  now  a  shallow  sea. 

But  to  return  to  the  first  case  where  the  first  effect  of  the  uplift 
is  just  sufficient  to  permit  the  sands  to  overlap  and  spread  over  the 
mud.  It  may  happen  that  the  uplift  is  followed  by  subsidence. 
In  this  case  the  shore-line  will  advance  on  the  land,  and  the  overlap 
will  be  landward,  thereby  permitting  mud  to  be  laid  down  on  the 
newly-formed  sands. 

The  succession  of  sediments  in  the  deeper  zone  of  deposition  will 
now  be  mud,  sand,  mud. 

If  the  land  is  oscillating  with  approximate  regularity,  we  shall 
get  many  alternating  layers  of  sand  and  mud.  And  since  the 
deposition  of  mud  is  relatively  slower  than  that  of  sand,  the  layers 
of  sand  will  be  thicker  than  those  of  mud. 

Geographical  Effect  of  Uplift. — If  the  upward  movement  continues, 
the  partially  land-surrounded  portions  of  the  sea  will  be  at  first 
converted  into  mediterranean  seas,  and  eventually  into  land-locked 
lakes.  If  this  takes  place  in  an  arid  region,  the  evaporation  of 
the  water  will  leave  a  deposit  of  salt  on  the  floor  of  the  dried-up 
basin.  When  the  infilling  of  the  lake-basin  takes  place,  as  the 
result  of  physical  and  climatic  changes,  the  deposit  of  salt  will  be 
covered  with  layers  of  sediment  that  will  protect  it  from  destruction. 
It  was  doubtless  in  this  way  that  the  valuable  deposits  of  salt  in 
England  and  Continental  Europe  were  formed. 

Simultaneous  Deposition  and  Erosion  during  Uplift. — When 
uplift  takes  place  the  sediments  first  laid  down  around  the  shore 
are  raised  into  a  position  where  they  become  subject  to  subaerial 


100  A  TEXT-BOOK  OF  GEOLOGY. 

and  marine  denudation  ;  and  being  for  the  most  part  loose  and 
incoherent,  they  are  easily  broken  up  and  removed.  But  the 
mere  uplift  of  the  land  does  not  stay  the  activity  of  the  processes 
of  denudation  that  were  in  operation  before  the  uplift  began.  On 
the  contrary,  the  rate  of  denudation  may  be  accelerated,  as  the 
obvious  effect  of  the  uplift  will  be  to  increase  the  gradient  of  the 
slopes,  whereby  the  erosive  power  of  the  streams  and  rivers  will  be 
correspondingly  increased. 

So  long  as  the  uplift  continues,  the  ordinary  products  of  denuda- 
tion plus  the  material  derived  from  the  breaking  up  and  re-sorting 
of  the  coastal  sediments,  which  are  now  subject  to  denudation  as 
the  result  of  the  uplift,  are  carried  further  seaward  and  spread  out 
as  a  sheet  that  overlaps,  but  lies  parallel  with  the  sediments  laid 
down  before  the  uplift  began. 

Thus  we  see  that  while  uplift  may  enable  the  edges  of  the  first 
and  consequently  oldest  layers  of  sediment  to  be  worn  away  and 
re-sorted,  deposition  will  still  be  in  progress  in  the  seaward  direction. 
Moreover,  there  will  be  no  physical  break  in  the  continuity  of  the 
layers  which  will  follow  one  another  in  a  conformable  sequence  or 
succession.  That  is,  the  layers  will  all  lie  in  the  same  plane,  like 
the  slates  or  wooden  shingles  on  the  roof  of  a  house. 

Deposition  during  Subsidence. — During  subsidence  the  sea 
advances  on  the  land,  and  the  overlap  of  the  sediments  is  land- 
ivard.  In  the  process  of  time  the  subsidence  of  the  land  permits 
coastal  valleys  to  be  submerged  and  estuaries  to  be  formed.  It  is 
almost  certain  that  the  piles  of  sediment  that  constitute  the  great 
geological  systems  were  formed  during  downward  movement. 

The  Cycle  of  Deposition. — The  typical  succession  of  sediments 
of  many  geological  systems  begins  as  a  basal  conglomerate  and 
closes  with  a  limestone,  the  complete  sequence  being  (a)  basal  con- 
glomerate, followed  by  (b)  marine  sandstones,  (c)  clays,  and  (d)  lime- 
stone in  ascending  order.  This  succession  is  obviously  the  result 
of  deposition  on  a  sinking  sea-floor. 

The  torrent-formed  gravels  composing  the  basal  conglomerate 
were  probably  shot  out  along  the  foot  of  mountain-land  fronting 
the  sea.  On  a  stationary  or  slowly  rising  sea-floor  the  gravels 
would  be  carried  further  and  further  until  they  eventually  reclaimed 
a  maritime  belt  of  land  from  the  sea.  This  belt  would  be  flat, 
swampy,  and  probably  deltaic. 

At  this  stage  a  general  subsidence  set  in  ;  and  as  the  downward 
movement  continued,  the  sea  encroached  further  and  further  on  the 
land.  The  terrestrial  gravels  thereby  in  time  became  covered  with 
marine  sands,  the  sands  with  muds,  and  the  muds  with  a  layer  of 
calcareous  organisms  that,  when  consolidated,  formed  the  closing 
limestone  member  of  the  series. 


THE    GEOLOGICAL   WORK    pP   't' 

Seams  of  coal  are  frequently  found  associated  with  the  basal 
conglomerate,  from  which  it  may  be  deduced  that,  duiing  the 
deltaic  period,  vegetation  established  itself  on  the  mud-flats,  and 
grew  so  rank  and  rapidly  that  sufficient  vegetable  matter  accumu- 
lated to. form,  when  consolidated,  valuable  seams  of  coal. 

Each  seam  of  coal  marks  an  old  land  surface,  and  when  we  find 
that  the  different  seams  are  separated  by  beds  of  sandstone  or 
conglomerate,  we  are  able  to  conclude  that  the  land  was  slowly 
oscillating  during  the  deltaic  period  ;  that  is,  before  the  general 
subsidence  began  that  led  to  the  terrestrial  gravels  and  their  seams 
of  coal  being  buried  beneath  the  succession  of  marine  beds. 

The  effect  of  progressive  subsidence  is  to  reduce  the  height  of 
the  dry  land  ;  while  the  continuous  denudation  tends  to  reduce  its 
surface  to  contours  of  low  relief.  Should  the  subsidence  continue, 
the  land  will  eventually  become  submerged.  While  the  subsidence 
continued,  the  sediments  spread  out  on  the  sea-floor  overlapped 
more  and  more.  Where  there  was  total  submergence  of  the  land 
we  may  even  find  a  calcareous  zone  or  limestone  riding  hard  on 
the  basement  rock  as  the  result  of  profound  overlap. 

PELAGIC  ORGANIC  DEPOSITS. 

We  have  seen  that,  as  we  leave  the  land,  the  materials  spread 
out  on  the  sea-floor  become  progressively  finer  and  finer  in  grain, 
until  a  limit  is  reached  beyond  which  the  sea  is  quite  clear  and  free 
from  sediment  derived  from  the  land. 

Pteropod  Ooze. — The  deep-sea  dredging  carried  out  by  the 
Challenger  Expedition  showed  that,  from  the  outer  edge  of 
the  mud  zone  down  to  a  depth  of  about  1500  fathoms,  the  sea-floor 
is  covered  with  a  calcareous  ooze  consisting  mainly  of  the  shells  of 
Pteropods  and  Foraminifera,  the  former  very  small  molluscs,  the 
latter  minute  protozoans  that  live  in  beautiful  chambered  shells 
full  of  small  pores  ;  hence  the  origin  of  the  name.  In  this  zone  the 
Pteropods  predominate. 

Globigerina  Ooze. — From  about  1500  fathoms  down  to  3000 
fathoms,  the  sea-floor  is  covered  with  a  calcareous  ooze  consisting 
almost  entirely  of  the  shells  of  Foraminifera,  the  commonest  of 
which  is  Globigerina,  from  which  this  ooze  is  named. 

Red  Clay. — At  greater  depths  than  3000  fathoms  the  calcareous 
oozes  are  absent,  their  place  being  taken  by  an  excessively  fine 
deposit  called  red  clay.  In  certain  areas  in  these  abyssal  depths 
there  are  deposits  of  Diatoms  and  Radiolaria,  both  of  which  are 
siliceous  organisms,  the  former  tiny  plants,  the  latter  minute 
protozoans. 

The  red  clay  would  appear  to  be  the  very  fine  dust  that  has  fallen 


A    TEXT-BOOK    OF    GEOLOGY. 


on  the  surface  of  the  sea.  It  consists  partly  of  fine  volcanic  dust, 
and  partly  of  wind-borne  desert  dust,  mixed  with  what  is  believed 
to  be  the  dust  of  meteors  that  have  been  broken  up  on  entering 
our  atmosphere. 

This  dust,  from  whatever  source  it  is  derived,  must  be  very  small 
in  quantity,  and  when  spread  over  the  many  millions  of  square 
miles  of  the  sea-floor,  must  take  thousands  of  years  to  form  a  layer 
even  an  inch  thick. 

The  surface  of  the  red  day  is  thickly  scattered  with  the  teeth  of 
sharks,  hundreds  of  which  have  been  skimmed  up  by  the  dredge 
and  brought  to  the  surface.  These  teeth  are  so  numerous  that  it 
must  have  taken  probably  thousands  of  years  for  them  to  accumu- 


Mud 
Sea   - 


Pteropod- 

zone 
level 


1500  fms. 

G/obigerina 
zone. 


8000  fms. 

Red  Clay 


FIG.  43. — Showing  zones  of  calcareous  ooze  and  red  clay 
in  the  abyssal  depths  of  the  sea. 

late,  and  yet  the  red  day  has  not  been  able  to  cover  them,  so  slow 
is  the  rate  at  which  it  is  being  deposited. 

Cause  of  Zonal  Arrangement  of  Calcareous  Organisms. — Both  the 
Pteropods  and  Foraminifera  are  organisms  that  swim  about  freely 
at  the  surface  of  the  sea.  They  exist  everywhere  in  countless 
millions,  and  there  is  a  continual  rain  of  their  dead  shells  which 
slowly  fall  through  the  water. 

The  empty  calcareous  shells  of  both  the  Pteropods  and  Forami- 
nifera are  soluble  in  sea-water  ;  and  where  the  depth  of  the  ocean 
exceeds  3000  fathoms  both  are  dissolved  before  they  reach  the  sea- 
floor.  Hence  the  absence  of  these  organisms  in  the  red-day  zone. 

Pteropods  are  absent  from  the  Globigerina  zone  because  their 
shells  are  more  soluble  than  those  of  the  Foraminifera.  Therefore, 
although  the  dead  shells  of  both  Pteropods  and  Foraminifera  begin 
their  long  downward  journey  together,  when  a  depth  of  1500 
fathoms  is  reached,  the  Foraminifera  are  left  to  continue  the  journey 


THE    GEOLOGICAL   WORK    OF   THE    SEA.  103 

alone,  the  shells  of  the  Pteropods  having  passed  into  solution  in 
the  sea-water. 

Pteropods  and  Foraminifera  are  abundant  near  the  shore  in  all 
classes  of  sediment,  but  they  are  not  easily  seen  except  when 
samples  of  the  mud  and  sand  are  carefully  washed. 

The  calcareous  ooze  of  the  deep  sea  is  the  material  of  which  the 
chalks  and  limestones  of  some  future  time  will  be  formed. 

Radiolarian  Ooze. — This  ooze,  which  also  contains  numerous 
Diatoms,  is  not  found  so  widely  spread  as  the  red  clay,  which  covers 
the  greater  portion  of  the  sea-floor  below  the  3000-fathom  line. 
At  the  greatest  depths  known  in  the  Pacific  Ocean  the  red  clay 
contains  deposits  of  Radiolarian  ooze.  The  Radiolaria  as  well  as 
the  Diatoms  live  at  the  surface  of  the  water,  but  as  they  are  com- 
posed of  silica,  which  is  but  feebly  soluble  in  sea-water,  they  are  able 
to  reach  the  bottom  even  at  the  greatest  depths. 

Coral  Reefs. — In  many  tropical  countries  where  the  shores  are 
bathed  by  warm  sea-currents,  colonies  of  coral  of  many  different 
genera  and  species  by  their  continuous  growth  form  rock  masses 
that  are  frequently  of  vast  extent. 

The  conditions  required  for  the  existence  of  vigorous  coral 
growth  are  (a)  a  mean  temperature  not  less  than  68°  Fahr.  ;  (b) 
absence  of  fresh  or  muddy  water  ;  and  (c)  warm  equatorial  sea- 
currents  to  provide  a  continuous  and  ample  supply  of  food  for  the 
coral-builders. 

The  reef -building  corals  cannot  live  at  depths  below  15  or  20 
fathoms,  and  appear  to  thrive  best  at  a  depth  of  about  7  fathoms. 
They  are  destroyed  by  exposure  to  the  sun  or  air,  even  for  a  very 
brief  time,  and  consequently  cannot  grow  above  the  lowest  tide- 
mark. 

Coral  reefs  of  great  extent  are  found  on  the  east  coast  of  Australia, 
in  the  South  Pacific  Islands,  Central  America,  and  east  coast  of 
Africa.  In  the  Indian  and  South  Pacific  Oceans,  hundreds  of  the 
islands  are  entirely  composed  of  coral  rock. 

Many  of  the  great  limestone  deposits  found  in  various  parts  of 
the  globe,  associated  with  the  older  geological  sedimentary  forma- 
tions, were  doubtless  formed  by  coral-growth,  although  the  proof 
of  this  is  not  always  obtainable,  as  the  original  organic  structure 
has  been  entirely  obliterated  by  the  internal  crystallisation. 

Coral  reefs  that  have  been  recently  elevated,  as  well  as  reefs 
submerged  by  the  subsidence  of  the  sea-floor,  as  shown  by  the 
borings  carried  out  at  Funafuti,  gradually  loose  their  organic 
structure.  They  acquire  a  crystalline  structure  like  an  ancient 
limestone,  owing  to  the  infiltration  of  water  through  their  mass. 
The  water  contains  dissolved  carbonate  of  lime,  which  it  deposits 
throughout  the  pores  and  crevices  of  the  mass  in  a  crystalline 


104  A   TEXT-BOOK    OF    GEOLOGY. 

form.     In  this  way  the  reef  is  consolidated  and  converted  into 
a  compact,  more  or  less  homogeneous,  crystalline  limestone. 

During  the  terrific  storms  that  at  certain  seasons  of  the  year 
sweep  over  the  tropical  seas,  the  waves  break  off  large  blocks  of 
coral  which  pound  one  another  into  coralline  sand  and  mud,  or 
are  hurled  with  destructive  effect  against  the  living  coral  reef,  from 
which  fresh  blocks  are  thereby  torn.  After  a  storm  the  shores  of 
a  coral  island  are  frequently  piled  up  with  sand  and  fragments 
of  coral — the  wreckage  of  the  submerged  reefs.  At  the  height  of 
the  storm,  and  even  for  some  hours  after  it  has  abated,  the  sea  is 
discoloured  with  coral-mud  for  a  distance  of  several  miles  beyond 
the  outer  reef.  The  detritus  that  is  not  heaped  up  on  the  beaches  is 
spread  as  a  sheet  on  the  sea-floor,  where  it  is  in  time  consolidated  by 
infiltration,  forming  a  foundation  for  the  renewed  growth  of  the 
coral-builders. 


FIG.  44. — Showing  growth  of  coral  reef  on  stationary 
or  slowly  rising  sea -floor. 

Formation  of  Coral  Reefs. 

Coral  reefs  may  grow  (a)  on  a  stationary  or  slowly  rising  sea- 
floor,  or  (6)  on  a  sinking  sea-floor.  When  they  grow  on  a  stationary 
or  slowly  rising  floor  they  extend  outward,  forming  tabular  masses, 
while  on  a  sinking  shore-line  they  grow  upward. 

Growth  of  Coral  Reefs  on  Stationary  or  Slowly  Rising  Floor.— 
The  living  coral  grows  vigorously  only  on  the  outside  or  seaward 
side  of  the  reef.  That  is  to  say,  it  grows  towards  the  source  of  the 
food-supply.  In  this  outward  growth,  being  unable  to  live  below 
the  20-fathom  line,  it  spreads  outward,  forming  huge  mushroom- 
shaped  masses.  In  time  the  overhanging  cornices  break  off  in 
large  slices,  partly  owing  to  the  stress  of  their  own  weight,  and 
partly  owing  to  the  force  of  the  waves  during  storms.  The  broken 
masses  fall  to  the  sea-floor.  In  this  way,  by  the  continuous  growth 
and  breaking  up  of  the  reef,  the  blocks,  mingled  with  sand  and  mud 
accumulate  until  they  rise  to  the  20-fathom  line,  where  they  form  a 
new  foundation  for  the  ever  active  myriads  of  coral  polyp. 


THE    GEOLOGICAL   WORK    OF   THE    SEA.  105 

On  a  stationary  sea-floor  the  coral  reef  thereby  gradually  spreads 
seaward,  forming  a  tabular  sheet  of  increasing  thickness. 

When  the  sea-floor  is  slowly  rising,  the  outward  growth  is 
relatively  rapid  ;  hence  the  calcareous  sheet  is  more  uniform  in 
thickness. 

In  Fiji  and  other  Pacific  islands,  consolidated  tabular  sheets 
of  uplifted  coral  reef  can  be  traced  inland  for  many  miles,  their 
crystalline  structure  resembling  that  of  many  of  the  ancient  sheets 
of  limestone,  intercalated  with  the  Wenlock,  Carboniferous,  and 
Jurassic  formations. 

Growth  of  Coral  Reef  on  Sinking  Sea-Floor. — If  the  land  is  sinking 
at  a  less  rate  than  the  growth  of  the  reef-building,  the  corals  will 
be  able  to  maintain  their  place  above  the  20-fathom  line,  and  hence 
will  grow  upward  and  outward.  If  the  water  outside  the  reef  is 
deep,  the  outward  growth  will  be  relatively  slower  than  the  up- 


FIG.  45. — Showing  formation  of  Barrier  Reef. 
A  and  B  are  old  shore -lines. 

ward.  If  the  rate  of  subsidence  competes  with  the  growth  of  the 
coral,  the  builders  will  just  be  able  to  maintain  their  position,  and 
the  floor  of  the  sea  will  be  covered  with  a  sheet  of  coral  reef,  the 
extent  of  which  will  depend  on  the  duration  and  amount  of  the 
subsidence. 

Formation  of  Barrier  Reefs  and  Atolls. — These  are  believed  to 
have  been  formed  by  the  upward  growth  of  coral  on  a  slowly  sinking 
shore-line.  This  view  was  first  advanced  by  Darwin,  and  is  now 
generally  accepted  by  geologists ;  but  the  theory  of  outward  growth 
advocated  by  Murray  has  many  supporters. 

When  the  coral  reef  is  marginal  to  a  sinking  continent  or  large 
land-area,  a  barrier  reef  is  formed.  For,  as  the  sea-floor  continues 
to  sink,  the  coral  reef  grows  upward,  the  shore-line  at  the  same  time 
receding  further  and  further  from  the  reef.  The  water  between 
the  reef  and  the  shore  thus  becomes  deeper  and  deeper  as  the  sub- 
sidence progresses  until  a  barrier  reef  is  formed,  as  shown  in  fig.  45. 

When  the  coral  reef  grows  around  the  shores  of  an  island,  the 


106 


A    TEXT-BOOK    OF    GEOLOGY. 


united  result  of  the  upward  growth  of  coral  and  progressive  sinking 
of  the  land  is  to  form  a  fringing  reef  marginal  to  the  old  shore. 
When  the  subsidence  continues  until  the  island  is  completely  sub- 
merged, all  that  remains  to  mark  the  former  existence  of  the  island 
is  a  ragged  ring  of  coral  reef,  forming  what  is  called  an  atoll. 

The  wreckage  of  the  reef,  piled  up  by  the  waves  and  wind,  in  the 
course  of  time  raises  the  reef  above  sea-level  ;  and,  as  the  land  con- 
tinues to  sink,  the  inside  lagoon  becomes  deeper  and  deeper. 

The  theory  of  outward  growth  and  dissolution,  as  advocated  by 
Murray,  assumes  that  atolls  were  built  up  on  marine  banks  or 
platforms,  on  which  the  coral  polyps  established  themselves  and 
formed  a  sheet  that  in  time  grew  to  the  surface.  At  this  stage  the 
growth  would  be  on  the  outside  fringe  of  the  reef,  exposed  to  the 


Lagoon  Coral-Peef 

i 


FIG.  46. — Showing  progressive  stages  and  formation  of  fringing 
reef  or  atoll.     B  and  A,  former  level  of  sea. 

break  of  the  ocean  waves,  from  which  the  polyps  derived  their 
food-supply ;  and  here,  by  the  action  of  the  waves,  the  rim  of  the 
reef  would  in  time  rise  above  the  surface.  The  corals  on  the  inside 
portion  of  the  sheet,  being  deprived  of  their  food-supply,  would  soon 
decay  and  dissolve,  thereby  forming  a  lagoon.  As  the  growth 
would  be  outward,  the  reef  would  gradually  extend  seaward,  the 
living  polyps  building  on  the  apron  of  broken  coral  on  the  outer 
slope  of  the  reef.  At  the  same  time,  the  lagoon  would  become 
larger  and  larger  by  the  continual  dissolution  of  the  dead  coral 
inside  the  living  barrier. 

Agassiz  pointed  out  the  frequent  occurrences  of  uplifted  coralline 
limestones,  which  might  be  worn  down  and  dissolved,  while  fringing 
reefs  grew  around  them,  thus  producing  barrier  reefs  and  atolls 
in  association  with  elevation  instead  of  subsidence. 

According  to  the  view  formulated  by  Darwin,  the  Pacific  Ocean, 


THE    GEOLOGICAL   WORK    OF   THE    SEA.  107 

with,  its  hundreds  of  atolls,  must  be  regarded  as  an  area  of  sub- 
sidence, the  existing  islands  marking  the  site  of  the  higher  peaks 
and  ridges  of  a  submerged  continent.  It  we  assume  the  truth  of 
this,  we  are  at  once  faced  with  the  question,  Why  did  the  coral 
polyps  refrain  from  building  till  the  peaks  and  summits  of  the 
mountains  were  just  on  the  point  of  complete  submergence  ?  If 
the  subsidence  view  be  true,  then  the  original  limits  of  this  ancient 
continent  ought  to  be  outlined  by  the  barrier  reefs  that  girdled  the 
receding  and  deeply  embayed  sinking  land,  like  the  great  barrier 
reefs  of  Australia  and  New  Caledonia.  But  of  such  mighty  barriers 
we  can  find  no  trace. 

Subsidence  and  upward  growth  seem  to  afford  a  satisfactory 
explanation  of  the  production  of  barrier  reefs  ;  and  if  accepted  as 
satisfactory  for  atolls,  we  must  assume  that  the  rate  of  subsidence 
of  the  ancient  continent  postulated  above  was  too  rapid  to  permit 
the  upward  growth  of  coral  reefs  on  the  site  of  the  sinking  strands 
till  all  but  the1  higher  points  of  the  land  were  submerged. 

Distribution  of  Coral  Reefs  and  Islands.— The  great  Barrier 
Reef  on  the  north-east  coast  of  Australia  is  1250  miles  long,  and 
varies  from  10  to  90  miles  wide.  It  is  usually  about  20  miles  from 
the  shore,  but  in  places  the  distance  increases  to  50  or  even  90 
miles.  The  depth  of  the  channel  varies  from  50  or  60  feet  to  as 
much  as  60  fathoms  in  the  southern  part,  where  the  reef  lies 
furthest  from  the  mainland. 

The  barrier  reef  of  New  Caledonia  is  400  miles  long,  and  the 
average  distance  from  the  land  is  about  10  miles.  The  length  of 
New  Caledonia  is  250  miles  ;  so  that  we  may  conclude  that  sub- 
sidence has  shortened  the  island  by  a  distance  of  150  miles  since  the 
coral  reef  began  to  grow  on  its  shore. 

Over  200  atolls,  or  true  coral  islands,  the  majority  of  them  only 
a  few  feet  above  sea-level,  are  scattered  throughout  the  Indian  and 
South  Pacific  Oceans.  The  principal  atolls  in  the  Indian  Ocean 
are  the  Laccadive  and  Maldive  Islands,  the  Chagos  Bank,  and  the 
Saya  de  Malha,  which  form  a  stretch  of  submerged  land  between 
India  and  the  north  of  Madagascar.  It  is  not  inconceivable  that 
such  a  group  of  islands,  before  they  were  submerged,  may  have 
formed  a  continuous  land  connection  from  Mozambique  to  the 
Malabar  coast  of  India.  Such  a  connection  would  help  us  to  under- 
stand the  presence  of  an  African  element  in  the  Indian  land  fauna. 

In  the  South  Pacific  the  principal  atolls  are  the  Low  Archipelago, 
Gilbert  Group,  Marshall,  and  Caroline  Islands. 

The  ring  of  coral  reef  forming  an  atoll  is  generally  breached  in 
one  or  many  places,  thereby  giving  access  to  the  lagoon  inside. 

Coral  reefs  of  great  extent  exist  on  both  shores  of  the  Red  Sea. 
They  extend  down  the  Zanzibar  coast  and  the  coast  of  Mozambique, 


108  A    TEXT-BOOK    OF    GEOLOGY. 

and  surround  the  Mauritius.  If  subsidence  of  the  east  coast  of 
Africa  were  to  take  place,  the  former  would  form  barrier  reefs 
running  parallel  with  the  present  coast-line. 

Lacustrine  Limestones. — The  limestones  formed  on  the  sea-floor 
have  their  correlatives  in  lakes.  Some  lacustrine  limestones  are 
composed  of  freshwater  shells,  or  the  calcareous  secretions  of  fresh- 
water algae  that  flourished  in  the  clear  water.  Others  were  formed 
by  precipitation  from  solution.  Many  of  these  calcareous  deposits 
have  been  consolidated  by  the  dissolution  of  bicarbonate  and  the 
re-deposition  of  carbonate  of  lime,  but  some  of  those  of  late  Tertiary 
date  are  still  loose  and  pulverulent. 


SUMMARY. 
The  Sea  as  a  Destructive  Agent. 

(1)  The  erosive  work  of  the  sea  is  chemical  and  mechanical. 

(2)  The  free  carbonic  acid  contained  in  sea-water  converts  lime- 
stones into  the  soluble  bicarbonate  of  lime  ;   dissolves  the  binding 
medium  in  all  kinds  of  calcareous  rocks  ;   and  attacks  the  felspar 
of  igneous  rocks,  such  as  granite  and  basalt.     Limestones  are  thus 
slowly  worn  away,  while  calcareous  and  igneous  rocks  are  first 
disintegrated  and  then  destroyed. 

The  free  oxygen  in  the  sea-water  continues  the  disintegration 
effected  by  the  carbonic  acid  by  oxidising  the  iron  in  the  iron- 
bearing  minerals. 

(3)  The  greatest  erosive  effect  of  the  sea  is  the  work  of  the  tides, 
sea-currents,  and  waves  set  in  motion  during  storms.     In  this  way 
the  edge  of  the  land  is  gradually  eaten  away,  the  softer  rocks  being 
shorn  back,  while  the  harder  are  undermined   until   they  finally 
become  shattered  and  broken  up.     The  blocks  of  hard  rock  accumu- 
late at  the  foot  of  the  cliffs,  where  they  at  first  form  a  protecting 
apron,  but  are  afterwards  broken  up  and  rounded,  in  time  forming- 
shingle,  and  finally  sand  and  silt.     During  great  storms  blocks  of 
hard  rock  are  flung  with  destructive  effect  against  the  cliffs. 

(4)  Soft  rocks  are  worn-  away  more  rapidly  than  hard  ;   thus  in 
time  the  former  are  worn  back  until  they  form  bays  and  gulfs,  while 
the  hard  rocks  remain  as  steep  cliffs  and  projecting  headlands. 

(5)  In  high  latitudes  masses  of  floating  ice  abrade  the  rocks,  and 
may  even  wear  away  the  edge  of  the  land  into  benches. 

(6)  The  recession  of  a  coast-line  shortens  the  course  of  streams  and 
rivers,  which  are  thus  enabled  to  regrade  and  cut  down  their  beds. 
In  this  way  rivers,  in  the  process  of  cutting  down  their  beds,  may 
excavate  terraces  in  the  lower  portion  of  their  course,  the  effect 
of  recession  being  the  same  as  an  elevation  of  the  land. 


THE    GEOLOGICAL    WORK    OF    THE    SEA.  109 

(7)  The  total  effect  of  all  the  processes  of  marine  denudation  is 
the  cutting  away  of  the  land  to  an  even  platform  called  a  plain  of 
marine  denudation.     Nearly  all  large  islands  and  continents  rise 
from    a    marine   shelf   or   platform   of   this   kind,   as   shown    by 
soundings  around  their  shores. 

The  Sea  as  a  Constructive  Agent. 

(8)  The  sea  is  the  final  destination  of  nearly  all  the  products  of 
denudation  of  the  dry  land.     Streams  and  rivers  continually  dis- 
charge an  enormous  load  of  gravel,  sand,  and  silt  into  the  sea,  where 
it  is  sorted  and  spread  out,  the  coarser  material  near  the  shore,  the 
sands  in  deeper  water,  and  the  silts  and  muds  still  further  seaward. 
Many  harbours  and  bays  in  time  become  filled  with  detritus,  and  in 
favourable  situations  sand-banks  and  shoals  of  sand  may  be  formed 
far  out  from  the  land.     Converging  sea-currents  may  form  sand- 
spits  of  great  length. 

(9)  In  estuarine  deposits  the  coarsest  are  laid  down  near  the 
entrance,  and  the  finest  at  the  utmost  limits  reached  by  the  flowing 
tide.     Where  streams  discharge  coarse  material  into  the  estuary, 
these  are  mingled  with  fine  harbour  muds. 

(10)  The  sea  is  the  cradle  and  preserver  of  life.     By  its  great 
extent  it  affords  unrivalled  means  for  the  migration  and  dispersion 
of  land  plants,  and  all  kinds  of  marine  life. 

(11)  The  remains  of  plants  and  animals  are  embedded  in  deposits 
of  all  kinds — marine,  estuarine,  fluviatile,  lacustrine,  and  volcanic — 
and  there  preserved  from  destruction.    They  form  valuable  records 
of  the  contemporary  life,  climate,  and  physical  geography  of  the 
period  of  deposition. 

(12)  When  deposition  takes  place  during  uplift  of  the  land  the 
marine  sediments  are  carried  further  and  further  seaward,  whereby 
seaward  overlap  takes  place. 

(13)  The  geographical  effect  of  continued  uplift  is  to  convert 
partially  land-surrounded  portions  of  the  sea  into  seas   of  the 
mediterranean  type,   and   eventually  into   land-locked   basins   or 
lakes,  in  which,  by  evaporation,  deposits  pf  salt  may  be  laid  down. 

(14)  During  uplift,  deposition  of  sediments  continues  without 
cessation,  the  sediments  being  merely  carried  further  and  further 
outward  or  seaward.     But  while  this  is  taking  place,  the  upward 
movement  of  the  land  has  raised  the  old  shore-line  above  sea-level, 
with  the  result  that  the  coastal  edges  of  the  sediments  first  laid 
down  are  subject  to  the  wear  and  tear  of  subaerial  and  marine 
agencies  of  denudation.     The  shoreward  portions  of  these   older 
layers  are  thereby  broken  up,  re-sorted,  and  spread  out  on  the  sea- 
floor  along  with  the  ordinary  products  of  denudation  of  the  land. 


110  A  TEXT-BOOK  OF  GEOLOGY. 

Thus  we  see  how  it  is  that  deposition  and  erosion  can*  proceed  at 
the  same  time  during  uplift  of  the  sea-floor. 

(15)  During  subsidence  the  sea  encroaches  on  the  land,  and  the 
sediments  overlap  one  another  in  a  landward  direction.     It  is  prob- 
able that  all  the  piles  of  sediments  forming  the  great  geological 
systems  were  laid  down  during  downward  movement  of  the  land. 

(16)  In  a  typical  cycle  of  deposition  we  find  that  gravels,  sands, 
and  muds  shot  down  on  a  stationary  or  slowly  rising  sea-floor  in 
time  reclaim  a  maritime  belt  from  the  sea,  producing  conditions 
that  are  frequently  deltaic.     When  a  progressive  subsidence  takes 
place,  these  terrestrial  deposits  are  covered  over  with  marine  sands, 
followed  by  marine  muds.     These  may  be  followed  by  a  layer  of 
calcareous  organisms  that,  when  consolidated,  will  form  a  bed  of 
limestone  which  will  close  the  cycle  of  deposition.    In  other  words, 
the  cycle  begins  with  terrestrial  sediments,  and  ends  with  a  deep- 
sea  deposit,  provided  the  subsidence  is  continued. 

During  the  deltaic  period,  if  the  climatic  conditions  are  favour- 
able, rank  vegetation  may  establish  itself  on  the  level  swampy 
coastal  lands,  and  if  it  remains  long  enough,  sufficient  decaying 
vegetable  matter  may  accumulate  to  form  valuable  seams  of  coal. 
When  the  general  downward  movement  begins,  this  vegetable 
matter  will  be  covered  over  with  sands  and  other  sediments,  and 
thereby  protected  from  destruction. 

The  presence  of  two  or  more  seams  of  coal  would  indicate  that 
the  land  was  slowly  oscillating  during  the  deltaic  period — that  is, 
before  the  general  downward  movement  was  fairly  started. 

(17)  The  abysmal  deposits  of  the  sea  consist  of  various  calcareous 
oozes  that  below  the  3000-fathom  line  give  place  to  the  red  clay,  com- 
posed of  volcanic,  desert,  and  meteoric  dust  that  settled  on  the 
surface  of  the  sea. 

(18)  The  continuous  growth  of  coral  in  warm  tropical  seas  forms 
large  masses  of  calcareous  rock,  which,  by  the  infiltration  of  cal- 
careous waters,  assume  a  crystalline  structure  resembling  the  older 
Palaeozoic  limestones. 

(19)  On  a  stationary  or  slowly  rising  sea-floor  the  coral  reef  grows 
outward,  forming  tabular  masses.     On  a  sinking  sea-floor  the  growth 
is  mainly  upward. 

(20)  Coral  reefs  growing  on  the  shores  of  a  sinking  continent  or 
large  land  area  form  barrier  reefs  ;   while  those  that  grow  around 
a  sinking  island  ioimfringing  reefs.    When  the  island  inside  the  reef 
finally  disappears  below  the  surface  of  the  sea,  an  atoll  or  true  coral 
island  is  formed. 

(21)  Marine  limestone  have  their  correlatives  formed  in  fresh- 
water lakes. 


CHAPTER   VII. 
ROCK-BUILDING. 

THE  CONSTRUCTION  OF  SEDIMENTARY  ROCKS. 

BY  far  the  greatest  visible  portion  of  the  Earth's  crust  is  composed 
of  sedimentary  or  aqueous  rocks.  We  will  therefore  now  consider 
the  original  constitution  of  these  rocks  as  resulting  from  the  condi- 
tions under  which  they  were  formed. 

Among  the  various  structures  to  be  considered  are  stratification, 
false-bedding,  and  lamination. 

Forms  of  Bedding. 

All  fragmentary  material  derived  from  the  denudation  of  the 
land  is  eventually  laid  down  on  the  bed  of  the  sea  or  on  the  floor 
of  some  lake  or  river,  where  it  is  sorted  by  the  action  of  the  water 
and  spread  out  in  layers  or  beds  which  are  also  termed  strata^ 

The  strata  or  beds,  according  to  the  conditions  in  which  they 
were  formed,  may  be  marine,  lacustrine,  or  fluviatile.  In  other 
words,  the  layers  of  gravels,  sand,  and  mud  laid  down  on  the  floor 
of  the  sea  form  what  are  termed  marine  beds ;  those  deposited  in  a 
lake-basin,  lacustrine  beds  ;  and  the  deposits  laid  down  in  a  river- 
bed, fluviatile  beds. 

Stratification. — We  found  in  the  last  chapter  that  the  detritus 
discharged  into  the  sea  is  sorted  and  spread  out  into  three  principal 
zones  running  nearly  parallel  with  the  shore,  the  coarsest  material 
being  deposited  nearest  the  shore,  and  the  finest  furthest  seaward. 

The  material  in  each  zone  is  approximately  uniform  in  size,  the 
sorting  or  grading  into  sizes  resulting  from  the  operation  of  the 
well-known  hydraulic  principle  that  equal  particles  falling  in  water 
offer  an  equal  resistance.  This  principle  can  easily  be  illustrated 
by  throwing  a  mixture  of  coarse  sand,  fine  sand,  and  silt  into  a 
tall  glass  jar  filled  with  water.  The  coarse  sand,  fine  sand,  and 
silt  will  after  a  little  time  be  found  to  have  settled  in  three  dis- 
tinct layers,  the  coarsest  being  at  the  bottom  because  it  fell  the 

1  Strata,  the  plural  of  stratum  =  a  layer  or  bed. 
Ill 


112 


A   TEXT-BOOK    OF    GEOLOGY. 


quickest,  with  the  finest  at  the  top  because  it  settled  the  slowest 
(fig.  47). 

The  deeper  the  water  into  which  the  mixture  is  thrown,  the  more 
complete  will  be  the  separation,  and  the  cleaner  the  products 
in  each  layer. 


FIG.  47. — Showing  sorting  action  of  still  water, 
(a)  Coarse  sand.  (6)  Fine  sand.  (c)  Silt. 

The  separation  or  sorting  just  described  takes  place  in  still  water, 
a  condition  which  is  seldom  or  never  met  with  in  nature. 

Let  us  now  assume  that  a  similar  mixture  is  thrown  into  the 
head  of  a  long  box-launder  or  chute  through  which  there  is  flowing 
a  slow  stream  of  clean  water.  The  flowing  water  possesses  the  same 


Direction    of  Flow 


FIG.  48. — Showing  sorting  and  spreading  action  of  moving  water. 
(a)  Coarse  sand.  (6)  Fine  sand.  (c)  Silt. 

sorting  action  as  still  water,  and  we  shall  again  obtain  three  sorted 
products  ;  but  instead  of  these  being  arranged  vertically  one  above 
another  as  in  our  first  experiment,  they  will  be  spread  out  in  the 
same  plane,  the  falling  particles  being  deflected  in  their  descent  in 
the  direction  of  the  flowing  water. 

The  particles  have  two  motions — a  vertical  and  a  horizontal; 
hence,  the  more  slowly  they  fall  through  the  water,  the  greater  will 
be  the  travel  or  deflection  before  they  settle  on  the  bottom.  And 


ROCK-BUILDING. 


113 


since  the  heaviest  particles  fall  first  and  the  finest  the  last,  we  shall 
get  the  coarse  sand  at  the  head  of  the  launder,  the  fine  sand  in  the 
middle,  and  the  silt  at  the  tail,  as  shown  in  fig.  48. 

Thus,  for  the  sorting  and  spreading  out  of  detrital  material  we 
require  the  water  to  be  in  motion,  and  of  course  this  condition  is 
always  provided  in  the  case  of  the  sea  by  the  daily  tides,  sea- 
currents,  and  the  wave  motion  generated  by  winds. 


FIG.  49. — Showing  conglomerates  without  bedding. 

The  gravel  and  shingle  lying  near  the  shore-line  form  conglomer- 
ates ;  the  sands  spread  further  seaward,  sandstones  ;  and  the  muds, 
still  further  out,  form  mudstones  and  shales. 

Conglomerates  frequently  exhibit  no  stratification  or  lines  of 
bedding,  and  when  they  do,  it  is  generally  caused  by  the  occurrence 
in  them  of  layers  of  finer  or  coarser  material.  Thus,  bands  of 


FIG.  50. — Showing  conglomerates  with  bands  of  sandstone, 
a-a,  imparting  a  bedded  appearance. 

sandstone  or  shale  impart  a  bedded  appearance  to  the  layers  of 
conglomerate  lying  between  them. 

Fig.  49  shows  a  section  of  a  bluff  of  conglomerate  which  exhibits 
no  appearance  of  bedding  ;  but  the  same  conglomerate  when  it 
contains  sandstone  bands  a-a,  is  seen  to  present  a  bedded  or  strati- 
fied appearance. 

It  should  be  noted  that  the  bands  of  sandstone  that  occur  in  a 
conglomerate  are  generally  extremely  variable  in  thickness  "and 
linear  extent.  This  is  what  might  be  looked  for  where  fine  material 
is  laid  down  among  coarse,  the  fine  being  in  most  cases  deposited 

8 


114 


A   TEXT-BOOK    OF    GEOLOGY. 


during  a  temporary  inset  of  the  coastal  sea-currents  by  a  continu- 
ance of  heavy  weather  or  seasonal  causes. 

The  floor  of  the  sea  around  the  coast-line  is  not  level,  but,  on  the 
contrary,  full  of  hollows,  ridges,  and  minor  inequalities,  resulting 
from  the  unequal  resistance  of  the  rocks  to  the  wear  and  tear  of 
the  sea. 

The  first  detritus  laid  down  on  the  floor  of  the  sea  will  be  spread 
out  as  a  sheet  which  will  be  of  variable  thickness.  The  tendency 


Sea -I  eve  I 


FIG.  51. — Showing  parallelism  of  beds  after  the  hollows  in 
the  sea-floor  are  filled  up. 

of  the  material  will  be  to  fill  up  the  hollows  and  depressions,  in 
consequence  of  which  the  sheet,  while  gradually  tapering  from  the 
shore-line  seaward,  will  be  thickest  in  the  hollows  and  thinnest  on 
the  ridges.  In  other  words,  the  first  sheet  of  material  will  conform 
to  the  contour  of  the  floor  on  which  it  rests. 

The  next  sheet  of  detritus  will  be  spread  over  the  first,  but  the 


FIG.  52. — Showing  seaward  overlap  as  a  result  of  deposition 
on  a  stationary  or  rising  sea-floor. 

hollows  will  receive  a  thicker  coating  than  the  other  portions  of 
the  floor.  As  layer  after  layer  is  laid  down,  the  hollows  will  be 
completely  filled  up  ;  and  after  that  happens,  the  succeeding 
layers  will  be  parallel  throughout. 

On  a  gently  shelving  sea-floor,  when  the  level  of  the  land  relatively 
to  the  sea  is  stationary,  or  when  the  land  is  rising  slowly,  the  detritus 
is  carried  further  and  further  seaward,  with  the  result  that  the 
successive  zones  of  sorted  material  do  not  lie  vertically  above  one 
another,  but  overlap  going  seaward,  the  upper  sheets  overlapping 
the  lower  as  shown  in  fig.  52.  The  result  of  this  overlapping  is 


ROCK-BUILDING. 


115 


that  different  grades  of  material  succeed  one  another  in  a  vertical 
line.  Thus  a  bed  or  stratum  of  mud  may  be  followed  by  a  bed  of 
sand,  and  a  bed  of  sand  by  one  of  gravel.  This  alternation  of 
different  grades  of  material  produces,  when  consolidation  takes 
place,  what  is  known  as  stratification. 

Alternations  of  thin  beds  of  sandstone  and  thinner  beds  of  slaty 
shale  are  frequently  seen  among  the  older  rock-formations,  and  this 
alternation  may  persist  through  a  thickness  of  many  thousand 
feet.  In  this  case  the  deposition  of  fine  silt  or  mud  on  the  sands 
may  have  been  due  to  small  seasonal  variations  in  the  velocity  of 
the  sea-currents,  or  to  the  seasonal  changes  in  the  prevailing  winds, 
exercising  an  influence  on  the  direction  and  strength  of  the  currents, 
or  to  the  varying  power  of  the  tides. 

Theoretically,  a  conglomerate  which  is  a  shore-line  deposit  ought 
to  graduate  going  seaward  into  a  sandstone,  and  from  a  sandstone 


Sc 


FIG.  53. — Showing  landward  overlap  as  the  result  of  deposition  of  detritus 
on  sinking  sea-floor. 

Sa,  tib,  and  Sc  mark  successive  sea-levels. 

into  a  mudstone ;  but  owing  to  the  mixing  up  of  the  material 
through  coastal  currents  and  gales  at  the  time  of  deposition,  this 
condition  is  perhaps  seldom  found  to  exist  in  nature. 

On  a  sinking  shore-line  with  an  advancing  sea,  the  overlap  of  the 
successive  layers  of  detritus  laid  down  on  the  sea-floor  will  be  on 
the  landward  side  as  shown  in  the  next  figure. 

Lamination. — When  a  rock  occurs  in  very  thin  layers  it  is  said 
to  be  laminated.  The  lamince  may  vary  from  the  hundredth  of  an 
inch  to  an  inch  thick.  A  laminated  structure  is  only  found  in 
rocks  composed  of  silt  or  mud.  The  laminae  frequently  vary  in 
colour.  Thus  one  lamina  may  be  greyish  blue ;  another  greenish 
grey,  yellowish  brown,  or  red.  Alternating  laminae  of  different 
colours  give  the  rock  a  ribbon-like  appearance  when  viewed  in 
sectional  elevation. 

Glacial  clays,  shales,  and  slates  frequently  possess  a  laminated 
structure.  These  are  composed  of  the  fine  sediments  carried  by 
rivers  into  seas  and  lakes,  being  deposited  where  there  is  little  or 


116  A   TEXT-BOOK   OF   GEOLOGY. 

no  movement  in  the  water,  or  of  muds  spread  over  the  floor  of 
estuaries  and  tidal  harbours. 

Investigation  has  shown  that  the  lamination,  even  when  paper- 
like,  is  due  to  minute  differences  in  the  size  of  the  particles.  The 
process  of  deposition  of  very  thin  layers  is,  like  stratification, 
dependent  on  the  principle  of  equal  falling  particles.  Let  us  assume 
that  a  silt-laden  river  like  the  Amazon  enters  the  sea  with  a  velocity 
of  one  foot  per  second.  It  is  obvious  that  the  suspended  particles 
will  settle  on  the  sea-floor  in  parallel  zones,  the  coarser  silts  first 
and  the  finest  furthest  seaward.  But  if  through  any  cause,  such 
as  the  daily  pulsations  of  the  tides,  the  velocity  of  the  current  is 
checked,  we  shall  get  frequent  alternations  of  normal  flow  and 
slack- water,  with  the  obvious  result  that  in  any  certain  zone  there 
will  be  deposited  alternating  layers  or  laminae  of  fine  and  excessively 
fine  silt,  laid  down  one  above  another.  This  process  of  lamination 
can  be  seen  in  operation  in  all  of  our  mud-filled  tidal  harbours. 
In  these  also  near  the  entrance,  muds  are  deposited  in  alternating 
layers,  due  to  the  varying  power  and  velocity  of  the  inrushing 
tides. 

Glacial  silts  laid  down  in  shallow  lakes  frequently  possess,  a 
laminated  structure.  The  lamination  in  this  case  is  due  to  the 
daily  and  seasonal  variations  in  the  flood-level  of  the  glacial  river. 
Glacial  rivers,  particularly  in  spring  and  summer,  exhibit  a  daily 
rise  and  fall,  the  maximum  rise  taking  place  in  the  afternoon. 
The  velocity  of  flow  varies  with  the  depth  of  the  water,  and  in 
consequence  we  get  at  the  point  of  discharge  an  overlapping  of 
sediments  of  different  grades  which,  as  we  have  seen,  induces  the 
structure  termed  lamination. 

Laminated  rocks  generally  split  readily  in  a  direction  parallel  to 
the  plane  of  the  laminae.  The  presence  of  finely  comminuted 
flakes  of  mica  adds  greatly  to  the  ease  with  which  the  splitting 
takes  place. 

The  differences  in  colour  frequently  met  with  in  laminated  rocks 
is  due  to  slight  variations  in  the  composition  of  the  sediments 
laid  down  at  various  times  or  seasons.  A  river  like  the  Amazon 
drains  nearly  half  a  continent.  Many  rock-formations  are  repre- 
sented within  its  watershed.  The  large  tributaries  are  not  always 
in  flood  at  the  same  times.  Thus  one  tributary  may,  when  in 
flood,  contribute  chalky  muds,  and  another  tributary  slaty  or 
micaceous  silt.  In  this  way  the  alternation  of  different-coloured 
sediments  is  obtained. 

Lamination  is  thus  seen  to  be  merely  a  minute  form  of  stratifica- 
tion, mainly,  but  not  exclusively,  the  work  of  water.  The  dust 
ejected  from  the  great  fissure-rent  during  the  Tarawera  eruption 
in  1886  was  a  mixture  of  various  grades  of  fine  material.  In  many 


ROCK-BUILDING. 


117 


places  it  settled  in  thin  laminae,  the  sorting  into  uniform  grades  of 
equal-falling  particles  being  effected  by  the  winnowing  action  of 
the  high  wind  prevailing  at  the  time.  The  wind  did  not  maintain 
a  steady  pressure,  but  came  in  powerful  blasts,  which  thus  allowed 
layers  of  dust  of  different  fineness  to  settle  one  after  another  in 
the  same  zone. 

The  sands  forming  coastal  dunes  frequently  possess  a  laminated 
or  banded  structure,  also  due  to  the  varying  velocity  of  the  pre- 
vailing winds. 


FIG.  54. — Showing  formation  of  false-bedding  at  head  of  lake. 

False-Bedding.— This  is  a  bedding  that  does  not  lie  parallel  to 
the  general  bedding  plane  of  the  formation  in  which  it  occurs.  It 
is  frequently  found  in  sands  and  fine  gravels  deposited  on  the  bed 
of  the  sea,  on  the  floor  of  a  lake,  or  in  the  channel  of  a  river  ;  and 
is  frequently  a  quite  local  phenomenon.  False-bedding,  also  termed 
current-bedding,  can  be  frequently  seen  in  process  of  formation  at 


FIG.  55. — Showing  false-bedding  in  lacustrine  detritus 
poured  into  a  lake-basin. 

the  head  of  valley-lakes  that  are  being  filled  up  with  river  detritus, 
and  also  in  the  broad  shingle  beds  of  mountain  stream's. 

The  gravels  and  sands,  as  they  are  discharged  into  the  lake- 
basin  in  times  of  normal  flow,  are  laid  down  in  an  inclined  position 
like  the  material  tipped  from  trucks  at  the  end  of  a  mine-dump. 
During  floods,  the  river  acquires  a  greater  velocity  and  is  thereby 
enabled  to  cut  away  the  crest  of  the  detritus  previously  laid  down 
at  the  head  of  the  lake.  As  the  flood  slackens,  a  sheet  of  detritus 
is  laid  over  the  truncated  edges  of  the  inclined  beds  a-b  ;  and 
when  the  flood  finally  subsides  and  normal  conditions  prevail,  the 
fresh  material  discharged  by  the  river  is  once  more  laid  down  in 


118 


A    TEXT-BOOK    OF    GEOLOGY. 


the  now  almost  still  water  in  an  inclined  position  like  the  first,  and 
we  get  the  appearance  shown  in  fig.  55. 

It  should  be  here  noted  that  the  detritus  is  not  carried  in  sus- 
pension, but  is  rolled  along  the  bottom  until  it  reaches  the  edge  of 
the  tip,  where  it  rolls  down,  at  once  adjusting  itself  to  the  natural 
angle  of  rest. 

False-bedding  is  frequently  seen  in  loose  river  gravels  at  places 


FIG.  56. — Showing  false-bedding  in  river-gravels. 

where  holes  are  scoured  in  the  bed,  or  old  shallow  channels  have 
been  gradually  filled  up  by  the  tipping  process  mentioned  above. 
When  filled  up  to  the  normal  flood-plane,  the  inclined  beds  tipped 
into  the  depression  are  overspread  with  sheets  of  gravel  lying 
parallel  to  the  plane  of  flow. 

False-bedding  is  also  seen  in  river-gravels  laid  down  in  what  is 


FIG.  57. — Showing  false-bedding  in  wind  blown  sands. 

termed  a  back-water  or  elongated  eddy  in  which  the  current  runs 
in  the  opposite  direction  to  that  of  the  general  flow  of  the  stream. 

The  false-bedding  of  estuarine  or  fluvio-marine  sediments  is 
of  frequent  occurrence.  It  generally  takes  place  in  the  same 
manner  as  in  lacustrine  deposits,  and  is  more  often  seen  in  sandy 
beds  than  in  conglomerates. 

Wind-blown  sands  frequently  exhibit  fine  examples  of  false- 
bedding.  The  sand  is  driven  along  before  the  wind  until  it 
reaches  the  lee-side  of  a  ridge  or  edge  of  a  declivity  where  it  im- 
mediately falls  down  the  sheltered  slope,  forming  layers  more  or 


ROCK-BUILDING.  119 

less  parallel  with  the  angle  of  rest,  as  shown  in  A  of  fig.  57,  in  which 
the  direction  of  the  wind  is  indicated  by  the  arrow. 

When  the  wind  blows  from  the  opposite  direction,  the  crests 
of  the  inclined  layers  of  sand  are  liable  to  be  truncated  along  the 
line  a-b.  When  this  happens,  fresh  layers  of  sand  are  sometimes, 
but  not  always,  laid  down  in  a  nearly  horizontal  plane,  as  shown 
in  B,  fig.  57.  It  sometimes  happens  that  the  wind  after  truncating 
the  inclined  layers  begins  to  build  up  a  new  set  of  layers  inclined 
towards  the  direction  in  which  the  wind  is  travelling. 

Current-laid  Stones. — In  rivers,  and.  in  all  currents  of  water 
that  run  continually  in  the  same  direction,  the  larger  stones,  parti- 
cularly those  of  a  slabby  shape,  tend  to  arrange  themselves  in  such 
a  way  as  to  offer  the  greatest  resistance  to  the  water  flowing  over 
them  (fig.  58).  This  arrangement  can  be  seen  in  almost  every 
gravel  terrace  composed  of  layers  of  fine  and  coarse  gravel.  It 
always  affords  a  valuable  clue  to  the  gold-miner  as  to  the  direction 
of  flow  of  the  ancient  river  that  formed  the  terrace,  thereby  enabling 

v.  Direction  of  flow 


FIG.  58. — Showing  arrangement  of  stones  in  a  river-bed 
to  resist  being  lifted. 

him  to  locate  with  some  degree  of  certainty  the  position  of  the 
gold-bearing  wash-dirt. 

Surface  Markings  on  Sediments. 

From  our  study  of  the  manner  in  which  sediments  are  formed 
on  the  sea-floor,  we  are  able  to  deduce  two  fundamental  truths 
that  have  an  extraordinary  importance  in  connection  with  the 
unravelling  of  the  history  of  the  Earth.  These  truths,  which  are 
now  recognised  as  geological  axioms,  may  be  expressed  as  under  : — 

(1)  That  all  mechanically  formed  sediments  are  composed  of 

the  waste  of  pre-existing  land. 

(2)  That  all  marine  detrital  sediments  were  laid  down  marginal 

to  land  areas. 

Hence,  in  his  endeavour  to  trace  out  the  geographical  distribution 
of  the  land  and  sea  at  the  different  stages  of  the  Earth's  history, 
the  geologist  searches  for  all  the  evidences  that  indicate  the  former 
existence  of  shore-line  conditions  of  deposition, 


120  A  TEXT-BOOK  OF  GEOLOGY. 

The  most  trustworthy  and  tangible  evidences  of  ancient  shore- 
lines are  beds  of  conglomerate  composed  of  beach-shingle,  and 
rocks  containing  the  remains  of  marine  life  that  are  known  to  live 
only  in  shallow  water.  These  outstanding  and  indestructible 
proofs  are  frequently  supplemented  by  facts  that  may  in  themselves 
appear  insignificant,  but  are  not  less  valuable  in  affording  clues 
as  to  conditions  of  deposition  on  which  special  emphasis  may  be 
safely  laid. 

Among  these  minor  proofs  are  ripple-marks,  sun-cracks,  rain- 
and  hail-prints,  and  animal  trails,  all  of  which  have  been  found 
in  rocks  composed  of  sediments  of  fine  texture. 

Ripple-Marks. — These  may  be  frequently  seen  in  the  sands 
laid  down  on  the  sandy  shore  of  a  lake  or  sea,  or  on  the  floor  of 
a  shallow  lake  or  estuary.  They  are  produced  by  the  pulsations 
of  a  slowly  retreating  tide.  They  are  also  formed  by  the  wind 
bearing  on  the  surface  of  shallow,  slowly-ebbing,  tidal  waters.  The 
ripple-marks  produced  by  one  ebbing  tide  will  be  obliterated  by 
the  next  flowing  tide,  which,  on  retreating,  will  form  a  new  series 
of  ripples. 

In  certain  situations  the  surface  of  sand  dunes  is  frequently 
covered  with  parallel  ripple-marks  formed  by  oscillations  in  the 
force  of  the  wind. 

Whtere  the  ripple-marks  are  formed  under  water  that  is  always 
receiving  fresh  accessions  of  sand,  a  ripple-marked  surface  may 
be  gently  overspread  with  a  layer  of  sand  and  be  thus  preserved 
(Plate  XII.).  Ripple-marked  sandstones  are  found  among  the 
geological  formations  of  all  ages. 

Sun-Cracks. — In  many  shallow  tidal  harbours,  estuaries,  and 
deltas,  a  marginal  strip  of  silt  or  mud  is  daily  left  high  and  dry 
between  the  high- water  and  low- water  lines.  At  the  upper  limits 
of  the  tide  the  sediments  may  be  exposed  to  the  drying  influence 
of  the  sun's  rays  for  many  hours  at  a  time  ;  and  when  this  happens 
the  muds  shrink,  and  in  doing  so  become  seamed  with  a  network 
of  cracks  that  produce  polygonal  cakes  somewhat  resembling 
the  pattern  of  an  ancient  Roman  pavement. 

Sun-cracked  muds  are  a  striking  feature  in  many  mangrove- 
covered  tidal  harbours  in  north  New  Zealand,  Australia,  East 
Indies,  and  other  tropical  and  semi-tropical  lands  where  the  blazing 
heat  of  the  summer  sun  at  certain  phases  of  the  tide  dries  up  the 
marginal  layers  of  mud  with  great  rapidity.  The  same  phenomenon 
on  a  miniature  scale  may  be  witnessed  in  almost  all  temperate 
lands  in  dried-up  mud-puddles  and  pools  on  the  roadside,  and 
in  cultivated  fields  after  heavy  rain  followed  by  sunshine  or  a 
drying  wind. 

Estuarine  and  tidal  sun-cracks  are  generally  obliterated  by  the 


ROCK-BUILDING.  121 

next  tide,  but  in  some  cases  they  are  gently  filled  with  fine  sediment 
and  thereby  preserved.  Fossil  sun-cracks  are  eloquent  witnesses 
of  tidal  muds  and  a  blazing  sun  in  past  geological  ages. 

Rain-  and  Hail-Prints. — Estuarine  muds  are  sometimes  pitted 
with  the  heavy  drops  of  a  passing  shower  of  hail  or  rain,  and  when 
these  prints  are  gently  covered  with  mud  by  a  slowly  rising  tide 
they  are  permanently  preserved,  thus  forming  valuable  meteoro- 
logical records. 

Where  the  mud  is  very  soft,  the  rain  only  makes  indistinct 


FIG.  59. — Showing  sun-cracks  in  harbour  mud. 

splashes,  and  where  it  is  too  hard,  it  fails  to  make  any  impression. 
But  in  tropical  and  semi-tropical  lands,  the  hail  frequently  falls 
as  large  as  hazel-nuts.  In  a  few  minutes  it  litters  the  ground  with 
leaves  stripped  from  the  fringing  mangrove  trees,  and  in  the  open 
estuary  descends  with  such  force  that  much  of  it  is  half-buried 
in  the  mud.  When  the  half-buried  hail  melts,  it  leaves  perfect 
dimples  or  prints  scattered  irregularly  over  the  surface  of  the  mud. 
Many,  if  not  the  majority,  of  the  supposed  fossil  rain- prints  are 
probably  hail-prints. 

Fossil-prints  have   been  found  in  many  geological  formations, 
and  their  values  lie  in  the  proofs  they  afford  that  the  meteoro- 


122  A  TEXT-BOOK  OF  GEOLOGY. 

logical  conditions  of  to-day  are  but  a  continuance  of  those  that 
existed  in  far-off  geological  times. 

Animal  Trails.— Crabs,  lobsters,  shellfish,  and  worms  as  they 
move  over  the  surface  of  the  partially  dried  silts  and  muds  exposed 
between  tide-marks,  leave  their  trails  and  burrows,  which,  under 
favourable  circumstances,  may  be  preserved  by  a  fresh  layer  of 
sediment.  Marks  such  as  these  have  been  found  in  rocks  composed 
of  fine  sand  and  mud  ;  and  are  regarded  with  much  interest  by 
geologists  as  they  afford  conclusive  proof  of  the  physical  conditions 
under  which  the  sediments  were  laid  down. 

Besides  these,  there  have  also  been  preserved  in  slabs  of  stone 
the  tracks  of  reptiles,  birds,  and  mammals  that  in  past  ages 
roamed  about  the  margins  of  the  sun-dried  estuaries  and  deltas 
in  search  of  food. 


FIG.  60. — Showing  animal  tracks. 

SUMMARY. 

The  rock-structures  that  have  been  considered  in  this  chapter 
are  (1)  Stratification,  (2)  Lamination,  and  (3)  False-bedding. 

(1)  Stratification  refers  to  the  arrangement  of  detrital  material 
in  parallel  layers  or  beds  commonly  termed  strata ;  a  word 
derived  from  the  Latin  stratum  =a  layer  or  bed.  When 
the  different  beds  (exposed  for  example  in  a  sea-cliff)  are 
distinctly  marked,  the  rocks  are  said  to  be  well- stratified. 
But  if,  on  the  other  hand,  the  bedding  is  indistinct  and 
difficult  to  determine,  the  rock  is  said  to  be  indistinctly 
stratified.  Thin  bands  of  any  material  occurring  at 
intervals  in  a  formation  that  possesses  no  bedding  planes, 
always  impart  a  stratified  appearance  to  the  rock.  A 
line  of  detached  nodules  or  pebbles  will  also  indicate  the 
original  deposition  plane  in  a  rock  that  otherwise  shows 
no  evidence  of  bedding. 


To  face  page  122. 


^ ;  \  [PLATE 


I 


A.    HORIZONTAL  PLIOCENE  STRATA,  NEW  ZEALAND. 


B.    INCLINED  STRATA. 


ROCK-BUILDING. 


123 


The  appearance  of  well-stratified  rocks  as  seen  in  many  sea- 
cliffs  is  shown  in  fig.  61,  and  in  Plate  XIII.  (A). 


(2) 


FIG.  6L — Showing  section  of  stratified  rocks. 

(a)  Conglomerate.  (c)  Thin-bedded  sandstone. 

(6)  Thick-bedded  sandstone.  (d)  Marly  clays. 

(e)  Marine  limestone. 

Lamination  refers  to  the  aqueous  deposition  of  silt  and  mud 
in  very  thin  layers  or  lamince.  Lamination  is  merely  a 
minute  form  of  stratification,  and  is  a  structure  found  only 
in  fine  sediments  deposited  in  still  or  slowly  moving 
waters  in  accordance  with  the  principle  of  equal-falling 
particles. 


1      11 L 


oo     O    o     o    o     o 


FIG.  62. — Showing  false-bedding  structure. 

(3)  When  the  planes  of  stratification  in  some  particular  bed  run 
at  some  other  angle  than  the  general  plane  of  the  beds 
above  and  below,  the  structure  is  termed  false-bedding 
or  current-bedding. 

The  appearance  of  false-bedding  in  consolidated  or  partially 
consolidated  rocks  is  shown  in  fig.  62. 

False-bedding   is    found   in    fluviatile,    lacustrine,  and    marine 


124  A  TEXT-BOOK  OF  GEOLOGY. 

deposits,  as  well  as  in  wind-blown  sands.  It  is  frequently  a  local 
phenomenon  due  to  the  action  of  eddies,  back-water  currents, 
and  the  tipping  of  sediments  into  comparatively  still  deep  water. 
It  should,  however,  be  noted  that  false-bedding  is  not  necessarily 
limited  to  local  occurrences.  False-bedding  can  sometimes  be 
traced  in  a  particular  stratum  for  miles,  For  example,  the  famous 
Desert  Sandstone  of  Northern  Australia  exhibits  this  peculiar 
structure  extending  over  hundreds  of  square  miles. 

(4)  Ripple-marks,    sun-cracks,    rain-prints,    and   animal   tracks 

are  found  preserved  in  rocks,  and  afford  conclusive  evidence 
of  shore-line  conditions  at  the  place  where  the  sediments 
were  laid  down.  They  also  afford  valuable  evidence  as 
to  the  meteorological  conditions  of  far-away  geological 
ages. 

(5)  It  is  a  geological  axiom  that  all  the  mechanically  formed 

sediments  laid  down  on  the  floor  of  the  sea  were  formed  of 
the'  waste  of  pre-existing  land. 

(6)  Another  axiom  of  extraordinary  value  in  solving  problems 

relating  to  the  distribution  of  land  and  sea  in  the  past 
geological  ages  is  that  all  sediments  laid  down  on  the 
sea-floor  were  marginal  to  land  areas  existing  at  the  time. 


CHAPTER   VIII. 
ROCK  STRUCTURES. 

THE  CONSOLIDATION  OF  SEDIMENTS. 

IN  the  last  chapter  we  were  principally  concerned  with  the  manner 
in  which  the  products  of  denudation  in  the  form  of  coarse  and  fine 
sediments  were  spread  out  in  parallel  layers  and  piled  up  on  the 
sea-floor  and  in  lake-basins.  These  sediments  are  the  materials 
of  which  stratified  rocks  are  formed. 

When  we  speak  of  a  rock  we  at  once  form  a  mental  picture  of 
something  hard  and  compact.  As  a  matter  of  fact,  some  rocks 
are  very  soft  and  others  very  hard.  A  marl  or  clay,  for  example, 
is  very  soft  and  friable  ;  while  a  sandstone  may  be  intensely  hard. 

Sedimentary  rocks,  as  generally  defined,  are  composed  of  sedi- 
ments that  are  more  or  less  hardened  or  consolidated  ;  and  although 
originally  laid  down  in  a  horizontal  or  approximately  horizontal 
position,  they  are  now  found  to  be  tilted  or  inclined  at  various  angles, 
and  thrown  into  folds  that  may  be  gentle  undulations  or  minute 
corrugations. 

Moreover,  closer  examination  soon  discloses  the  fact  that  the 
rocks  have  not  only  been  pushed  into  folds  and  corrugations,  but 
also  fissured  with  many  small  cracks  or  joints,  and  occasionally 
traversed  by  great  dislocations  GUI  fractures  termed  faults. 

Everywhere  there  is  evidence  that  the  rocks  have  been  at  one 
time  or  another  subjected  to  enormous  stress  or  pressure  whereby 
they  have  been  folded,  crumpled,  tilted,  fissured,  or  fractured  as 
mentioned  above. 

Our  aim  in  this  chapter  will  be  to  consider  the  various  processes 
by  which  soft  and  incoherent  sediments  may  be  consolidated  or 
hardened  into  what  is  called  rock. 

Hardening  of  Sediments. 

The  hardening  of  sediments  may  be  effected  (a)  by  pressure  or 
(6)  by  a  cementing  medium. 
Hardening  by  Pressure. — This  is  the  simplest  and  most  obvious 

125 


126  A  TEXT-BOOK  OF  GEOLOGY. 

means  of  consolidation,  and  whether  the  pressure  is  effected  by 
natural  or  artificial  agency,  the  results  are  always  the  same.  Thus, 
when  clay  is  placed  in  a  mould  and  subjected  to  great  pressure,  it 
is  compressed,  partially  dehydrated,  and  converted  into  a  brick  or 
tile  as  compact  as  an  ordinary  shale. 

The  same  thing  takes  place  in  Nature.  When  clay,  mud,  or  fine 
silt  is  subjected  to  the  pressure  of  hundreds  of  feet  of  overlying 
sediment,  it  is  converted  into  a  shale  or  claystone. 

Coarse  sediments  are  not  easily  consolidated  by  mere  pressure 
alone  ;  but  if  they  consist  of  particles  of  various  sizes  that  will  fill 
up  the  interstices  between  the  larger  grains  or  pebbles,  or  if  they 
contain  an  admixture  of  muddy  paste,  they  may  be  consolidated 
into  a  fairly  hard  rock  by  pressure  alone. 

The  principle  underlying  consolidation  by  pressure  is  that  fine 
sediments  afford  a  larger  adhesive  surface  relatively  to  the  size  of 
the  constituent  particles  than  coarse  sediments. 

Hardening  by  a  Cementing  Medium. — The  process  of  hardening 
by  a  cementing  medium  may  be  easily  illustrated  by  a  simple 
experiment.  Take/bw  ounces  of  Portland  cement,  six  ounces  of 
clean  sand,  and  six  ounces  of  pebbles  the  size  of  peas. 

Place  these   constituents   on   a   flat   plate   or  board  and  mix 

thoroughly  in  a  dry  state. 

Pile  the  mixture  into  the  form  of  a  flat  truncated  cone.  Make 
a  hole  in  the  middle  of  the  cone  and  into  it  pour  two  ounces 
of  clean  water. 

With  two  spatulas,  one  in  each  hand,  work  the  mixture  and 
water  into  a  thick  paste,  adding  a  little  more  water  if  re- 
quired. Turn  the  paste  over  for  several  minutes  until  the 
water  is  thoroughly  incorporated,  and  then  press  it  firmly 
into  a  mould  of  any  shape.  A  small  cigar-box  will  do  very 
well.  After  two  or  three  hours  remove  the  hardened  mass 
from  the  box  and  you  will  have  a  slab  of  hard  rock  arti- 
ficially formed. 

This  experiment  is  not  intended  to  illustrate  the  chemical  process 
of  setting  so  much  as  the  part  played  by  the  cement  in  binding  the 
sand  and  grit  into  a  hard  aggregate.  The  cement  is  merely  an 
artificial  matrix  in  which  the  sand  and  grit  are  embedded.  If 
moistened  with  water  and  placed  in  the  mould,  it  would  form  a 
slab  of  fine-grained  artificial  stone  much  stronger  than  that  obtained 
in  our  experiment. 

In  the  making  of  concrete,  which  is  merely  an  artificial  stone, 
it  is  found  that  the  greater  the  proportion  of  constituent  aggregates, 
such  as  sand,  gravel,  or  broken  rock,  to  the  cement  or  matrix,  the 
weaker  is  the  resulting  concrete  ;  and  such  also  is  found  to  be  the 
case  in  Nature. 


ROCK    STRUCTURES.  127 

Take  again  the  case  of  the  frozen  gold-bearing  gravels  in  Siberia, 
Alaska,  and  Alpine  New  Zealand.  The  contained  water  freezes 
for  a  depth  of  a  few  inches  or  many  feet,  according  to  the  length 
and  severity  of  the  winter  frosts,  and  hardens  the  whole  mass  into 
a  rock-like  mass  resembling  a  conglomerate.  Here  the  frozen 
water  is  the  cementing  medium  or  matrix,  and  although  the 
hardening  is  only  temporary  it  very  well  illustrates  the  formation  of 
conglomerates. 

The  permanent  hardening  of  sediments  by  a  cementing  medium 
is  effected  in  Nature  by  the  deposition  of  mineral  matter  between 
the  constituent  particles  from  waters  slowly  circulating  through 
the  mass. 

The  commonest  natural  cementing  media  are  carbonate  of  lime, 
oxide  of  iron,  and  silica. 

Carbonate  of  lime  and  some  iron  compounds  are  soluble  in  water 
charged  with  carbonic  acid  gas,  and  these  may  be  deposited  as 
carbonates  if  the  water  evaporates,  or  if  the  carbonic  acid  becomes 
disengaged  as  it  does  quite  readily,  being  what  is  known  as  a  weak 
acid— that  is  an  acid  which  possesses  but  a  feeble  hold  of  the 
substances  with  which  it  combines. 

The  deposit  or  precipitate  of  carbonate  of  lime  or  iron  acts  as  a 
cementing  medium  and  binds  the  surrounding  particles  into  a 
coherent  mass.  In  this  way  sands  are  converted  into  sandstones, 
and  gravels  into  conglomerates. 

When  the  carbonate  of  iron  is  deposited  among  porous  sands 
or  gravels,  it  becomes  in  the  presence  of  water  converted  into  the 
rusty  brown  hydrous  oxide  called  limonite.  Hence  we  find  that 
the  cementing  medium  of  ferruginous  sandstones,  gritstones,  and 
conglomerates  is  in  almost  all  cases  the  hydrous  oxide  of  iron. 

On  the  other  hand,  when  carbonate  of  iron  is  precipitated  in  a 
fine  impervious  sediment,  it  remains  as  the  carbonate,  forming  the 
well-known  clay-band  ironstone — a  valuable  ore  of  iron  that  occurs 
in  formations  of  all  geological  ages. 

Silica  is  perhaps  more  abundant  as  a  cementing  matrix  than 
carbonate  of  lime,  particularly  among  the  older  rocks.  It  is  soluble 
in  waters  containing  potash  or  soda,  especially  at  high  temperatures. 
Deep-seated  waters  are  generally  alkaline  from  the  presence  of  dis- 
solved salts  of  potash  or  soda.  These  waters  in  their  passage 
through  the  rocks  dissolve  silica,  and  when  they  come  to  the 
surface  or  when  they  reach  a  cooler  stratum,  the  silica  is  deposited 
around  and  between  the  particles,  which  are  thereby  cemented  into 
a  compact  rock. 

Siliceous  waters  are  frequently  abundant  in  volcanic  regions  of 
waning  activity,  where  they  appear  in  the  form  of  hot  springs  and 
geysers.  The  potash  or  soda  has  a  greater  liking  or  affinity  for  the 


128         A  TBXT-BOO'K  OF  GEOLOGY. 

carbonic  acid  of  the  atmosphere  than  for  the  silica.  It  conse- 
quently forms  a  new  partnership,  thereby  liberating  the  silica  which 
is  deposited  as  an  incrustation  of  sinter  around  the  outlet  of  the 
spring  or  geyser  from  which  the  waters  issue.  Moreover,  sands 
and  gravels  that  happen  to  lie  near  are  cemented  into  hard 
rock. 

These  siliceous  waters  also  possess  a  petrifying  power,  such 
organic  substances  as  leaves,  twigs,  and  even  animal  remains  being 
replaced  by  silica,  the  replacement  taking  place  so  slowly  as  fre- 
quently to  reproduce  in  stone  the  exact  form  and  structure  of  the 
original  organism. 

From  the  foregoing  we  find  that  a  stratum  of  sand  may  be 
cemented  with  carbonate  of  lime,  oxide  of  iron,  or  silica. 

The  carbonate  of  lime  forms  a  calcareous  sandstone  ;  the  oxide  of 
iron,  a  ferruginous  or  limonitic  sandstone  ;  and  silica,  a  siliceous 
sandstone.  That  is,  the  nature  of  the  sandstone  and  of  the  qualify- 
ing adjective  depends  on  the  character  of  the  cementing  matrix. 
Similarly,  we  may  have  a  calcareous  conglomerate,  a  limonitic  con- 
glomerate, or  a  siliceous  conglomerate. 

When  a  pile  of  sediments  consists  of  alternating  layers  of  mud 
and  sands,  the  sandy  beds  are  in  many  cases  found  to  be  cemented 
into  a  hard  resistant  rock,  while  the  layers  of  mud  or  clay  remain 
relatively  soft.  Such  hard  bands  may  be  seen  standing  out  as 
projecting  ledges  in  many  sea-cliffs  and  scarps. 

The  hardening  of  the  sandy  beds  was  in  most  cases  due  to  the 
deposition  in  them  of  a  cementing  medium.  The  clayey  beds 
being  impervious  to  water  only  attained  the  moderate  degree  of 
hardness  that  could  be  imparted  by  pressure  and  dehydration  ; 
whereas  the  sandy  beds  being  porous  offered  a  free  passage  for  the 
flow  of  the  mineralised  waters  which  left  behind  them  a  deposit 
of  cementing  material. 

The  coral  reefs  and  coralline  beach-sands  of  the  tropics,  as  well 
as  the  shelly  sands  and  shell-banks  found  on  the  strands  of  nearly 
all  lands,  have  been  in  many  places  converted  into  compact  lime- 
stones by  the  partial  or  complete  dissolution  and  replacement  of 
the  corals  and  shells  by  carbonate  of  lime  in  a  semi-crystalline  or 
crystalline  form  in  which  there  is  generally  no  trace  of  the  original 
organisms. 

Other  Cementing  Media. — Among  other,  but  less  common, 
cementing  materials  deposited  among  sediments  by  mineralised 
waters  are  carbonate  of  magnesia,  sulphate  of  lime,  sulphate  of 
barium,  and  oxides  of  manganese. 

Carbonate  of  lime  is  a  comparatively  soft  substance  ;  consequently 
the  rocks  in  which  it  is  the  cementing  medium  are  seldom  capable 
of  withstanding  much  wear  and  tear.  On  the  other  hand,  silica 


fiOCK    STBUCTUItES.  129 

is  an  exceeding  hard  substance,  and  hence  siliceous  sandstones  and 
conglomerates  are  always  hard  rocks  capable  of  withstanding  a 
great  amount  of  mechanical  or  chemical  erosion. 

Limonitic  sandstones  are  frequently  soft  and  friable,  but  many 
limonitic  quartz  conglomerates  and  grits  possess  great  resisting 
power. 

Concretions. — It  frequently  happens  that  only  isolated  portions 
of  a  bed  or  stratum  are  hardened  by  the  cementing  medium. 
These  hardened  portions  or  concretions  have  been  formed  where 
the  supply  of  carbonate  of  lime  or  evaporation  has  been  greatest. 
They  may  occur  close  together  or  widely  scattered. 

Concretions  x  are  generally  spheroidal  in  form,  and  for  that  reason 
are  sometimes  mistaken  for  pebbles  or  water- worn  boulders. 
They  may  range  in  size  from  a  few  inches  up  to  ten  feet  or  more 
in  diameter.  They  are  frequently  composed  of  concentric  layers 


FIG.  63. — Showing  tabular  concretions. 

that  peel  off  like  the  layers  of  an  onion  until  a  solid  spheroidal  core 
is  reached.  This  spheroid  on  close  examination  is  frequently 
found  to  have  formed  round  some  organic  body,  such  as  a  shell  or 
saurian  bone,  as  a  nucleus. 

In  many  cases  the  internal  portion  of  the  concretion  has  con- 
tracted more  than  the  external,  thereby  giving  rise  to  numerous 
radiating  cracks  that  have  subsequently  become  filled  with  calcite. 
Concretions  possessing  this  structure  are  sometimes  called  septarian 
boulders,  or  simply  septaria. 

Concretions  are  commonly  found  in  shales  and  clays.  They  are 
especially  abundant  in  the  shaly  clays  of  the  Cretaceous  Period  in 
many  parts  of  the  globe. 

Concretions  in  which  carbonate  of  iron  is  the  cementing  medium 
are  sometimes  quite  common  in  clays  and  shales,  but  they  seldom 
attain  the  dimensions  of  septaria. 

Clays  sometimes  contain  nodular-shaped  concretions  composed 
of  limestone,  oxide  of  iron,  or  iron  pyrites.  These  concretions 
frequently  assume  grotesque  forms  that  sometimes  bear  a  quaint 

1  From  the  Latin  con  =  together,  and  cretus  =  grown. 

9 


130  A  TEXT-BOOK  OF  GEOLOGY. 

resemblance  to  organic  bodies  or  to  objects  fashioned  by  the  hand 
of  primitive  man.  Many  of  these  nodules  are  hollow,  the  cavity 
being  lined  with  crystals. 

The  hardened  portions  of  a  stratum  are  frequently  lenticular  or 
tabular  in  form,  and  lie  so  close  together  as  almost  to  form  a  con- 
tinuous band  of  hard  rock,  as  shown  in  fig.  63  on  p.  129.  This 
structure  is  due  to  the  separation  of  the  carbonate  of  lime  from  the 
remainder  of  the  rock,  and  its  aggregation  in  layers  or  tabular 
masses. 

Some  concretions  were  probably  formed  during  the  accumula- 
tion of  the  sediments  in  which  they  lie,  but  the  majority  have 
arisen  from  a  rearrangement  and  concentration  of  like  kinds  of 
mineral  matter. 

Flints. — These  occur  as  grey  and  black  nodules  dispersed  in 
certain  layers  in  the  Upper  Chalk  of  England,  north-west  Europe, 
America,  and  New  Zealand,  and  elsewhere.  They  frequently 
enclose  some  organism,  such  as  a  sponge,  echinoderm,  or  shell, 
the  organism  being  the  nucleus  round  which  the  siliceous  concentra- 
tion took  place. 

In  some  places,  as  in  Marlborough  and  Kaipara  in  New  Zealand, 
the  flint  forms  distinct  beds  many  feet  thick. 

Flint  possesses  a  perfect  conchoidal  fracture,  The  dark  colour 
of  the  black  variety  arises  from  the  presence  of  carbonaceous  matter, 
which  can  be  dispelled  by  heat. 

Flint  nodules  are  formed  by  the  interchange  of  carbonate  of  lime 
and  silica.  Many  marine  plants  and  animals  secrete  silica  from 
sea- water  for  the  building  up  of  their  organisms.  The  water 
present  in  the  chalk  dissolves  these  organisms,  and  as  it  does  so, 
replaces  them  with  carbonate  of  lime.  The  water  now  charged  with 
silica  deposits  the  silica  elsewhere,  preferably  where  some  of  it 
already  exists,  as,  for  example,  on  sponge-skeletons,  which  consist 
of  siliceous  spicules. 

In  cases  where  the  calcareous  shell  of  an  echinoderm  or  a  coral 
has  been  replaced  by  silica,  it  would  seem  that  the  dissolution  of 
the  carbonate  of  lime  was  accompanied  by  direct  replacement 
with  silica,  molecule  by  molecule,  the  process  being  similar  to  the 
replacement  of  shells  by  iron  pyrites,  which  has  so  frequently  taken 
place  in  clays  and  shales. 

Such  replacement  of  one  mineral  by  another  is  called  pseudo- 
morphism* 

Fulgurites. — As   a  geological   agent  lightning  is   not   of   much 

importance.     It  is  reputed  to  be  capable  of  shattering  solid  rocks, 

but  its  usual  effect  is  to  perforate  their  surface  with  small  holes 

lined  with  a  glassy  enamel.     When  the  electric  spark  is  discharged 

1  From  the  Gr.  pseudos= false,  and  morphe=iorm. 


ROCK    STRUCTURES.  131 

into  sand  or  loose  soil,  it  may  form  short,  tapering,  fragile  tubes  of 
partially  fused  sand- grains  called  fulgurites ;  or  blebs  of  well-fused 
glass,  usually  shaped  like  buttons  or  dumbbells.  From  their  pre- 
valence in  the  desert  regions  of  Australia  these  fused  blebs  or  drops 
have  been  called  Australites. 


CHAPTER  IX. 
EARTH-MOVEMENTS, 

EARTH-MOVEMENT  may  take  the  form  of  uplift  or  subsidence,  rock- 
folding  or  faulting,  shearing  or  horizontal  displacement.  More- 
over, it  may  be  local  or  continental,  slow  or  rapid. 

Local  movements  are  usually  due  to  volcanic  agency,  earth- 
quakes, or  faulting.  They  are  relatively  rapid,  and  may  cause 
sharp  folding  and  faulting  of  strata  in  the  neighbourhood  of  the 
disturbance. 

The  movement  which  affects  continents  or  large  areas  is  usually 
slow,  and  may  not  amount  to  more  than  a  few  inches  in  a  century. 
Such  slow  regional  movement  is  called  secular  movement,  as  it  is 
more  or  less  continuous  over  a  number  of  years. 

An  upward  crustal  movement,  which  eventually  results  in  the 
formation  or  building  up  of  a  land-surface  of  continental  dimensions, 
is  called  epeirogenic  ;  while  an  upward  linear  folding  of  the  strata, 
which  eventually  uplifts  mountain-chains,  is  called  orogenic.1 

SLOW  ELEVATION  AND   SUBSIDENCE  OF  THE   LAND. 

Elevation. 

When  we  find  strata  containing  marine  shells  forming  masses 
of  dry  land,  hundreds  or  may  be  thousands  of  feet  above  the  sea- 
level  datum,  we  are  compelled  to  conclude  that  the  land  has 
emerged  from  the  sea.  These  shelly  beds  represent  the  uplifted 
sea-floor  of  some  past  geological  age. 

Among  the  best  evidences  of  recent  uplift  are  what  are  called 
raised-beaches,  which  is  only  another  name  for  uplifted  sea-strands. 
These  occur  on  the  shores  of  many  lands  in  both  hemispheres,  being 
strikingly  conspicuous  on  the  coasts  of  Scotland,  England,  Norway, 
Sweden,  Spain,  South  Italy,  Sicily,  Morocco,  Algeria,  Egypt,  Pacific 
side  of  North  and  South  America,  India,  Australia,  and  New 
Zealand.  They  form  benches  or  terraces  that  in  some  cases  can 
be  traced  along  the  coast-line  for  scores  and  even  hundreds  of  miles, 

1  Gr.  oras=a  mountain,  and  genesis = production. 
132 


EARTH-MOVEMENTS.  133 

curving  round  headlands  and  following  the  various  indentations 
of  deep  bays  and  long  fiords. 

Raised-beaches  consist  of  shingle  and  sand  mixed  with  sea-shells, 
the  majority  of  which  belong  to  species  still  living  in  the  adjacent 
seas.  They  are  generally  backed  by  cliffs  that  are  frequently  wave- 
worn  and  undercut  or  hollowed  into  caves.  In  some  cases  the 
rocks  are  covered  with  barnacles  or  perforated  with  holes  bored  by 
marine  shells. 

In  the  fiords  of  Norway  the  raised-beaches  occur  at  heights 
varying  from  50  to  600  feet  above  the  sea,  and  singularly  enough 
they  are  not  horizontal  but  slope  gently  towards  the  sea,  which 
is  an  evidence  that  the  rate  of  uplift  is  more  rapid  on  the  landward 
than  seaward  side.  This  unequal  rate  is  what  is  termed  differential 
uplift. 

In  Scotland  raised-beaches  can  be  traced  at  25,  40,  50,  60,  75, 
and  100  feet  above  the  present  sea-level. 

The  occurrence  of  marine  shells  in  rocks  involved  in  the  great 
earth-folds  which  comprise  the  Alps,  Himalayas,  and  other  moun- 
tain-chains, affords  incontestable  proof  of  uplift  in  bygone  geological 
times.  Raised  beaches  are  records  of  comparatively  recent  eleva- 
tion ;  and  we  have  abundant  evidence  that  elevation  is  still  in 
progress  in  some  parts  of  the  globe.  It  is  doubtful  if  the  land  is 
ever  in  a  state  of  complete  rest  for  any  considerable  time.  If  it  is 
not  rising,  it  is  probably  sinking  ;  and  it  frequently  happens  that 
uplift  on  one  side  of  a  continent  is  compensated  by  subsidence 
on  the  other.  Marks  placed  on  the  coast  of  Sweden  in  1820  have 
shown  that  the  land  is  still  rising  at  the  rate  of  2  or  3  feet  in  a 
century. 

Subsidence. 

The  evidences  of  subsidence  are  not  always  so  obvious  as  those 
of  uplift,  as  they  are  mostly  to  be  found  submerged  in  the  sea. 
Among  the  most  conclusive  proofs  are  submerged  coal-seams, 
submerged  forests,  and  buildings.  To  these  might  be  added  the 
formation  of  atolls  and  drowned  valleys. 

Submerged  Coals. — At  the  present  time  seams  of  coal  are  being 
worked  many  hundred  feet  below  sea-level  in  New  South  Wales 
and  New  Zealand.  Now  coal,  as  we  know,  consists  of  the  remains  of 
vegetation  that  required  sunshine  and  air  for  its  growth.  It  was 
formed  on  the  dry  land,  and  is  what  is  called  a  terrestrial  deposit. 
When,  therefore,  we  find  it  hundreds  and  in  some  cases  thousands 
of  feet  below  the  present  level  of  the  sea,  we  are  safe  in  concluding 
that  a  subsidence  of  the  old  land-surface,  on  which  the  coal  vegeta- 
tion grew,  has  taken  place  at  some  remote  period. 
|f  Submerged  Forests. — The  erect  stumps  of  forest  trees,  frequently 


134  A  TEXT-BOOK  OF  GEOLOGY. 

associated  with  peaty  matter  containing  twigs,  leaves,  and  fruits, 
are  found  below  sea-level  in  many  lands.  Good  examples  are  seen 
at  Formby  Point  on  the  coast  of  Lancashire  ;  at  Leasowe,  in 
Cheshire ;  and  at  Freshwater  West,  in  Pembrokeshire.  Such 
submerged  forests  are  an  evidence  of  subsidence  in  quite  late 
geological  times  or  of  coastal  sag. 

Drowned  Valleys. — The  celebrated  fiords  of  Norway  and  New 
Zealand,  that  stretch  far  back  among  the  neighbouring  mountain- 
chains,  are  merely  valleys  that  have  been  invaded  by  the  sea. 
In  California  and  other  lands  the  soundings  show  that  some 
of  the  existing  valleys  can  still  be  traced  far  seaward.  That  is, 
river- valleys  are  found  to  be  continuous  with  valleys  in  the  sea- 
floor.  This  is  rightly  held  to  be  proof  of  comparatively  recent 
subsidence  of  the  coast-line. 

The  broken,  deeply  indented,  and  ragged  coasts  of  British 
Columbia,  Alaska,  North-East  Canada,  and  Greenland  have 
originated  from  a  general  subsidence  of  the  previously  deeply 
dissected  maritime  lands  in  these  regions. 

Barrier  Reefs  and  Atolls. — According  to  Darwin's  view,  these 
are  coral  reefs  that  have  grown  upward  on  a  sinking  sea-floor. 
The  borings  conducted  at  the  island  of  Funafuti  proved  the  exist- 
ence of  coral  reefs  and  coral  limestone  down  to  a  depth  of  1114  feet 
below  sea-level ;  and  since  the  coral  polyp  can  only  live  in  com- 
paratively shallow  water,  extensive  subsidence,  probably  amount- 
ing to  800  feet,  must  have  taken  place  since  the  foundations  of  the 
existing  coral  reefs  were  formed  by  the  coral-builders. 

Rapid  Earth-Movement. 

Earthquakes  may  be  due  to  the  sudden  jolts  arising  from  the 
settlement  of  the  ground  along  the  plane  of  great  faults  or 
dislocations  in  the  crust,  or  they  may  be  propagated  by 
sudden  subterranean  explosions  of  steam  during  a  volcanic 
eruption. 

Shocks  of  great  intensity  may  crack  and  overthrow  buildings, 
fracture  rocks,  fissure  the  ground,  propagate  earth- waves  and  tidal 
waves. 

Earthquakes  that  originate  at  fault-planes  may  throw  down 
forests,  shatter  rocks,  and  cause  other  destruction  for  hundreds  of 
miles  along  the  line  of  fracture  or  dislocation.  Shocks  resulting 
from  volcanic  explosions  are  frequently  sharp  and  destructive,  but 
the  effects  are  generally  local  and  confined  to  the  volcanic  zone, 
which  may  be  bounded  by  fault-planes. 

The  standing  pillars  of  the  Temple  of  Jupiter  Serapis  in  the 
Bay  of  Baiae,  a  few  miles  north  of  Naples,  are  an  example  of  the 


EARTH-MOVEMENTS .  135 

extreme  steadiness  of  the  subsidence  and  elevation  that  may  take 
place  even  in  a  volcanic  region  where  the  movement  is  relatively 
rapid. 

The  ruins  stand  within  a  stone's-throw  of  the  water's  edge,  and 
on  all  sides  are  extinct  craters.  Less  than  a  mile  to  the  north-east 
lies  the  well-known  crater  of  Solfatara,  still  in  the  expiring  stages 
of  volcanic  activity  ;  and  about  two  miles  to  the  north-west  is  the 
beautifully  symmetrical  cone  of  Monte  Nuovo. 

Three  standing  marble  pillars  rise  from  a  level  pavement. 
About  10J  feet  from  the  base  the  columns  are  pitted  with  holes 
made  by  boring  molluscs.  The  length  of  column  pitted  in  this 
way  is  a  little  over  8  feet.  Near  the  top  of  the  perforated 
portion  there  is  a  slight  annular  indentation,  in  all  probability  the 
work  of  marine  corrosion,  which  would  indicate  that  the  land 
remained  stationary  at  that  point  for  some  time. 

The  presence  of  the  borings  proves  that  the  pillars  must  have 
been  submerged  in  sea-water  for  some  considerable  time  and 
afterwards  elevated  to  their  present  level.  The  original  height 
of  the  temple  above  sea-level  is  not  known.  The  level  of  the 
platform  is  still  a  little  below  high-water  mark,  and  a  second 
platform  exists  5  feet  below  the  first,  indicating  that  an  earlier 
subsidence  had  rendered  it  necessary  to  construct  a  new  floor 
at  a  higher  level.  Therefore,  within  historic  times,  we  have  proof 
of  an  up-and-down  vertical  movement  amounting  to  45  feet,  on 
the  assumption  that  the  original  lower  floor  was  only  2  feet  above 
high-water  mark  when  first  constructed. 

Raised  shelly  strands  and  other  evidences  of  recent  elevation 
may  be  seen  all  along  the  west  coast  of  Italy  as  far  south  as  the 
Straits  of  Messina  ;  and  similar  evidences  of  elevation  are  found 
on  the  shores  of  Tunis  and  Algeria. 

Thus,  as  between  the  east  and  west  coasts  of  Italy  we  have  a 
tilting  movement  in  progress,  the  axis  of  which  runs  parallel  with 
the  peninsula.  The  Adriatic  shores  of  Italy,  however,  are  under- 
going submergence. 

The  most  recent  uplift  of  which  we  have  authentic  evidence  took 
place  during  the  great  Yukutat  earthquake  in  Alaska,  in  September 
1899,  as  the  result  of  displacement  along  pre-existing  fault-planes. 
The  vertical  uplift  varied  from  7  to  47  feet,  as  attested  by  the 
presence  of  barnacles  and  bunches  of  mussel  shells  attached  to  the 
ledges  of  rock  high  above  the  present  tide-mark. 

Tilting  of  Strata. 

Dip. — When  a  bed  or  stratum  is  tilted  so  as  to  be  inclined  in 
some  direction,  the  direction  of  the  inclination  is  called  the  dip  ; 


136  A  TEXT-BOOK  OF  GEOLOGY. 

and  it  is  always  the  steepest  line  of  the  inclined  surface  that  shows 
the  true  direction  of  the  dip. 

The  amount  of  the  inclination,  or  the  angle  of  dip  as  it  is  generally 
called,  is  always  measured  from  the  plane  of  the  horizon. 

Thus  we  have  : — 

The  direction  of  inclination  =  dip. 

The  amount  of  inclination  =  angle  of  dip  measured  from  the 
plane  of  the  horizon. 


FIG.  64.— To  illustrate  dip  and  strike  of  strata. 

The  direction  as  well  as  the  amount  of  the  dip  is  always  observed 
and  recorded.  The  direction  of  the  dip  is  determined  with  a 
compass,  and  the  amount  of  dip  with  a  clinometer.  The  ordinary 
geological  box-compass  is  generally  provided  with  a  pendulum-bob 
for  recording  angles  of  inclination  on  a  graduated  scale. 


w. 

FIG.  65. — Showing  angle  of  dip=x. 

The  amount  of  the  dip  may  vary  from  0°  to  90°.  At  90°  the  beds 
are  said  to  be  vertical. 

In  fig.  65  the  bed  of  limestone  marked  a  dips  to  the  west  at  an 
angle  of  inclination  =  x. 

Strike. — The  horizontal  line  along  the  tilted  stratum  is  called 
the  strike,  and  it  is  always  at  right  angles  to  the  dip. 

In  metalliferous  mining  the  strike  of  a  lode  is  commonly  spoken 
of  as  the  course  of  the  lode.  So  that  the  horizontal  direction 
pursued  by  the  course  of  a  lode  or  bed  is  the  strike.  If,  for  example, 
a  seam  of  coal  or  a  metalliferous  lode  crops  out  on  a  plain  or 


EARTH-MOVEMENTS.  137 

level  ridge,  the  line  joining  the  various  outcrops  is  the  line  of  strike 
or  course  of  the  seam  or  lode. 

If  a  tent-fly  raised  on  a  ridge-pole  be  taken  to  represent  a 
folded  stratum  of  rock,  then  the  direction  in  which  the  ridge  lies 
will  represent  the  strike. 

Furthermore,  if  the  tent-fly  represents  an  anticline,  then  the 
ridge  is  the  axis  of  the  anticline,  as  previously  described. 

Folding  of  Strata. 

The  majority  of  sedimentary  or  clastic  rocks  were  laid  down  as 
horizontal  layers  on  the  floor  of  some  sea  or  lake.  When,  therefore, 


FIG.  66. — Tent-fly  on  ridge-pole,  illustrating  strike  of  strata. 

we  find  such  rocks  forming  hills  and  high  mountain-chains,  we 
are  compelled  to  conclude  that  they  have  been  elevated  by  some 
powerful  agency.  And  when  on  closer  examination  we  find  that 
these  strata  are  not  always  horizontal,  but  in  many  places  tilted 
and  folded,  we  are  further  compelled  to  conclude  that  they  have 
been  subjected  to  enormous  side-pressure. 

It  must  always  be  remembered  that  although  rocks  are  so  hard 
and  resistant,  they  can  be  crumpled  up  and  corrugated  like  a  thin 
sheet  of  iron  when  sufficient  lateral  pressure  is  exerted  on  them. 

The  mechanics  of  the  folding  of  strata  can  be  illustrated  in  a 
graphic  manner  by  the  following  simple  experiment : — 

Take  fifty  strips  of  cloth  of  different  colours,  about  2  feet  long 
and  6  inches  wide,  and  pile  them  one  over  another  on  a  flat  table 
(fig.  67).  The  strips  of  cloth  represent  a  series  or  succession  of 
horizontal  strata, 


138 


A    TEXT-BOOK    OF    GEOLOGY. 


Next  place  a  board  on  top  of  the  pile  and  apply  vertical  pressure 
on  the  board.  It  will  be  observed  that  the  horizontal  position 
of  the  layers  is  not  disturbed. 

Suppose,  however,  that  we  now  place  two  light  weights  on  the 
board,  and,  holding  a  small  piece  of  cardboard  in  each  hand,  apply 
pressure  on  the  ends  of  the  layers,  slowly  bringing  our  hands 
towards  each  other  under  the  board.  The  cloth  will  now  be  found 
to  be  puckered  up  into  a  number  of  folds  of  various  form  and 
size.  Moreover,  the  distance  between  the  ends  will  be  greatly 
reduced. 

In  this  experiment  we  have  a  good  example  of  what  takes  place 


FIG.  67. — Showing  pile  of  cloth. 

when  horizontal  strata  are  folded.  The  lateral  pressure  throws  the 
strata  into  folds,  some  of  which  may  be  gentle  undulations,  and 
others  sharp  corrugations  according  to  the  force  exerted  and  the 
character  of  the  rocks. 

It  is  important  to  note  that  the  strata  when  folded  cover  a 
smaller  area  than  when  lying  horizontal. 


FIG.  68. — Showing  pile  of  cloth  folded  by  lateral  pressure. 

In  strata  that  have  been  deeply  involved  in  folds  it  is  not  un- 
usual to  find  the  contained  fossils  and  pebbles  deformed,  elongated, 
and  even  sheared  in  the  direction  of  the  lateral  movement,  which 
is  sometimes  called  lateral  thrust. 

The  folding  and  crumpling  of  strata  are  generally  believed  to 
be  due  to  lateral  pressure  arising  from  the  sinking  of  crustal 
segments  upon  the  cooling  and  contracting  interior. 

Thus,  when  a  block  of  horizontal  strata  is  squeezed  into  a  smaller 
segment  the  effect  is  to  crumple  the  strata  into  folds. 

The  arch  of  a  fold  is  termed  an  anticline,1  and  the  trough  a 
syncline.2 

1  Gr.  anti  =  opposite,  and  klino  =  l  incline. 

2  Gr.  syn  =  together,  and  klino  =  l  incline. 


EARTH-MOVEMENTS. 


139 


In  a  sheet  of  corrugated  iron  the  ridges  will  represent  anticlines, 
and  the  troughs  synclines. 

The  line  running  along  the  crest  of  a  ridge  is  called  the  axis  of 
the  ridge,  i.e.  the  anticlinal  axis,  and  the  line  along  the  bottom  of 
a  trough  the  synclinal  axis.  For  example,  the  ridge  of  a  tent-fly 
supportedjby  a  ridge-pole  is  the  axis  of  the  roof. 


B  A 

FIG.  69.— Showing  folded  strata. 
A,  Anticlines.  B,  Syncline. 

The  sides  of  an  anticline  or  of  a  syncline  are  called  the  limbs. 
In  an  anticline  we  speak  of  arch-limbs  because  they  form  the  arch ; 
and  in  a  syncline,  of  trough-limbs  because  they  slope  down  so  as 
to  form  a  trough. 


drch  or 
Anticline 


Trough  or 
Syncline 


FIG.  70. — Diagram  showing  parts  of  a  fold.     (After  Lapworth.) 

(ac)  Core  of  anticline.  (al)  Arch-limb. 

(tc)  Trough-core.  (tl)  Trough-limb. 

(ml)  Middle  limb. 

It  is  obvious  that  in  the  case  of  a  syncline  following  an  anticline, 
the  adjacent  limb  will  belong  to  both,  and  is  therefore  called  the 
middle  limb,  as  shown  in  fig.  70. 

Different  Forms  of  Folds. — According  to  the  character  of  the 
strata  and  the  amount  of  compression,  folds  may  assume  different 
forms.  The  corrugations  on  a  sheet  of  corrugated  iron  are  sym- 
metrical, but  symmetrical  folding  is  not  very  common  in  Nature. 
More  frequently  one  side  or  limb  of  a  fold  is  steeper  than  the 


140 


A    TEXT-BOOK    OF    GEOLOGY. 


other,  and  as  this  is  the  common  type  of  fold,  it  is  called  normal 
folding. 

In  fig.  71  we  have  a  series  of  beds,  including  a  seam  of  coal 
arranged  in  two  anticlines  and  two  synclines,  the  folding  being 
normal. 

In  cases  of  sharp  folding,  one  limb  may  be  vertical  or  actually 
pushed  over  beyond  the  vertical.  A  turned- over  fold  is  called 
an  over  fold  or  inverted  fold. 


FIG.  71. — Showing  folded  strata  in  cross-section  and  plan. 

When  an  overturned  fold  is  pushed  over  so  far  that  the  limbs 
are  parallel  and  nearly  horizontal,  we  have  what  is  termed  a 
recumbent  fold. 

In  what  is  termed  a  monoclinalfold,  the  strata  are  bent  from  the 


FIG.  72. — Section  showing  fold  with  vertical  limb  a, 
and  overfold  at  6. 


normal  direction  for  a  distance  and  then  resume  the  original  plane. 
In  sharply  bent  monoclinals,  the  strata  in  the  middle  limb  are 
generally  drawn  out,  compressed,  or  deformed. 

A  notable  example  of  monoclinal  folding  is  seen  in  the  Isle  of 
Wight  where,  on  the  south  side  of  the  island,  the  Cretaceous  rocks 
are  tilted  till  they  are  almost  vertical,while  the  Lower  Tertiary  strata 
follow  with  a  similar  inclination,  but  rapidly  flatten  down,  going 
northwards  till  they  become  horizontal  on  the  north  coast. 

A  succession  of  closed  overturned  folds  forms  what  is  termed 
an  isoclinal.  Folds  of  this  type  are  frequently  met  with  among 


EARTH-MOVEMENTS . 


141 


the  older  schists  and   gneisses,  and   sometimes  among    Mesozoic 
rocks. 

Strata  that  have  been  uplifted  in  the  form  of  a  dome  so  as  to 
incline  outwards  in  all  directions,  are  said  to  have  a  qua-qua- 
versal  dip. 


FIG.  73. — Showing  monoclinal  folding  of  Lower  Tertiary  strata  in  section  of 
the  Isle  of  Wight,  Totland  Bay  to  Headon  Hill.     (After  H.  W.  Bristow.) 

a.  Chalk. — Cretaceous. 


b.  Reading  Beds. 

c.  London  Clay. 

d.  Lower  Bagshot  Beds. 

e.  Bracklesham  Beds. 
/.  Barton  Clay. 

g.  Barton  Sand. 


h.  Headon  Beds.  } 

i.  Osborne  Beds.  (  ^. 

w  ft.  Bembridge  and         { Oligocene. 

Eocene'  Hamstead  Beds.  ) 

m.  Gravels. — Recent. 


FIG.  74. — Showing  isoclinal  or  closed  folds. 

Great  arches  or  great  troughs  with  minor  corrugations  on  their 
flanks  are  termed  geanticlinals  (or  anticlinoria)  and  geosynclinals 
(or  synclinoria)  respectively. 

In  the  central  massif  of  the  Alps,  the  strata  are  arranged  in  a 
singular  radial  form,  with  great  flanking  corrugations.  This  is 
termed  fan-folding,  of  the  Alpine  type. 


142 


A    TEXT-BOOK    OF    GEOLOGY. 


Radial  folding  on  a  minor  scale  is  sometimes  seen  in  volcanic 
regions,  arising  from  rapid  local  subsidence  accompanied  by  lateral 
pressure. 

Plication  of  Strata. — Plication  is  merely  a  form  of  minute  folding. 
It  is  frequently  seen  among  gneisses,  mica-schist,  and  other  meta- 
morphic  rocks  that  have  been  subjected  to  enormous  lateral 


FIG.  75. — Showing  example  of  fan-folding  in  European  Alps. 
(After  Heim.) 

pressure.     A  great  many  plications  are  sometimes  seen  in  the 
space  of  an  inch. 

Complicated  plication  has  given  rise  to  the  term  contorted,  which 
is  frequently  applied  to  banded  rocks  that  have  been  crumpled 
up  into  minute  folds.  Thus  some  gneisses  and  mica-schists  are 


FIG.  76. — Showing  thinning  of  coal-seam  due  to  contortion  of  coal- 
measures  in  the  Saint  Eloy  Basin,  France.     (After  De  Launay.) 

spoken  of  as  highly  contorted,  which  usually  means  that  the  rocks 
are  finely  plicated. 

The  hardest  and  most  resistant  rocks,  under  the  influence  of 
great  stress,  behave  as  semi-plastic  bodies.  In  the  process  of 
folding,  the  limbs  of  the  plications  are  frequently  found  to  have 
been  squeezed  until  they  have  become  thin.  Where  this  has 
happened,  the  crests  and  troughs  of  the  folds  usually  show  a  corre- 
sponding thickening,  the  flowage  being  from  the  region  of  greatest 
stress  in  the  limbs  to  the  places  of  least  stress  in  the  arches  and 


EARTH-MOVEMENTS . 


143 


troughs.     The  hardest  mineral  substances,  even  quartz,  seem  to 
be  capable,  of  flowage  under  the  influence  of  sufficient  pressure. 

Shales,  sandstones,  and  nearly  all  sedimentary  rocks  exhibit 
the  same  thinning  in  the  limbs  of  sharp  folds  due  (a)  to  compression 
and  consolidation  of  the  constituents,  or  (6)  to  the  elongation 
arising  from  the  shearing  which  has  so  frequently  accompanied 
sharp  folding  and  crumpling  of  the  strata. 


S.W.  N.L 

FIG.  77. — Showing  effect  of  overthrust  folding. 

Overthrust  and  Shearing. — When  an  overturned  fold  is  thrust 
against  a  boss  of  granite  or  a  mass  of  any  hard  resistant  rock, 
the  lateral  pressure  may  cause  the  fold  to  override  the  strata 
lying  against  the  resistant  boss.  In  this  way  rocks  may  be  gathered 


FIG.  78. — Showing  outcrop  of  horizontal  strata  in  a  gorge. 

up  in  great  earth-folds  and  pushed  for  many  miles  from  the  place 
where  they  were  originally  formed.  Notable  examples  of  over- 
thrust  are  found  in  the  Alps  and  in  the  Highlands  of  Scotland. 

When  the  force  exerted  in  the  folding  exceeds  the  elastic  limit  of 
the  rock,  rupture  takes  place  usually  in  the  apex  of  the  fold,  followed 
by  shearing  and  acute  faulting. 

Outcrop. — The  edges  of  strata  which  appear  at  the  surface  are 
called  the  outcrop.  The  exposed  edges  of  hard  resistant  rocks 


144  A   TEXtf-BOOK   OF   GEOLOGY. 

sometimes  form  conspicuous  escarpments  that  can  be  traced  for 
many  miles. 

When  the  strata  are  horizontal,  the  outcrop  of  the  different 
beds  will  only  be  seen  in  a  sea-cliff,  valley,  or  gorge. 

On  sloping  ground  the  extent  of  the  outcrop  does  not  represent 
the  true  thickness  of  the  beds.  For  example,  in  fig.  78  the  thick- 
ness of  the  bed  lying  between  the  two  bands  of  limestone  is  not 
a-b,  but  the  line  x  at  right  angles  to  the  plane  of  the  bed. 

Outcrop  Sag  or  Curvature. — In  dissected  areas,  weak  rocks,  such 
as  mica-schist  and  shales,  are  frequently  found  to  be  bent  or 
curved  at  the  outcrop.  This  sag  sometimes  renders  it  difficult  to 
obtain  trustworthy  observations  as  to  the  true  dip  of  the  strata. 
It  is  caused  by  the  drag  or  sag  of  the  outcrop  ends  of  the  beds 
arising  from  the  stress  due  to  their  own  weight  (fig.  79). 

In  regions  that  were  at  one  time  covered  with  a  thick  sheet  of 


B 


FIG.  79. — Showing  effects  of  outcrop  sag  of  strata. 
A,  In  horizontal  beds.         B,  In  tilted  beds* 


ice,  it  is  not  uncommon  to  find  weak  rocks  bent,  or  even  crumpled 
up  and  shattered,  for  many  yards  below  the  surface. 

Denudation  of  Folds. — Many  beds  that  have  been  folded  do  not, 
as  we  now  see  them  exposed  at  the  surface,  show  complete  anti- 
clines and  synclines.  In  most  cases  the  crowns  or  crests  of  the 
folds  have  been  removed  by  denudation,  so  that  it  is  only  by 
plotting  the  dip  and  strike  of  the  different  outcrops  that  we  are 
able  to  tell  that  the  folds  exist. 

It  is  probable  that  the  folding  of  the  strata  and  denudation 
proceeded  at  the  same  time.  In  this  case  the  crown  of  the  fold 
would  be  worn  away  in  such  a  manner  that  the  complete  arch,  as 
indicated  by  the  dotted  lines  in  fig.  80,  probably  never  existed. 

It  is  not  often  that  the  apex  of  an  anticline  is  seen,  except  in  the 
case  of  small  folds. 

Strata  arranged  in  the  form  of  an  anticline  will  be  worn  away 
faster  than  the  same  beds  disposed  in  the  form  of  a  syncline.  In 
the  anticlinal  arrangement  the  limbs  dip  away  from  each  other, 
which  permits  the  rain  to  find  its  way  readily  along  the  bedding 
planes,  where  it  disintegrates  the  rock,  thereby  assisting  the  force 


EARTH-MOVEMENTS. 


145 


of  gravity  in  breaking  up  the  outcrops.  The  rain-water  lying 
between  the  bedding  planes  by  its  hydraulic  pressure  also  exerts  a 
strong  disruptive  force  which,  in  cold  climates,  will  receive  effective 
help  from  frost. 

In  the  case  of  the  synclinal  arrangement,  the  limbs  dip  towards 
one  another,  the  different  beds  resting  on  each  other  like  a  pile  of 
saucers.  The  beds  thus  support  one  another,  and  consequently 
their  exposed  edges  alone  are  subject  to  the  effects  of  denudation. 


ENE 


FIG.  80. — Showing  denuded  crown  of  anticlinal  fold  of  Silurian 
rocks  in  the  Valley  of  Woolhope,  Herefordshire. 

Hence,  we  frequently  find  that  the  valleys  have  been  excavated 
along  the  course  or  axis  of  anticlines,  while  in  the  adjacent  ridges 
the  beds  are  arranged  in  synclinal  folds  as  shown  in  fig.  81. 

Outliers  and  Inliers. — These  commonly  arise  from  the  denuda- 
tion of  horizontal  or  gently  undulating  strata.  Many  formations 
that  at  one  time  formed  a  continuous  sheet  over  extensive  areas 


FIG.  81. — Showing  valley  excavated  along  course  of  an  anticline,  and 
ridges  composed  of  beds  arranged  in  synclinal  folds. 

have  been  greatly  reduced  in  size,  and  in  some  cases  they  are 
now  represented  by  only  a  few  interrupted  sheets  and  isolated 
patches. 

A  notable  example  of  the  gradual  destruction  of  a  formation  is 
exhibited  by  the  Desert  Sandstone  which  at  one  time  covered  over 
400,000  square  miles  of  the  surface  of  Queensland,  but  has  now 
been  worn  away  to  about  a  twentieth  of  its  former  extent,  and  is 
represented  only  as  a  series  of  isolated  ridges,  peaks,  plateaux,  and 
mesas l  scattered  over  the  interior. 


1  Span.  Mesa  =  a.  table. 


10 


146  A  TEXT-BOOK  OF  GEOLOGY. 

An  outlier  is  simply  an  isolated  remnant  of  more  extensive  beds, 
and  is  usually  denned  as  a  detached  mass  of  rock  surrounded  on 
all  sides  by  older  rock. 

Outliers  occur  among  all  kinds  of  rock,  including  loose  gravel. 
The  beds  forming  them  may  be  horizontal,  inclined,  or  folded. 
Examples  are  frequently  met  with  in  front  of  prominent  escarp- 
ments of  limestone  and  basalt. 

Outliers  of  Jurassic  and  Cretaceous  strata  are  common  in  Central 
England.  Table-topped  outliers  of  basalt  are  numerous  in  New 
Zealand  and  Victoria,  and  in  all  countries  where  dissected  sheets  of 
basaltic  lava  cover  the  ancient  plateaux. 

An  inlier  is  the  converse  of  an  outlier.  It  consists  of  an  isolated 
mass  of  rock  on  all  sides  surrounded  by  younger  rocks.  An  inlier 
is  the  result  of  denudation,  hence  most  frequently  met  with  in 
valleys,  or  in  places  where  the  arch  of  an  anticline  has  been  partially 
worn  away. 

Inlier  Outlier 


SSW  HNE. 

FIG.  82. — Section  along  west  side  of  Weka  Pass,  N.Z.,  showing  outlier. 
(1)  Weka  Pass  stone.  (2)  Amuri  limestone.  (3)  Greensands. 

An  example  of  an  inlier  is  seen  at  the  point  marked  a  in 
fig.  81. 

Crustal  Folding  and  Mountain  Building. — All  the  principal 
mountain  chains  on  the  planet  owe  their  existence  to  the  uprising 
of  crustal  folds,  and  mountain-making  may  be  defined  as  the  result 
of  localised  folding  in  regions  where  the  uplift  is  faster  than  the  rate 
of  denudation.  If  the  rate  of  denudation  were  equal  to  the  rate  of 
uplift,  it  is  obvious  that  the  truncated  and  dissected  folds  would 
never  form  highlands  or  features  of  bold  relief.  A  potential 
mountain-making  fold  suppressed  by  denudation  would  in  time 
become  buried  in  the  waste  derived  from  its  own  destruction,  and 
the  ultimate  result  would  be  a  worn-down  stump  indistinguishable 
from  the  stump  of  an  alpine  chain  worn  down  by  long-continued 
subaerial  denudation. 

Alpine  chains  may  therefore  be  regarded  as  the  expression  of 
relatively  rapid  folding. 

It  is  seldom  that  a  mountain  chain  consists  of  a  simple  synclinal 
fold.  More  often  the  great  chains  consist  of  a  series  of  deeply 
dissected  isoclinal  folds  forming  a  confused  alpine  complex,  flanked 


EARTH-MOVEMENTS.  147 

by  many  more  or  less  parallel  ranges.  The  great  height  and  rugged 
contours  of  the  Pyrenees,  Alps,  Himalayas,  Andes,  and  Rocky 
Mountains  are  an  evidence  of  their  comparative  youth.  Their 
uplift  has  been  so  recent  and  rapid,  that  denudation  has  merely 
succeeded  in  eroding  the  crests  of  the  folds  into  narrow  serrated 
ridges,  deep  valleys,  and  profound  gorges. 

There  is  evidence  that  great  alpine  chains  existed  in  the  remotest 
geological  ages.  All  of  these  chains  were  vigorously  attacked  by 
the  contemporary  agents  of  denudation,  their  waste  furnishing  the 
sediments  that  built  up  the  later  formations.  The  only  vestige 
that  remains  of  these  primitive  alps  is  their  worn-down  stumps, 
many  of  which  have  lain  for  countless  aeons  buried  beneath  piles  of 
sedimentary  strata.  Here  and  there  the  buried  stumps  have 
become  exposed  by  recent  denudation,  or  disclosed  by  deep  boring. 

When  a  renaissance  of  the  folding  movements  takes  place  along 
the  segment  occupied  by  a  worn-down  and  buried  alpine  chain,  a 
second  alpine  chain  may  rise  on  the  ruins  of  the  first.  Most  of  the 
existing  mountain  chains  occupy  the  ruins  of  Palaeozoic  chains, 
the  stumps  of  which  have  sometimes  become  involved  in  the  later 
folds. 

In  the  complex  structure  of  Europe,  Lapworth  and  Suwess  have 
recognised  three  primary  folded  chains  all  overfolded  towards  the 
north,  namely  : — 

(1)  The  Caledonian,  S.W.-N.E.—  Pre-Devonian. 

(2)  The  Armorican,  W.N.W.-E.S.E.— Pre-Permian. 

(3)  The  Variscan,  W.S.W.-E.N.E.,— Pre-Pliocene. 

The  Caledonian  is  the  northernmost  of  these  ancient  alpine  chains. 
It  is  composed  of  a  massif  of  Archaean  granites,  gneisses,  schists, 
and  older  Palaeozoic  slates,  sandstones,  and  quartzites  that  extend 
from  the  west  of  Scotland  north-eastward  to  the  northern  limits  of 
Scandinavia.  It  causes  little  surprise  to  find  that  rocks  of  such 
antiquity  have  been  crushed  into  many  complex  folds,  overthrust, 
and  profoundly  faulted. 

The  Armorican  folded  chain,  so  named  from  the  ancient  name  of 
Brittany,  is  the  result  of  movements  that  ridged  up  the  Devonian 
and  older  Carboniferous  formations  between  the  close  of  the  Early 
Carboniferous  and  the  advent  of  the  Permian.  The  truncated 
remains  of  this  western  alpine  chain  can  still  be  traced  from  Ireland 
to  Central  France,  buried  beneath  a  pile  of  Mesozoic  and  younger 
formations. 

The  Variscan  or  eastern  fold  is  of  Cainozoic  date.  It  extends 
from  the  Atlantic  border  through  Southern  France  to  Northern 
Bohemia,  and  includes  the  Pyrenees,  Alps,  and  Carpathians. 

In  Asia,  the  Caucasus,  Hindu  Kusch,  Altai,  Thian  Shan,  and 


148  A  TEXT-BOOK  OF  GEOLOGY. 

Himalaya  Mountains  are  composed  of  folds  of  the  Variscan  type, 
the  axes  of  which  lie  approximately  parallel  with  the  Equator. 

The  great  Ural  chain  consists  of  folds  running  north  and  south. 

In  the  American  continents,  the  Pacific  Ocean  is  bordered  by 
high  mountain  chains  that  are  surmounted  by  many  active  and 
extinct  volcanoes.  These  are  typical  folded  chains  of  the  meridional 
type,  but  turn  their  folds  towards  the  abysses  of  the  sea.  The 
segment  of  the  Aleutian  Islands  forms  an  independent  fold. 

It  is  significant  that  all  the  great  chains  of  Europe,  Asia,  and 
North  Africa,  with  the  exception  of  the  Urals,  are  composed  of  east 
and  west  folds  ;  while  the  Andes,  Rocky  Mountains,  and  Sierras 
in  America  run  parallel  with  arcs  of  the  meridian. 

Volcanic  activity  is  frequently  associated  with  areas  of  vertical 
displacement  resulting  from  faulting  or  intense  folding;  and  all 
great  earthquakes  are  the  jolts  propagated  by  the  revival  of  move- 
ment along  ancient,  but  in  most  cases  well-defined  fault-lines. 

SUMMARY. 

Elevation. — (1)  The  occurrence  of  rocks  containing  marine  shells 
at  a  height  above  sea-level  is  an  evidence  of  elevation  of  the  land 
in  past  geological  times.  The  existence  of  raised-beaches  or  sea- 
strands  is  a  proof  of  comparatively  recent  elevation. 

Subsidence. — (2)  The  best  proofs  of  subsidence  are  submerged 
forests  and  coal-seams,  drowned  valleys,  and  atolls. 

The  forests  grew  on  the  dry  land  near  the  sea,  and  could  only 
become  submerged  by  the  sinking  of  the  coastal  region.  Likewise 
coal-seams  are  composed  of  the  remains  of  a  terrestrial  vegetation 
that  required  air  and  sunlight  for  its  growth.  Therefore,  when 
seams  are  found  thousands  of  feet  below  sea-level,  we  know  that 
subsidence  to  that  extent  has  taken  place. 

The  fiords  of  Norway  and  New  Zealand  are  merely  submerged 
mountain- valleys.  In  California  and  elsewhere  some  of  the  existing 
river- valleys  can  be  traced  by  soundings  far  seaward  of  the  present 
shore-line — clearly  a  proof  of  subsidence  in  quite  recent  times. 

According  to  the  view  of  Darwin,  atolls  and  barrier  reefs  were 
formed  by  the  upward  growth  of  the  coral-building  polyp  on  a 
slowly  sinking  sea-floor.  The  borings  at  Funafuti  showed  the 
existence  of  coralline  and  foraminiferal  limestones  at  a  depth  of 
1114  feet  below  sea-level ;  and  since  the  coral  polyp  cannot  live 
in  water  deeper  than  say  150  feet,  a  subsidence  of  over  900  feet  must 
have  taken  place  in  that  area. 

As  the  land  sinks  below  the  sea,  the  coral  reefs  grow  upward, 
and  their  distance  from  the  shore-line  increases  until  in  the  case  of 
a  continent  or  large  island  a  barrier  reef  is  formed,  and  in  the  case 


EARTH-MOVEMENTS.  149 

of  a  small  island  an  encircling  reef.  When  the  land  encircled  by  a 
coral  reef  finally  disappears  below  the  surface  of  the  sea,  an  atoll  is 
formed. 

(3)  Bapid  earth-movement  may  be  due  to  volcanic  eruptions 
or  to  the  sudden  jolts  arising  from  earthquakes. 

Tilting  of  Strata. — (4)  The  direction  in  which  a  stratum  or  bed  is 
inclined  is  called  the  dip  ;  and  the  amount  of  the  inclination 
measured  from  the  plane  of  the  horizon  is  the  angle  of  dip. 

The  strike  is  the  horizontal  line  along  the  tilted  stratum,  and  it  is 
always  at  right  angles  to  the  dip. 

Folding  of  Strata. — (5)  The  majority  of  sedimentary  or  aqueous 
rocks  were  laid  down  in  a  horizontal  position,  but  many  of  them 
have  since  been  pushed  into  folds  by  lateral  pressure  that  in  all 
probability  arose  from  the  cooling  and  contracting  of  the  Earth's 
crust. 

The  arches  of  folds  are  called  anticlines,  and  the  troughs,  synclines. 
Simple  symmetrical  folding  is  rare.  More  commonly  the  folds  are 
unsymmetrical,  the  limbs  being  shorter  and  steeper  on  one  side 
than  on  the  other. 

Folds  that  have  been  subjected  to  great  lateral  pressure  from  one 
direction  are  sometimes  pushed  over  and  form  what  are  called 
overturned  or  inverted  folds.  When  an  overturned  fold  is  pushed 
over  until  the  limbs  are  closed  and  nearly  horizontal,  it  is  called 
a  recumbent  fold. 

A  succession  of  closed  folds  that  are  overturned,  but  not  recum- 
bent, form  an  isoclinal. 

Great  anticlinal  arches  with  minor  corrugations  on  the  flanks  are 
called  geanticlinals,  and  great  troughs,  geosynclinals .  In  the  Alps 
of  Switzerland,  Heim  has  identified  what  is  thought  to  be  a  fan  or 
radial  form  of  folding. 

Plication  of  Strata.- — (6)  Plication  is  a  form  of  minute  folding 
frequently  seen  among  the  older  altered  rocks,  such  as  gneiss  and 
mica-schist.  Plication  may  be  very  complicated.  Kocks  that  are 
strongly  plicated  are  frequently  spoken  of  as  contorted.  Contorted 
schists  sometimes  exhibit  a  thinning  or  drawing  out  of  the  limbs 
of  the  folds  with  a  corresponding  increase  in  the  arches  and  troughs. 
This  indicates  that  a  certain  flowage  of  the  rock-constituents  took 
place  under  the  stress  of  enormous  pressure.  In  other  words, 
under  great  pressure  the  rock-constituents  behave  as  plastic 
bodies. 

Shearing. — (7)  When  rocks  are  subjected  to  a  travelling  lateral 
pressure  that  is  arrested  by  some  massif  of  hard  resistant  rock,  such 
as  a  granite-boss,  they  are  frequently  forced  into  sharp  overturned 
folds  which  may  become  fractured  and  sheared,  the  upper  portion 
of  the  fold  being  pushed  over  the  lower.  This  overthrusting  of 


150  A  TEXT-BOOK  OF  GEOLOGY. 

crustal  segments  has  been  observed  in  the  Alps  by  Heim,  and  in 
the  Highlands  of  Scotland  by  Peach. 

Outcrop. — (8)  The  edges  of  strata  that  appear  at  the  surface 
are  called  the  outcrop.  Weak  rocks  may  be  bent  or  curved  at  the 
outcrop  by  the  weight  of  their  own  mass  or  by  the  stress  of  a  sheet 
of  moving  ice.  Such  outcrop  curvature,  as  it  is  called,  is  common 
in  shales,  phyllite,  and  mica-schist. 

Denudation  of  Folds. — (9)  The  majority  of  folds  of  considerable 
size  have  been  denuded  to  a  greater  or  less  extent;  hence  their 
existence  is  generally  deduced  by  construction  from  the  observed 
dips  and  strikes  as  recorded  in  the  field. 

(10)  Outliers  are  the  isolated  remnants  of  a  rock-formation  that 
at  one  time  formed  a  continuous  sheet  over  an  extensive  area. 
They  may  be  defined  as  detached  masses  of  rock  surrounded  on  all 
sides  by  older  rock. 

An  inlier  is  the  converse  of  an  outlier.  It  consists  of  an  isolated 
patch  of  rock  surrounded  on  all  sides  by  younger  rock.  Hence 
inliers  are  most  frequently  found  in  the  exposed  crowns  of  anticlines. 

Crustal  Folding  and  Mountain  Building. — (11)  Mountain  chains 
are  composed  of  great  uplifted  crustal  folds  that  have  been  deeply 
dissected  during  the  progress  of  the  uplift. 


CHAPTER  X. 
JOINTS,   FAULTS,   CLEAVAGE. 

Joints. 

Joint  Structure. — Joints  are  simple  cracks  or  fissures.  They  are 
found  in  rocks  of  all  kinds  and  of  all  ages. 

Sedimentary  rocks  are  usually  traversed  by  two  systems  or  sets 
of  joints,  both  perpendicular  to  the  stratification  planes,  and 
commonly  intersecting  one  another  at  right  angles.  The  joints 
in  each  set  are  approximately  parallel  to  one  another. 

As  a  rule,  one  set  of  joints  is  more  pronounced  than  the  other, 
and  may  be  traced  for  many  yards.  The  joints  in  the  major  set 
are  commonly  called  master-joints. 

The  course  of  the  master- joints  is  usually  parallel  with  the  strike 
of  the  main  lines  of  uplift ;  that  is,  parallel  with  the  axes  of  the 
anticlines. 

The  two  sets  of  joints  and  the  bedding  planes  give  three  planes 
nearly  at  right  angles,  which  divide  the  rock  into  cuboidal  or  pris- 
matic blocks  and  columns. 

Rocks  that  have  been  much  disturbed  are  sometimes  intersected 
with  three  or  four  systems  of  joints.  Generally  speaking,  rigid 
rock  is  more  jointed  than  one  that  is  more  yielding. 

The  joints  in  each  set  may  be  many  feet  or  yards  apart,  or  in 
exceptional  cases  only  an  inch  or  less. 

In  horizontal  strata,  the  joints  are  usually  approximately 
vertical ;  but  in  regions  where  the  rocks  have  been  subject  to  great 
disturbance,  the  joint  planes  may  occupy  any  position. 

Joints  are  sometimes  mistaken  for  bedding  planes,  but  these  can 
usually  be  distinguished  by  lines  of  material  of  different  texture  or 
colour,  or  by  lines  of  nodules  and  hard  bands. 

Joints  are  of  necessity  confined  to  the  zone  of  fracture  ;  and  in 
the  majority  of  cases,  an  individual  joint  when  followed  along  its 
course  seems  to  die  out  in  less  than  a  score  of  yards,  to  be  succeeded 
after  a  longer  or  shorter  interval  by  another  joint  following  the  same 
general  direction. 

Many  joints  end'at  the  contact  of  two  kinds  of  rock,  but  master- 

151 


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A    TEXT-BOOK    OF    GEOLOGY. 


joints  may  pass  through  a  whole  series  of  rocks.  For  example, 
throughout  the  whole  of  Yorkshire  the  Mountain  Limestone 
Series  is  traversed  by  master- joints  passing  downward  through  the 
limestone,  sandstone,  and  shale  in  nearly  the  same  direction. 

Joints  are  more  or  less  open,  or  usually  filled  with  silt  and  mud 
carried  into  them  by  water.  In  many  cases,  more  especially  in 
limestones  and  other  calcareous  rocks,  they  have  been  enlarged 
into  gaping  fissures  or  caves  by  the  action  of  underground  waters. 
On  the  surface  they  frequently  become  enlarged  through  the 
solvent  effect  of  rain,  aided  by  the  ordinary  processes  of  weathering. 

Joint  planes  sometimes  show  polished  and  grooved  surfaces, 
which  would  tend  to  show  that  a  certain  amount  of  sliding  move- 
ment had  taken  place  parallel  to  the  polished  faces.  Evidence  of 
displacement  along  joint  planes  is  perhaps  exceptional. 

PMtion  of  Strata  tty  Matter  Joint$. 


Limetton* 


FIG.  83. — Showing  master- joints  passing  through  different  rocks. 

The  majority  of  the  older  coal-seams  are  traversed  by  two  sets 
of  vertical  joints  called  cleats,  crossing  one  another  at  right  angles. 
The  face  cleats  run  parallel  with  the  strike  of  the  seam  and  are 
usually  the  more  pronounced.  The  end  or  butt  cleats  are  shorter 
and  not  as  a  rule  so  well  defined. 

The  cleats  are  of  great  importance  in  facilitating  the  getting  of 
the  coal ;  hence  the  direction  of  the  working  faces  or  breasts  with 
reference  to  the  cleats  is  a  matter  of  supreme  importance. 

The  joints  in  igneous  rocks  are  not  generally  so  regular  or  well 
defined  as  in  sedimentaries.  But  in  exceptional  cases  they  are  so 
symmetrically  disposed  as  to  produce  the  well-known  prismatic 
columnar  structure  which  is  sometimes  seen  in  flows  of  basaltic 
rock,  and  less  often  in  andesites  and  rhyolites. 

Granite  is  frequently  intersected  with  two  sets  of  joints,  one  of 
which  is  sometimes  well  defined.  When  the  joints  are  far  apart, 
large  blocks  of  stone  can  be  obtained,  but  when  they  are  close 
together,  the  rock  is  broken  up  into  a  rubble  of  small  fragments 
(Plate  XIII.). 

The   master- joints,   in  whatever  rock  they  occur,   are  always 


JOINTS,    FAULTS,    CLEAVAGE.  153 

utilised  by  the  workmen  to  facilitate  the  hewing  of  the  stone  in 
blocks  that  can  be  turned  to  commercial  account. 

Causes  of  Jointing. — The  mechanics  of  jointing  has  not  yet  been 
satisfactorily  explained,  although  many  suggestions  have  been 
advanced  by  different  writers.  The  generally  accepted  opinion 
is  that  the  joint-cracks  are  the  result  of  the  various  stresses  con- 
nected with  the  contraction  and  folding  of  rock-masses. 

Among  the  stresses  referred  to  are  shrinkage  arising  from  the 
drying,  or  cooling  of  the  rock,  tension  and  shearing  due  to  folding. 

Thick  sheets  of  mud  when  drying  in  the  sun  develop  vertical 
cracks  due  to  the  dehydration  and  contraction  of  the  mass.  Sheets 
of  lava  in  the  portions  exposed  to  the  cooling  effects  of  the  atmo- 
sphere or  of  the  surface  on  which  they  rest,  as  they  cool  also  develop 
well-defined  cracks  that  only  in  exceptional  cases  show  the  sym- 
metrical arrangement  known  as  columnar  structure.  , 

In  sedimentary  rocks  the  master-joints  may  pass  downward 
through  different  kinds  of  rock  ;  and  in  passing  through  a  con- 
glomerate may  even  sever  the  constituent  pebbles  in  two.  Clearly 
then  the  jointage  in  these  cases  took  place  after  the  consolidation 
of  the  rocks. 

The  orientation  or  general  direction  of  the  master-joints  in  clastic 
rocks  is  usually  parallel  with  the  axes  of  the  folds,  which  would 
lead  us  to  the  conclusion  that  it  was  in  some  way  genetically 
connected  with  the  processes  or  mechanics  of  folding.  It  would 
seem  as  if  the  stresses  arising  from  the  bending  of  the  hardened  and 
rigid  rock-mass  were  relieved  by  the  formation  of  innumerable 
short  cracks  or  rents  running  parallel  with  the  main  line  of  uplift ; 
that  is,  parallel  with  the  strike.  In  other  words,  when  the  bending 
exceeded  the  elastic  limit  of  the  rock,  parallel  fractures  would  be 
formed. 

But  anticlinal  folds  have  a  beginning  and  an  end.  Some  may  be 
short  and  plunge  steeply  at  the  ends.  Others  may  extend  for 
scores  or  even  hundreds  of  miles  before  they  die  out. 

The  formation  of  an  anticlinal  fold  can  be  best  understood  by 
reference  to  a  simple  experiment.  Suppose,  for  example,  that  we 
place  a  long  pillow  or  bolster  lengthwise  on  a  table  and  over  it 
throw  a  sheet  or  table-cloth. 

It  will  at  once  be  seen  that  the  anticline  of  cloth  rises  gradually 
from  one  end  of  the  table  until  it  becomes  well  defined  along  the 
body  of  the  pillow ;  and  at  the  other  end  plunges  or  pitches,  as  it  is 
called,  until  it  finally  dies  out  as  the  horizontal  surface  of  the  table 
is  reached. 

By  a  similar  experiment  with  two  pillows  and  a  sheet  it  can  be 
shown  that  synclines  also  have  a  beginning  and  an  end. 

Let  us  now  suppose  that  the  table-cloth  is  replaced  with  a  sheet 


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A    TEXT-BOOK    OF    GEOLOGY. 


of  hardened  rock  bent  into  a  simple  fold.  It  is  obvious  that  the 
greatest  bending  stress  will  be  exerted  parallel  to  the  axis  of  the 
anticlinal  uplift.  If  the  rock  is  rigid  and  refuses  to  bend  easily, 
the  stress  will  be  relieved  by  the  formation  of  master-joints  running 
parallel  with  the  strike  or  axis  of  the  fold.  These  joints  will  be 
tension-cracks,  developed  in  the  tension-zone. 

There  will  also  be  a  tensional  stress  at  the  ends  of  the  folds  due 
to  the  extension  of  the  strata  as  it  rises  from  the  horizontal  position. 
This  stress  will  not  be  so  great  as  that  parallel  with  the  axis,  and  as 
a  consequence  it  will  be  relieved  by  the  formation  of  smaller  and 
shorter  joints. 

Joints  formed  by  anticlinal  folding  are  obviously  the  result  of 
tension  in  the  upper  layers  of  the  uplifted  mass  ;  while  those 
arising  from  synclinal  folding  are  the  result  of  tension  in  the  lower 
layers,  in  accordance  with  a  well-known  law  in  mechanics. 


Zone 

of         Compression 

Zone 

of         Tension 

c 

1 

t 

| 

11 

I 

1 

FIG.  84. — Showing  distribution  of  stresses  in  a  loaded  beam. 

For  example,  if  we  take  a  beam  of  wood,  supported  at  both  ends 
and  loaded  at  the  centre,  the  upper  layers  will  be  in  compression 
and  the  lower  in  tension,  as  shown  in  fig.  84. 

The  portion  of  the  beam  lying  above  the  neutral  axis  or  line  of 
no  stress,  c  c,  will  be  in  compression,  and  the  portion  below  the 
neutral  axis  in  tension.  The  magnitude  of  the  stresses  is  greatest 
at  the  upper  and  lower  surfaces  of  the  beam,  and  diminishes  as  the 
neutral  axis  is  approached,  as  graphically  shown  by  the  shaded 
portions  of  the  stress  diagram  at  a  and  6  (fig.  841). 

If  the  force  were  applied  from  below  so  as  to  cause  upward  bend- 
ing, or  a  tendency  to  bend,  the  upper  layers  would  obviously  be  in 
tension  and  the  lower  in  compression. 

Now,  bending  in  any  direction  is  always  accompanied  by  a 
horizontal  shearing  stress,  although  in  a  homogeneous  beam  or  mass 
this  stress  is  not  always  obvious.  But  its  existence  can  easily  be 
proved  experimentally. 

If  we  replace  the  solid  beam  with  a  pile  of  thin  boards,  supported 


JOINTS,    FAULTS,    CLEAVAGE. 


155 


at  both  ends  and  loaded  with  a  weight  W  at  the  centre,  the  boards 
will  not  only  be  bent,  but  they  will  also  slide  over  one  another  as  a 
result  of  the  horizontal  shearing  stress,  as  shown  in  fig.  85. 

Stratified  rocks  that  have  been  sharply  folded  frequently  exhibit 
evidence  that  slipping  or  shearing  has  taken  place  along  the  bedding 
planes  during  the  process  of  folding.  The  presence  of  a  layer  of 
clay,  as  well  as  grooves  and  striae  on  the  bedding-plane  surfaces,  are 
among  the  most  obvious  of  these  evidences.  In  cases  where  the 
shearing  stress  cannot  be  relieved  by  the  movement  of  the  beds, 
relief  may  be  found  by  the  formation  of  joints  or  cracks  at  right 
angles  to  the  line  of  force. 

The  jointing  of  rocks  may,  therefore,  be  set  down  mainly  to  the 
influence  of  tension  and  shearing  resulting  from  crustal  disturbance, 
sometimes  supplemented  by  the  stresses  introduced  into  rock- 
masses  by  shrinkage  during  the  processes  of  drying  and  cooling. 


FIG.  85. — Showing  effect  of  a  horizontal  shearing  stress. 


Faults. 

A  crack  or  rent  without  movement  of  the  rock  on  either  wall  is 
a  simple  fracture.  In  the  majority  of  large  cracks  there  has  not 
only  been  fracture,  but  also  displacement.  In  other  words,  the 
rents  have  become  what  are  known  to  geologists  and  miners  as 
faults. 

Definition  of  Fault. — A  fault  may  be  defined  as  a  fracture,  on  one 
side  of  which  movement  has  taken  place,  whereby  the  rocks  on  that 
side  have  been  displaced  relatively  to  those  on  the  other  side. 

Origin  of  Faults. — Faults  are  caused  by  crustal  stresses  arising 
from  the  slow  secular  folding  movements  that  build  up  mountain- 
chains,  or  from  the  sharper  movements  propagated  by  the  intrusion 
of  igneous  magmas  or  by  earthquakes.  That  is,  the  disturbing 
agents  may  be  orogenic  or  hypogenic.1 

A  fault  may  be  the  result  of  a  single  continuous  movement,  slow 
or  fast,  or  of  a  succession  of  slight  movements,  with  intervals  of 
quiescence.  The  renaissance  of  movement  on  an  ancient  fault- 

1  Gr.  hypo  =  under,  and  genesis  =  production. 


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A    TEXT-BOOK    OF    GEOLOGY. 


plane  may  be  responsible  for  the  production  of  earthquakes  and 
other  seismic  phenomena. 

In  regions  where  the  rocks  have  not  undergone  much  disturbance, 
but  approximately  occupy  the  original  position  in  which  they  were 
laid  down,  the  faults  are  probably  due  to  mere  subsidence  of 
crustal  blocks,  arising  from  the  action  of  vertical  shearing  stress. 
In  a  loaded  beam  this  stress  tends  to  fracture  the  beam  in  a  vertical 
direction,  and  in  every  crustal  segment  there  must  be  the  same 
tendency.  When  the  stress  exceeds  the  ultimate  strength  of  the 
rocks  composing  the  segment,  fractures  will  be  formed,  producing 
the  effect  known  as  step-faulting,  shown  in  fig.  100. 

Coal-mining  operations  have  shown  that  the  coal-measures  in 
most  lands  are  intersected  by  numerous  faults,  and  it  is  probable 
that  all  portions  of  the  Earth's  crust  are  dislocated  in  the  same 
way. 

Many,  if  not  the  majority,  of  the  great  faults  that  traverse  the 
crust  seem  to  be  connected  withholding  and  mountain-building. 


FIG.  85 A. — Showing  effect  of  vertical  shear. 

They  are  frequently  zones  of  dislocation  or  thrust-planes  rather 
than  true  faults. 

The  inclination  of  faults,  measured  from  the  horizon,  is  generally 
high,  being  in  most  cases  over  40°.  The  inclination  of  thrust- 
planes,  on  the  other  hand,  is,  as  a  rule,  quite  low,  seldom  exceeding 
an  angle  of  20°.  This  seems  to  be  a  consequence  of  their  origin, 
for  it  is  only  when  the  folds  have  been  thrust  into  a  nearly  recum- 
bent position  that  fracture  and  shearing  take  place. 

Relationship  of  Faults  and  Joints. — Joints  may  be  taken  as  the 
expression  of  the  internal  stresses  arising  in  disturbed  rock  masses  ; 
and  faults  as  the  expression  of  the  rupture  by  which  crustal  folds 
achieve  relief  when  the  stress  exceeds  the  limit  of  relief  afforded 
by  joints. 

Therefore,  while  joints  and  faults  are  essentially  different,  they 
can  both  be  traced  to  the  same  cause,  and  in  this  respect  they 
may  be  said  to  have  a  close  genetic  relationship. 

Relationship  of  Faults  and  Folding. — The  stress  of  sharp  folding 
is  frequently  relieved  by  the  formation  of  powerful  fractures  which 
by  movement  or  shearing  may  develop  into  faults.  In  fig.  86  we 


JOINTS,    FAULTS,    CLEAVAGE. 


157 


have  an  example  of  folding  without  fracture,  and  of  folding  with 
fracture,  accompanied  by  shearing,  resulting  in  the  formation  of 
a  shear-plane  or  fault.  An  example  of  overthrust  folding,  followed 
by  fracturing  and  faulting  of  the  folded  rocks,  is  shown  in  fig.  77. 

Linear  Extent  of  Faults. — -Faults,  like  joints,  have  a  beginning 
and  an  end.  They  begin  gradually,  attain  a  maximum,  and  then 
gradually  die  out.  Many  of  the  small  faults  met  with  in  mining 
regions  are  short,  frequently  less  than  a  hundred  yards  long.  Some 
large  faults,  however,  can  be  traced  for  scores  of  miles.  Such  great 
dislocations  should  perhaps  be  best  described  as  thrust-planes 
rather  than  mere  faults. 

The  course  of  large  faults  is  usually  more  or  less  sinuous,  and 
some  fault-faces  exhibit  many  minor  corrugations. 

Evidences  of  Faulting. — The  opposing  walls  or  surfaces  of  a  fault- 


FIG.  86. — Showing  effects  of  folding  and  fracture 
accompanied  by  faulting. 

plane  are  generally  polished,  scratched,  and  grooved  by  the  rubbing 
which  took  place  when  they  moved  against  one  another.  Such 
polished  and  striated  surfaces  are  called  slickensides. 

In  many  cases  the  fault-fissure  is  filled  with  crushed  rock  frag- 
ments, or  lined  with  a  layer  of  clay  which  is  known  to  miners  as 
pug.  This  clay  is  merely  rock-flour,  and  it  may  be  soft,  or  so  hard 
as  to  resemble  a  slaty  shale. 

In  powerful  faults  in  which  great  movement  has  taken  place, 
the  wall-rock  may  be  crushed  and  brecciated — that  is,  broken  into 
angular  fragments — for  a  width  of  many  yards.  For  example,  the 
plane  of  the  Moanataiari  fault  which  intersects  the  Thames  Gold- 
field  in  New  Zealand  is  in  places  occupied  by  a  zone  of  crushed 
rock  and  soft  clay,  varying  from  40  to  100  feet  wide. 

Many  fault-fractures  became  channels  for  the  circulation  of 
mineralised  waters,  which  deposited  in  them  quartz  or  other 
crystalline  minerals  together  with  ores  of  great  economic  value. 


158 


A   TEXT-BOOK   OF   GEOLOGY. 


It  was  in  this  way  that  many  of  the  most  valuable  ore- veins  were 
formed. 

Age  of  Faults.— A  fault  is  obviously  younger  than  the  rocks  which 
it  intersects,  and  when  the  rocks  are  traversed  by  two  systems  of 
faults,  one  system  will  generally  be  found  to  displace  the  other, 
thereby  affording  conclusive  evidence  that  it  is  the  younger. 

A  fault  that  traverses,  for  example,  a  pile  of  Cretaceous  and 
Miocene  strata  must  be  younger  than  Miocene.  If  the  Miocene 
beds  are  overlain  by  Glacial  Drift  which  is  not  disturbed  by  the 
fault,  then  the  date  of  faulting  took  place  some  time  after  the 
Miocene,  and  before  the  advent  of  the  Glacial  Period. 

Or  again,  if  the  fault  traverses  the  Cretaceous  beds  and  not  the 


FIG.  87.— To  illustrate  age  of  faults. 

A,  Where  faulting  took  place  after  deposition  of  beds  a. 

B,  Where  faulting  took  place  before  deposition  of  beds  a. 

(a)  Miocene  beds.  (6)  Cretaceous  beds. 

overlying  Miocene,  as  in  fig.  87,  B,  then  we  know  that  the  fault  is 
younger  than  Cretaceous,  but  older  than  Miocene. 

Movement  is  believed  to  be  still  in  progress  along  the  planes  of 
some  of  the  great  faults  of  late  Tertiary  date.  The  joltings  caused 
by  the  sudden  settlement  of  the  ground  on  the  downthrow  side 
of  the  San_Juan^  fault  are  held  by  some  to  have  been  responsible 
for  the  disastrous  earthquakes  that  ruined  San  Francisco  in  1906. 

Fault- Structure. — Faults  intersect  rocks  of  all  kinds  and  all  ages. 
Some  of  them  are  crustal  dislocations  of  great  magnitude  that  can 
be  traced  on  the  surface  for  scores  and  even  hundreds  of  miles. 

Faults  may  run  in  any  direction,  but  the  major  faults  of  a  region 
frequently  possess  the  same  general  bearing. 

A  fault  may  run  parallel  with  the  strike  of  a  bed  or  lode,  or  it  may 
cross  the  strike  at  right  angles  or  at  any  other  angle. 

Some  regions  are  intersected  by  a  number  of  parallel  faults,  and 
in  some  places  two  or  more  independent  systems  of  faults  may 
mutually  intersect  one  another. 


JOINTS,    FAULTS,    CLEAVAGE. 


159 


Hade  of  Faults. — faults  are  not  oftenl*  vertical,  but  generally 
incline  to  one  side  or  the  other.  A  fault  is  said  to  hade  when  it 
inclines  from  the  vertical  plane.  The  hade  of  a  fault  is,  therefore, 
the  angle  which  the  fault  makes  with  the  vertical  plane.  Hade 
and  angle  of  dip  are  thus  only  the  same  when  both  are  45°. 

The  hade-line  of  a  fault  is  the  resultant  of  two  principal 
component  forces,  namely,  gravitational  stress  acting  vertically 
towards  the  centre  of  the  Earth,  and  lateral  thrust  mainly  due 
to  subsidence. 

Classification  of  Faults.— Faults,  according  to  the  direction  of 
the  vertical  displacement,  are  divided  into  two  classes,  namely : — 

(a)  Normal  faults. 

(b)  Reversed  or  overlap  faults. 

In  normal  faults  the  downthrow  of  the  beds  or  lode  is  towards 


W. 


FIG.  88. — Cross-section  showing  direction  of  downthrow  in  normal  fault. 

the  side  to  which  the  fault  inclines  or  hades.  Thus,  in  fig.  88  the 
hade  and  downthrow  are  both  in  the  same  direction.  This  is  the 
most  common  type  of  fault ;  hence  the  name  normal. 

Here  the  seam  of  coal  has  been  faulted  down  from  a  to  b  in  the 
direction  indicated  by  the  arrow. 

In  reversed  or  overlap  faults  the  downthrow  of  the  beds  is  on  the 
under  or  foot-wall  side  of  the  fracture,  as  shown  in  fig.  89. 

In  the  above  figure  the  hade  is  towards  the  west,  but  the  down- 
throw is  on  the  under  or  foot-wall  side.  That  is,  the  seam  has 
been  displaced  from  b  to  a  ;  or  if  we  assume  that  a  is  the  original 
position  of  the  seam,  then  it  has  been  moved  from  a  to  b ;  that  is, 
contrary  to  the  direction  of  the  hade. 

In  fig.  77  we  have  an  example  of  fracturing  and  faulting  in  the 
middle  limb  of  an  overturned  fold  arising  from  the  resistance  of 
the  granite  boss  to  the  lateral  thrust  from  the  south-west. 

Displacements  caused  by  Faults. — Faults  cause  different  effects 


160  A  TEXT-BOOK  OF  GEOLOGY. 

according  to  the  direction  of  their  strike  and  dip  relatively  to  the 
strike  and  dip  of  the  beds,  seam,  or  lode  which  they  intersect. 

A  fault  which  runs  parallel  with  the  strike  of  the  beds  is  termed  a 
strike-fault.  It  may  dip  with  the  bed  or  against  it. 

A  fault  which  runs  in  the  same  direction  as  the  dip  of  the  beds — 
that  is,  at  right  angles  to  their  strike— is  called  a  dip-fault.  A 
fault,  however,  may  pursue  any  course  between  the  strike  and  dip 
of  a  bed ;  consequently  the  distinction  between  strike-faults  and 
dip-faults  is  sometimes  not  very  well  marked.  For  example, 
when  the  course  of  the  fault  is  midway  between  the  dip  and  strike 
of  the  bed,  the  fault  may  be  termed  either  a  dip-fault  or  a  strike- 
fault. 


FIG.  89. — Cross-section  showing  reversed  fault. 

Faults  according  to  their  direction  in  respect  of  the  beds  or 
lodes  they  intersect  may  cause  : — 

(1)  A  vertical  displacement  =  throw. 

(2)  A  horizontal  displacement  = shift  of  faulted  bed. 

(3)  An  apparent  lateral  displacement  =  heave. 

The  vertical  displacement  may  vary  from  the  fraction  of  an 
inch  to  thousands  of  feet.  For  example,  the  great  10-yard  seam  of 
coal  in  Staffordshire  has  been  thrown  down  450  feet. 

The  horizontal  shift  of  the  dissevered  portion  of  a  bed  may 
amount  to  thousands  of  feet,  and  is  dependent  on  the  amount  of 
throw  and  the  angle  of  inclination  of  the  fault-plane,  as  will  be 
shown  later. 

The  apparent  lateral  displacement  caused  by  faulting  is  depend- 
ent on  the  throw  and  the  amount  of  denudation  the  country  has 
suffered  since  the  faulting  took  place. 

When  a  fault  displaces  stratified  rocks,  the  lines  of  bedding 
afford  a  measure  of  the  vertical  displacement ;  but,  in  the  absence 
of  some  rock  marked  by  a  distinctive  peculiarity  of  colour  or 


JOINTS,    FAULTS,    CLEAVAGE. 


161 


composition,  there  is  no  means  of  estimating  the  amount  of  dis- 
turbance. 

Effects  of  Faults  on  Horizontal  Strata. — When  a  vertical  fault 
intersects  a  horizontal  bed,  such  as  a  seam  of  coal,  the  only  displace- 
ment is  a  vertical  one,. but  inclined  faults  cause  both  vertical  and 
horizontal  displacement. 

In  fig.  90  a  horizontal  seam  of  coal  is  intersected  by  faults  A,  B, 
and  C  ;  A  being  vertical,  B  steeply  inclined,  and  C  relatively 
flat. 

It  is  obvious  that  the  vertical  displacement  or  downthrow,  com- 
monly called  throw  by  miners,  equal  to  m  n,  is  the  only  displacement 
caused  by  the  vertical  fault  A.  There  is  no  horizontal  displace- 
ment. 

Fault  B  is  inclined  to  the  east,  and  causes  a  vertical  displace- 


W  £ 

FIG.  90. — Showing  effects  of  normal  faults  on  a  horizontal  bed  or  seam. 

ment  d  s,  and  a  horizontal  displacement  d  e,  which  represents  the 
horizontal  disseverment  of  the  ends  e  and  s. 

Fault  C  produces  the  same  amount  of  downthrow  as  fault  B, 
but  being  much  flatter,  it  causes  nearly  four  times  as  much  hori- 
zontal displacement ;  that  is,/&  =  4  e  d.  It  is  obvious  that  the  flatter 
the  plane  of  the  fault,  the  greater  will  be  the  horizontal  displacement. 

The  expressions  downthrow  and  upthrow  as  used  by  miners  are 
merely  co-relative  terms  applied  to  the  vertical  displacement.  Thus, 
if  the  mine  workings  were  advancing  from  n  to  s,  the  direction 
of  the  faulted  seam  e  k  would  be  spoken  of  as  an  upthrow.  If, 
on  the  other  hand,  the  direction  of  the  workings  was  from  k  to  e, 
then  when  the  fault  was  encountered  the  position  of  the  seam  at 
s  n  would  be  said  to  be  the  result  of  a  downthrow. 

The  faults  A,  B,  C,  shown  in  fig.  90,  are  examples  of  normal 
faulting. 

Summarising  the  foregoing,  we  find  that  when  an  inclined  fault 
intersects  a  horizontal  bed  or  seam  the  displacements  are  : — 

11 


162  A  TEXT-BOOK  OF  GEOLOGY. 

(a)  A  vertical  downthrow  (or  upthrow)  =  throw. 

(b)  A  horizontal  disseverment  due  to  the  faulted  portion  sliding 

down  the  fault-plane.     For  the  same  throw,  the  flatter 
the  fault-plane,  the  greater  will  be  the  horizontal  shift. 

Do  not  fail  to  note  that  no  lateral  displacement  or  heave  has  taken 
place  in  the  examples  of  faulting  shown  in  fig.  90,  where  we  have 
only  vertical  downthrow  with  fault  A.  and  downthrow  and  dis- 
severment with  faults  B  and  C. 

Effects  Of  Strike-Faults. — A  strike-fault  runs  parallel  with  the 
strike  of  the  bed  or  seam.  It  may  dip  with  the  bed  or  against 
it,  and  according  to  the  direction  of  the  hade  it  may  be  a  normal 
or  reversed  fault. 

A  strike-fault  causes  vertical  and  horizontal  displacements  of 
the  beds  intersected,  as  shown  in  fig.  91. 


FIG.  91. — Showing  effects  of  strike-fault  dipping  contrary 
to  the  dip  of  the  strata. 

In  this  figure  the  vertical  downthrow=a  b,  and  the  horizontal 
shift  =  a  c. 

A  strike-fault  causes  a  repetition  of  inclined  beds,  as  shown  in 
fig.  92. 

In  regions  that  have  suffered  considerable  denudation,  a  faulted 
bed  or  seam  of  coal  may  be  partly  removed  on  one  or  both  sides  of 
the  fault,  as  shown  in  fig.  93. 

Thrust-Planes. — Strike-faults  may  dip  or  hade  in  the  same 
direction  as  the  beds  they  intersect,  and  the  angle  subtended 
between  the  bedding  planes  and  fault-plane  may  be  so  small  that 
the  plane  of  movement  eventually  follows  the  bedding  as  offering 
the  line  of  least  resistance. 

When  the  dip  and  strike  of  the  fault  coincide  with  the  dip  and 
strike  of  the  beds,  there  is  no  apparent  disturbance  in  the  relation- 
ship of  the  rocks  on  each  side  of  the  fault-fracture. 

The  only  evidence  of  the  existence  of  such  a  fault  is  the  smooth, 
polished,  and  slickensided  surfaces  on  the  plane  of  movement. 

In  some  cases  the  movement  along  a  thrust-plane  has  crushed 
the  wall-rocks  into  fragments,  forming  what  is  called  a  friction- 


JOINTS,    FAULTS,    CLEAVAGE. 


163 


A 


B 


B 


FIG.  92. — Showing  repetition  of  inclined  beds.  Upper  diagram  is  map 
of  beds  traversed  by  strike-fault.  Lower  diagram  is  a  cross-section 
along  line  A  B,  showing  repetition  of  dislocated  beds. 


FIG.  93. — Showing  coal-seam  partly  removed  by  denudation 
on  one  side  along  line  of  strike-fault. 


164 


A    TEXT-BOOK    OF    GEOLOGY. 


breccia.  The  friction-breccia  produced  in  this  way  may  be  a  few 
inches  or  many  feet  wide.  The  fragments  are  generally  held  to- 
gether in  a  matrix  of  clay  or  pug  resulting  from  the  attrition  of 
the  walls  ;  and  not  infrequently  many  of  the  rock  fragments  are 
partially  rounded  and  even  sometimes  scratched  and  striated  by 
the  grinding  effect  of  the  wall-movement. 

Effect  Of  Dip-Faults. — The  course  of  dip-faults  is  parallel  with 
the  dip  of  the  beds  or  veins  intersected. 

On  the  slickensided  faces  of  great  faults,  the  striae  caused  by 
the  rubbing  of  one  rock-surface  upon  the  other  usually  follow  a 
vertical  plane.  In  other  words,  there  is  no  side  shift  of  the  faulted 
bed.  Consequently,  when  the  faulted  beds  are  vertical,  there  is 


FIG.  94. — Showing  dip-fault  intersecting  inclined  coal-seam 
before  faulting  (represented  by  wooden  model). 

no  lateral  displacement  or  heave,  as  the  dissevered  ends  merely 
slide  upon  one  another  in  a  vertical  plane. 

The  apparent  heave  or  lateral  displacement  is  produced  by  the 
dip  of  the  seam  carrying  the  faulted  portion  of  the  seam  to  the 
right  or  left ;  and,  manifestly,  the  natter  the  dip,  the  greater  will 
be  the  apparent  displacement.  When  the  seam  or  lode  is  vertical, 
there  can  obviously  be  no  heave ;  for  since  the  movement  is  vertical, 
the  fractured  faces  will  merely  slide  on  one  another.  • 

Let  a  b  in  fig.  94  represent  an  inclined  coal-seam,  and//a  fracture. 
When  faulting  takes  place,  the  effect  will  be  as  shown  in  fig.  95. 

Here  m  n  represents  the  downthrow  or  vertical  displacement  ; 
and  it  will  be  observed  that  there  is  no  horizontal  shift,  since  the 
fault-plane  is  vertical.  Moreover,  it  will  be  observed  that  the 
outcrop  m  is  vertically  above  the  faulted  outcrop  n. 

When  the  ground  on  the  high  side  is  worn  down  by  denudation 
to  the  level  of  d  e,  there  is  displayed  an  apparent  horizontal  dis- 
placement of  the  dissevered  seam  from  s  to  n,  fig.  96,  and  s  n  =the 


JOINTS,    FAULTS,    CLEAVAGE. 


165 


heave,  which  is  not  real  but  the  result  of  the  vertical  movement 
followed  by  denudation.  And  the  portion  of  the  seam  exposed  at 
s  by  denudation  does  not  correspond  with  the  portion  cropping  out 
at  n. 


FIG.  95. — Showing  displacement  caused  by  dip-fault  after  faulting. 

If  the  fault  inclines  to  one  side,  as  in  fig.  97,  then  we  shall  have  a 
vertical  downthrow  =  a  b,  and  a  horizontal  shift  b  c,  which  will 
represent  the  horizontal  disseverment  due  to  the  faulted  portion 


FIG.  96. — Showing  apparent  heave. 

sliding  on  a  sloping  plane.  And,  obviously,  the  flatter  the  dip  of 
the  fault,  the  greater  will  be  the  shift  for  a  given  throw  or  vertical 
displacement. 

If  now  we  suppose  that  the  elevated  portion  of  the  seam  is 
denuded  down  to  the  level  of  d  c,  then  there  will  be  an  apparent 
heave —sn  ;  but,  obviously,  the  portion  of  the  seam  at  n  will  not 
correspond  to  the  portion  at  s,  but  to  the  summit  of  the  portion 
of  m)  now  removed  by  denudation. 


166 


A   TEXT-BOOK   OF   GEOLOGY. 


Furthermore,  when  the  upthrow  side  is  denuded  down  to  the 
level  of  d  c,  the  evidence  of  the  horizontal  shift  b  c  will  be  removed. 


FIG.  97  — Showing  effects  of  inclined  dip-fault  on  tilted  coal-seam. 


FIG.  98. — Showing  effect  of  dip-fault  on  a  syncline.  Diagram  A  shows  the 
appearance  of  the  outcrops  after  faulting  and  denudation ;  diagram  B, 
after  faulting,  but  before  denudation. 

Effect  of  Dip-Fault  on  Syncline. — When  a  block  of  strata  arranged 
in  the  form  of  a  syncline  is  traversed  by  a  dip-fault  and  the  ground 
on  the  high  side  is  denuded  down  to  the  level  of  the  low  side,  the 


JOINTS,    FAULTS,    CLEAVAGE. 


167 


lines  of  outcrop  on  the  high  side  will  appear  inside  the  lines  on  the 
low  side,  since  they  represent  a  narrower  portion  of  the  syncline, 
as  shown  in  fig.  98. 

Effect  of  Dip-Fault  on  Anticline. — The  effect  in  this  case  is  the 
opposite  of  that  produced  on  a  syncline  ;   that  is,  the  outcrops  of 


B  A 

FIG.  99. — Showing  effect  of  dip-fault  on  anticline. 
A,  After  denudation.  B,  Before  denudation. 

the  denuded  portion  will  appear  outside  the  lines  of  outcrop  on  the 
downthrow  side,  as  shown  in  fig.  99. 

Step-Faults.  —  Extensive    subsidence    or   elevation    is    usually 
accomplished  by  the  production  of  a  number  of  parallel  faults. 


FIG.  100. — Showing  effect  of  step-faulting. 

When  the  dip  of  the  different  faults  is  in  the  same  direction,  there 
is  frequently  produced  a  succession  of  downthrows  which  in  cross- 
section  resemble  the  steps  of  a  stair ;  hence  the  name  step-fault. 

The  displacement  caused  by  step-faults  is  usually  small,  and 
is  best  seen  when  the  faults  dislocate  a  coal-seam. 

Trough-Faults. — When  two  parallel  fractures  dip  towards  one 
another,  permitting  a  block  of  strata  to  be  thrown  down  between 
them,  they  form  what  is  spoken  of  as  a  trough-fault.  A  well-known 


168 


A    TEXT-BOOK    OF    GEOLOGY. 


example  is  the  trough-fault  of  Dudley  Port  Mine  in  Staffordshire, 
which  has  thrown  down  the  great  10-yard  seam  of  coal  a  vertical 
distance  of  450  feet  (fig.  101). 

When  the  area  depressed  by  trough-faulting  is  of  considerable 
linear  extent,  it  forms  what  German  geologists  call  a  Grdben}- 

Field  Evidence  of  Faults. — Many  faults,  perhaps  the  majority, 
give  rise  to  no  surface  feature  by  which  we  might  be  led  to  suspect 
their  existence.  The  surface  evidences  of  the  dislocation  caused 
by  minor  faults  and  those  of  great  antiquity  have  been  obliterated 
by  the  wear  and  tear  which  the  land  has  undergone  since  the 
faulting  took  place.  Only  powerful  faults  of  late  date  modify 
the  topographical  features  in  such  a  way  as  to  proclaim  their 
presence. 

The    great    Moanataiari    Fault,    which    taverses    the    Thames 


FIG.  101. — Showing  effect  of  trough-fault, 
(a)  Seam  of  coal.  (6)  Sheet  of  basalt. 

Goldfield,  and  displaces  all  the  gold-bearing  lodes  lying  in  its  path, 
is  of  such  recent  date  that  its  course  may  be  traced  on  the  surface 
for  many  miles,  being  marked  by  a  distinct  line  of  depression,  as 
well  as  by  the  downthrow  and  displacement  of  the  spurs  which  it 
crosses.  It  dips  to  the  southwest  at  a  uniform  angle  of  45°,  and 
wherever  it  is  cut  in  the  mine  workings  its  course  is  marked  by  a 
layer  of  friction-breccia  and  clay,  varying  from  20  to  100  feet  thick. 
Its  vertical  displacement  amounts  to  about  400  feet. 

Faults  are  rarely  visible  at  the  surface  except  in  bare  cliffs  and 
artificial  cuttings.  As  a  rule  they  are  obscured  with  a  sheet  of 
younger  detritus.  Even  the  clean  fault-fractures  so  frequently 
seen  in  cliffs  and  railway-cuttings  may  be  mere  local  dislocations, 
or  branches  radiating  from  some  greater  fault. 

The  majority  of  the  faults  that  traverse  the  coal-fields  of  Great 
Britain,  North  France,  Belgium,  Pennsylvania,  New  South  Wales, 
1  Ger.  Graben  =  ditch  or  trench, 


JOINTS,    FAULTS,    CLEAVAGE. 


169 


New  Zealand,  and  other  countries,  were  unknown  until  their 
presence  was  disclosed  by  the  progress  of  underground  mining. 

When  once  the  position,  course,  and  dip  of  a  fault  are  ascertained, 
its  position  in  contiguous  areas  can  be  predicted  with  a  certain 
degree  of  accuracy,  provided  no  later  faulting  or  dyke  intrusion 
has  diverted  it  from  its  normal  course. 

Since,  then,  the  topographical  effects  and  actual  fractures   of 


FIG.  102. — Showing  existence  of  fault  inferred  from  presence 
of  abutting  limestone  and  conglomerate. 

faults  are  seldom  seen  at  the  surface,  the  geologist  is  compelled 
to  depend  on  the  inference  to  be  drawn  from  certain  field  occur- 
rences as  to  the  existence  of  faults.  Thus,  when  two  members  of 
the  same  formation  are  found  abutting  against  one  another,  as 
shown  in  fig.  102,  it  is  inferred  that  a  fault  exists  at  the  line  of 
contact. 


FIG.  103. — Showing  existence  of  fault  inferred  from  repetition  of  beds. 

Again,  the  repetition  of  a  series  of  beds,  or  of  some  of  the  beds, 
in  the  absence  of  folding,  is  always  held  to  be  an  evidence  of 
faulting,  as  shown  in  fig.  103. 

Where  a  younger  series  of  strata  occupying  the  floor  of  a  valley 
or  inland  basin  is  tilted  on  end,  may  be  for  scores  of  miles  against 
an  older  formation,  as  frequently  happens  along  the  foot  of  a 
mountain-chain,  the  evidence  is  held  to  indicate  profound  disloca- 
tion or  faulting  of  an  orogenic  character.  Such  faulting  has  taken 
place  in  the  Great  Basin  of  the  Western  States  of  America  and  in 


170  A  TEXT-BOOK  OF  GEOLOGY. 

the  inland  basins  of  New  Zealand  in  connection  with  the  uplift 
of  the  block  mountains  in  those  regions. 

The  existence  of  an  unseen  fault  may  be,  as  a  rule,  determined 
by  the  detailed  examination  and  mapping  of  a  district.  By  its 
effect  on  the  geological  structure,  the  position  and  course  of  the 
fault,  as  well  as  its  vertical  displacement,  can  be  worked  out 
without  the  actual  fracture  being  seen  in  a  single  section  on 
the  surface. 

In  coal  areas  and  goldfields,  the  faults  proved  to  exist  by  the 
underground  workings  always  afford  a  valuable  aid  to  the  field- 
geologist. 

Many  valleys  have  been  excavated  along  the  course  of  faults ; 
hence  persistent  escarpments  on  the  valley-walls  are  always  sug- 
gestive of  faulting. 

Lines  of  springs  frequently  follow  the  course  of  faults,  and  should 


W 


FIG.  104. — Showing  faulting  of  young  Tertiaries  against 

mica -schist  in  New  Zealand, 
(a)  Young  Tertiary  lacustrine  beds.  (6)  Palaeozoic  mica-schist. 

be  carefully  noted.  The  sheet  of  stiff  clay  which  lies  along  the 
walls  of  fault-fissures  arrests  the  flow  of  underground  water  which 
eventually  finds  its  way  to  the  surface  in  the  form  of  springs.  The 
existence  of  a  mineral  vein  may  also  be  indicated  by  a  line  of 
springs. 

Cleavage. 

Cleavage  Structure. — Shales  and  other  rocks  composed  of  fine 
sediments  possess  a  tendency  to  split  into  laminae  parallel  to  the 
original  bedding  planes,  and  this  is  the  natural  thing  to  expect 
from  the  manner  in  which  the  sediments  were  laid  down.  Many 
of  the  older  fine-grained  rocks,  however,  possess  a  tendency  to  split 
into  plates  or  thin  flags  at  right  angles  to  the  original  stratification 
planes.  This  peculiar  structure  is  best  exemplified  in  roofing 
slates,  and  is  called  cleavage  or,  more  correctly,  slaty  cleavage,  to 
distinguish  it  from  the  natural  cleavage  possessed  by  many  cry- 
stalline minerals. 

Although  cleavage  is,  as  a  rule,  best  developed  when  at  right 


JOINTS,    FAULTS,    CLEAVAGE.  171 

angles  to  the  bedding  planes,  it  may  intersect  these  at  any  angle, 
or  may  even  be  parallel  with  them.  For  example,  the  slates  at 
Collingwood  in  New  Zealand  possess  a  distinct  cleavage  that  in 
different  places  intersects  the  stratification  planes  at  various  angles 
from  30°  to  45°. 

When  examined  under  the  microscope,  in  thin  slices,  the  con- 
stituent particles  of  a  slate  are  found  to  be  elongated  in  a  direction 
parallel  with  the  cleavage-planes.  It  is  this  parallelism  of  the 
grains  which  enables  a  slate  to  split  readily  into  thin  plates. 

Where  the  cleavage  is  well  developed,  the  original  stratification 
planes  become  obscure,  or  they  may  be  altogether  obliterated. 
In  highly  altered  slates,  crystalline  minerals,  such  as  mica  and 
rutile  (the  former  most  abundantly),  are  frequently  developed  along 
the  cleavage-planes.  In  this  way  we  are  able  to  trace  the  alteration 
of  shale  to  slate,  of  slate  to  phyllite  or  mica-slate,  of  phyllite  to 
mica-schist,  and  of  mica-schist  to  gneiss. 

Origin  of  Cleavage. — Cleavage  is  the  result  of  enormous  lateral 
pressure.  It  is  generally  best  developed  where  the  rocks  have 
been  subjected  to  intense  folding  combined  with  sufficient  super- 
incumbent weight  to  prevent  the  loss  of  lateral  stress  by  the 
upward  yielding  of  the  strata.  Near  the  surface  the  rocks  will 
yield  and  fracture  before  the  lateral  pressure  becomes  sufficient 
to  cause  the  component  particles  to  be  elongated  or  rearranged 
at  right  angles  to  the  compressing  force.  Hence  it  is  found  that 
slaty  cleavage  is  always  best  developed  in  ancient  sediments  that 
have  been  subjected  to  prolonged  compression  in  deep  crustal  folds. 

Sandstones,  conglomerates,  and  altered  igneous  rocks  frequently 
exhibit  an  incipient  form  of  cleavage  that  is,  however,  usually 
short  and  irregular. 

Slaty  cleavage  is  not  confined  to  rocks  of  any  particular  age, 
but  is  seldom  met  with  in  formations  younger  than  the  Jurassic. 
The  fine  roofing  slates  of  Wales,  of  Cambrian  age,  are  remarkably 
fissile  and  homogeneous  in  texture. 

Cleavage,  in  all  respects  similar  to  that  induced  in  slates,  has 
been  imitated  by  mechanical  means  in  various  mixtures  of  clay 
by  Sorby  and  other  experimenters. 

Slates  that  have  been  subjected  to  a  torsion  or  twisting  stress 
through  the  obstruction  offered  by  an  unyielding  buttress  of  granite 
lying  in  the  path  of  the  compressive  force  are  found  to  break  up 
readily  into  thin  prismatic  pencils. 

SUMMARY. 

Joint- Structure. — (1)  The  majority  of  sedimentary  rocks,  both 
altered  and  unaltered,  are  traversed  by  two  sets  of  simple  cracks 


172  A  TEXT-BOOK  OF  GEOLOGY. 

called  joints,  that  are  usually  perpendicular  to  the  original  bedding 
planes  and  at  right  angles  to  one  another,  thereby  dividing  the 
rock-mass  into  cuboidal  or  prismatic  blocks. 

Joints  are  commonly  confined  to  the  particular  rock  in  which 
they  occur,  but  in  some  cases  they  are  found  to  pass  from  one  rock 
to  another.  The  best-developed  joints  are  known  to  workmen 
as  master-joints. 

Some  igneous  rocks  are  traversed  by  two  sets  of  joints  that 
divide  the  rock-mass  into  symmetrical  columns,  giving  rise  to 
the  well-known  columnar  structure  which  is  particularly  well 
developed  in  some  basalts. 

Joints  are  necessarily  confined  to  the  zone  of  fracture,  and  in 
the  majority  of  cases  they  are  not  continuous  but  die  out  when 
followed  in  any  given  direction,  being  succeeded  after  an  interval 
by  others  having  the  same  general  course. 

Joint-planes  sometimes  show  polished  and  striated  faces  which 
indicate  rubbing  or  attrition  due  to  some  movement. 

The  joints  that  are  so  frequently  found  traversing  seams  of  the 
older  coals  are  termed  cleats.  The  face  cleats  run  parallel  with  the 
strike  and  are  generally  the  most  pronounced.  The  butt  cleats 
are  perpendicular  to  the  face  cleats.  The  master- joints  in  rocks 
and  the  cleats  in  coals  are  utilised  by  the  workmen  to  facilitate 
the  breaking  of  the  material. 

Origin  of  Joints. — (2)  Joints  are  in  all  probability  caused  by 
tension  stresses  arising  from  folding  and  earth-movements  resulting 
from  shrinkage  and  shearing.  The  master- joints  usually  run 
parallel  with  the  axis  of  elevation,  which  points  to  a  genetic  relation- 
ship between  joints  and  folding. 

In  anticlinal  folds,  the  upper  layers  of  rock  will  be  in  tension  and 
the  lower  in  compression  ;  while  in  a  syncline,  the  lower  layers 
will  be  in  tension  and  the  upper  in  compression. 

Faults. — (3)  A  fault  is  a  simple  crack  or  fissure  on  one  side  of 
which  movement  has  taken  place  so  as  to  shift  the  rocks  on  each 
side  relatively  to  one  another. 

Faults  are  caused  by  crustal  stresses  of  greater  magnitude  than 
those  which  originated  jointage.  Joints  and  faults  are  closely 
related,  and  both  are  the  visible  expression  of  mechanical  stresses. 
Sharp  folding  results  in  fracturing  and  faulting  whenever  the  stress 
exceeds  the  elastic  limit  of  the  rock-mass. 

Faults  begin  gradually,  somewhere  along  their  course  attain 
a  maximum  displacement,  and  then  gradually  die  out.  Their 
length  may  vary  from  a  few  hundred  feet  to  hundreds  of  miles, 
and  their  vertical  displacement  from  a  fraction  of  an  inch  to  many 
thousand  feet. 

The  faces  of  fault-planes  are  frequently  polished,  grooved,  and 


JOINTS,    FAULTS,    CLEAVAGE.  173 

striated — that  is,  slickensided.  In  many  cases,  perhaps  the  majority, 
the  fault-fissure  is  filled  with  a  sheet  of  clay  resulting  from  the 
attrition  of  the  rock-surfaces.  In  other  cases  they  are  filled  with 
fragments  of  rock.  In  many  cases  fault-fissures  have  formed 
channels  for  the  circulation  of  underground  waters  which  have 
deposited  mineral  matter  and  metallic  ores  in  them.  Many 
faults  have  in  this  way  become  changed  into  valuable  lodes. 

(4)  In  what  is  called  a  normal  fault  the  downthrow  is  towards 
the  side  to  which  the  fault  inclines  ;    and  in  a  reversed  or  overlap 
fault  the  downthrow  is  on  the  footwall  side. 

(5)  A  fault,  according  to  the  direction  it  pursues  in  relation  to 
the  strike  of  the  rocks  it  traverses,  may  be  a  strike-fault  which  runs 
parallel  with  the  strike,  or  a  dip-fault  which  runs  at  right  angles 
to  the  strike.     But  it  must  be  remembered  that  faults  may  run 
at  any  angle  between  the  strike  and  dip. 

A  strike-fault  causes  both  vertical  and  horizontal  displacement 
of  the  beds  it  intersects,  and  if  the  throw  is  considerable,  may  cause 
a  repetition  of  the  surface  outcrops  of  a  succession  of  beds. 

(6)  Dip-faults   cause  a   vertical   and   an  apparent   lateral  dis- 
placement, the  last  due  to  the  dip  of  the  faulted  beds  carrying 
the  faulted  portion  to  the  right  or  left. 

Where  parallel  faults  cause  a  displacement  in  the  same  direction, 
they  form  what  are  called  step-faults  ;  and  where  two  faults  dip 
towards  each  other  so  as  to  permit  a  block  of  rock  to  drop  down 
between  them,  they  form  a  trough-fault. 

(7)  Among  the  best  field-evidences  of  faulting  are  (a)  the  side 
displacement  of  beds,  and  (6)  the  repetition  of  beds  where  there  is  no 
reason  to  suspect  the  existence  of  isoclinal  folding.     Few  faults 
are  recognisable  on  the  surface,  as  in  the  majority  of  cases  denuda- 
tion has  kept  pace  with  the  rate  of  displacement.     Their  existence 
can,  however,  be  deduced  from  the  deposition  and  arrangement  of 
the  rocks,  as  shown  by  a  careful  geological  survey.     Faults  are 
easily  recognised  in  coal  and  metal  mines  by  the  displacement  of 
the  seams  and  lodes  which  they  intersect. 

Cleavage.— (8)  This  is  the  tendency  possessed  by  many  rocks, 
particularly  those  of  fine  texture,  to  split  into  thin  plates  in  some 
direction  not  parallel  to  the  original  bedding  plane.  Cleavage  is 
best  seen  in  clay  slates.  It  can  be  induced  in  artificial  mixtures 
of  clays,  iron  oxide,  etc.,  by  the  application  of  enormous  lateral 
pressure.  The  cleavage-plane  is  always  perpendicular  to  the  line 
of  pressure.  It  is  believed  that  slaty  cleavage  is  the  result  of  lateral 
pressure  or  compression  arising  from  crustal  folding. 


CHAPTEK   XL 

COMPOSITION   OF  EARTH'S   CRUST. 
Constitution  and  Physical  Properties  of  Minerals. 

THE  crust  of  the  Earth  is  composed  of  rocks  and  minerals  which, 
in  their  ultimate  constitution,  consist  of  elementary  substances 
called  elements. 

Some  Chemical  Principles. — Elements  are  simple  substances, 
and  of  these,  chemical  research  has  identified  about  seventy  in  the 
various  rocks,  minerals,  and  compounds  that  constitute  the  access- 
ible portion  of  the  crust.  The  majority  are,  however,  compara- 
tively rare. 

Elements  and  their  compounds  exist  naturally  in  three  conditions, 
namely,  the  gaseous,  liquid,  and  solid. 

Most  solids  can  be  rendered  liquid  by  the  application  of 
heat ;  and  by  applying  still  more  heat,  the  liquid  form  can  be 
changed  to  the  gaseous.  Conversely,  by  the  application  of  suffi- 
cient cold  and  pressure,  the  gases  can  be  made  first  liquid  and 
then  solid. 

Of  the  metallic  minerals,  mercury  is  the  only  one  that  is  liquid 
at  ordinary  temperatures.  It  can  be  converted  into  a  solid  by 
subjecting  it  to  the  influence  of  intense  cold. 

The  majority  of  the  elements  do  not  exist  in  a,  free  or  uncom- 
bined  state,  but  two,  three,  or  more  combine  with  one  another 
to  form  various  compounds.  A  compound  may  be  a  gas,  like 
carbon  dioxide  ;  a  liquid,  like  water  ;  or  a  solid,  like  calcite  or 
limestone. 

Among  the  elements  that  exist  in  a  free  state  we  have  the  gases 
oxygen,  nitrogen,  and  chlorine  ;  the  liquid,  mercury  ;  and  the 
solids,  gold,  silver,  platinum,  copper,  iron,  carbon,  sulphur,  and 
some  others. 

Practically,  all  the  elements  resent  an  existence  in  a  free  state, 
and  hence  are  always  on  the  alert  to  form  alliances  with  other 
elements  or  compounds. 

The  chemical  affinities  or  likings  of  some  elements  for  certain 
other  elements  are  very  powerful  and  for  others  feeble.  The  gas 

174 


COMPOSITION    OF    EARTH'S    CRUST.  175 

fluorine,  for  example,  is  so  active  that  it  can  only  be  separated 
from  its  compounds  with  the  greatest  difficulty,  and  when  separated 
it  requires  the  exercise  of  extraordinary  precautions  to  keep  it  from 
combining  with  other  elements.  On  the  other  hand,  nitrogen, 
when  free,  is  not  very  active,  and  it  is  for  this  reason  that  it  con- 
stitutes so  large  a  proportion  of  the  atmosphere. 

An  element  may  possess  the  power  to  combine  with  many 
different  elements  with  various  degrees  of  intensity.  With  those 
to  which  it  is  strongly  attracted,  it  will  form  stable  compounds, 
and  with  those  to  which  it  is  feebly  attached,  feeble  combinations 
that  are  easily  broken  up. 

Thus  silicon  has  a  powerful  attraction  for  oxygen,  and  when 
once  these  two  elements  are  united,  as  we  find  them  in  silica  (Si02), 
which  occupies  such  an  important  place  among  the  constituents 
of  the  Earth's  crust,  it  is  almost  impossible  to  disassociate  them. 
On  the  other  hand,  iron  and  oxygen  have  a  mutual  attraction, 
forming  oxides  of  iron,  but  the  oxygen  can  easily  be  displaced  from 
the  iron  by  presenting  carbon  to  it  under  suitable  conditions.  In 
fact,  the  oxides  of  almost  all  the  metals  can  be  broken  up  by  carbon, 
and  this  is  the  principle  that  underlies  the  reduction  or  smelting 
of  the  base  metals. 

What  has  been  said  of  the  elements  is  also  true  of  many  com- 
pounds, particularly  of  the  gaseous  compounds  and  the  salts 
soluble  in  water.  That  is,  they  possess  the  power  to  unite  with 
elements  or  other  compounds  to  form  new  compounds.  They  also, 
like  the  elements,  possess  certain  affinities,  preferring  to  unite  with 
certain  elements  and  compounds  in  preference  to  others.  Like- 
wise with  certain  elements  and  compounds  they  are  capable  of 
forming  stable  combinations,  while  with  others  they  form  feeble 
combinations.  Thus  the  union  of  lime  (CaO)  and  carbonic  acid 
(C02)  is  a  comparatively  stable  compound  forming  calcite,  lime- 
stone, or  chalk  ;  but  the  soluble  bicarbonate  of  lime,  which  is 
formed  when  carbonic  acid  dissolved  in  water  acts  on  limestone 
(CaC03),  is  a  feeble  combination,  the  excess  of  carbon  dioxide 
being  easily  displaced. 

The  inveterate  natural  propensity  and  continual  struggle  of 
certain  elements  and  compounds  to  form  new  and  attractive  com- 
binations more  to  their  liking,  is  the  dominant  principle  underlying 
the  weathering  and  disintegration  of  rocks  which  play  so  important 
a  role  in  the  general  processes  of  denudation. 

The  three  compounds  responsible  for  the  greater  part  of  this 
disturbance  are  silica  (chemically  called  silicic  acid],  carbonic 
acid,  and  sulphuric  acid.  Next  to  these  we  have  the  elements 
oxygen  and  chlorine,  both  active  and  powerful  allies  of  the 
acids. 


176  A  TEXT-BOOK  OF  GEOLOGY. 

The  acids  unite  with  the  oxides  of  the  metals  called  bases  to  form 
new  compounds.  Thus  :  — 

Silicic  acid,  i.e.  silica,  forms  silicates. 

Carbonic  acid,  i.e.  carbon  dioxide,  forms  carbonates. 

Sulphuric  acid  forms  sulphates. 

Oxygen  unites  with  metals  to  form  oxides,  or  unites  with 

lower  oxides  to  form  higher  oxides. 
Chlorine  unites  with  metals  to  form  chlorides. 

The  silicates,  carbonates,  and  sulphates  are  important  in  any 
study  of  the  crust  on  account  of  the  dominant  part  they  play  as 
rock-forming  minerals. 

Of  the  eighty  or  more  elements  distinguished  by  chemical 
science,  about  twelve  constitute  about  97  per  cent,  of  the  mass 
of  the  accessible  crust.  These  twelve  are  as  follows  :  — 


Oxygen,      .  .47 

Silicon,       .         .         .         .         .  .28 

Aluminium,        .         .  8 

Iron,           .  6 

Calcium,     .         .         .         .        .         .  .         4 

Magnesium,         .         .                  ...  .         2 

Sodium,      ........         2 

Potassium, 

Carbon,  chlorine,  barium,  and  manganese,  about  2 

Some  Physical  Properties  of  Minerals. 

A  Mineral  Defined.—  The  term  mineral  embraces  such  a  wide 
range  of  natural  substances  that  it  is  difficult  to  formulate  a  defi- 
nition sufficiently  comprehensive  and  exact  to  satisfy  scientific 
requirements.  A  mineral  may,  however,  be  defined  as  a  natural, 
homogeneous,  inorganic  substance  possessing  a  definite  chemical 
composition. 

This  definition  includes  water  and  its  solid  form  ice,  but  excludes 
coal  and  some  other  substances^!  vegetable  origin  that  are  by 
common  usage  regarded  as  minerals. 

The  most  important  physical  properties  of  minerals  are  the 
following  :  — 

1.  Crystalline  form.  5.  Tenacity. 

2.  Cleavage.  6.  Specific  gravity. 

3.  Fracture.  7.  Lustre  and  feel. 

4.  Hardness.  8.  Colour  and  streak. 


177 

Formation  of  Crystals. — Crystals  may  be  formed  in  Nature  in 
three  different  ways,  namely  : — 

(1)  By  sublimation  from  gases. 

(2)  By  chemical  precipitation  from  solutions. 

(3)  By  separation  from  a  fused  or  molten  mass. 

In  volcanic  regions  the  sides  of  the  vents  of.  fumaroles  and  of  all 
the  cavities  or  vughs  to  which  the  gases  have  access,  are  frequently 
lined  with  beautiful  incrustations  of  sulphur  crystals  formed  by 
sublimation  through  the  mutual  interaction  of  sulphuretted- 
hydrogen  and  sulphurous  acid  gases  emanating  from  the  ground. 

The  formation  of  crystals  by  precipitation  from  aqueous  solutions 
is  a  subject  of  which  we  have  many  familiar  examples.  If  a  hot 
saturated  solution  of  brine  be  allowed  to  cool,  crystals  of  rock-salt 
will  separate  out  from  the  mother  liquor.  Or  when  a  string  is 
suspended  in  a  saturated  cooling  solution  of  sugar,  it  soon  becomes 
covered  with  the  beautiful  crystals  called  rock-sugar  or  barley- 
sugar.  The  crystalline  gangue  or  matrix  of  mineral  lodes  is  now 
believed  to  have  been  formed  by  precipitation  from  mineralised 
waters  that  at  one  time  circulated  in  the  fissures. 

A  fused  mass  of  rock  is  merely  a  solution  of  a  thick  and  viscous 
kind,  and,  on  cooling,  crystals  separate  out  from  it  just  as  they  do 
from  an  aqueous  saline  solution. 

CRYSTALLINE  FORM. 

All  minerals  have  a  tendency  to  occur  in  certain  definite  geo- 
metrical forms  which  are  called  crystals.  There  are  hundreds  of 
crystal  forms,  but  all  can  be  referred  to  six  groups  to  which  the 
name  crystallographic  systems  is  applied.  They  are  as  follows  : — 

1.  Cubic  or  Isometric.  4.  Monoclinic. 

2.  Dimetric.*  5.  Triclinic. 

3.  Trimetric.  6.  Hexagonal. 

In  every  crystal  the  flat  surfaces  or  faces  are  called  planes,  and 
these  may  be  flat,  rough,  or  curved.  The  line  formed  by  the 
meeting  of  two  planes  is  an  edge,  and  the  point  where  three  or 
more  planes  meet  is  called  a  solid  angle. 

All  crystals  may  therefore  be  regarded  as  solid  geometrical 
figures  bounded  by  planes  or  faces  ;  and  although  the  size  of  the 
planes  may  vary,  as  they  do  in  large  and  small  crystals,  the  angles 
between  corresponding  planes  in  different  crystals  of  the  same 
mineral  are  always  the  same.  A  few  minerals  are  dimorphic. 

In  all  crystals  the  planes  are  referred  to  certain  imaginary  lines 

12 


178 


A    TEXT-BOOK    OF    GEOLOGY. 


called  axes  running  through  the  crystal.  This  construction  is 
easily  understood  by  the  examination  of  wooden  or  glass  models, 
but  can  also  be  made  clear  by  a  few  simple  experiments,  now  to 
be  described. 


FIG.  105. — Showing  cube  with  its  three  axes  at  right  angles  to  one  another. 
(a-b)  Pinakoid  faces. 

Cubic  or  Isometric x  System. — Take  a  piece  of  soap,  transparent 
if  procurable,  and  cut  it  into  a  cube  about  two  inches  square. 


FIG.  106. — Showing  octahedron,     (c,  c)  Pyramid  faces. 

Through  the  centre  of  each  pair  of  opposite  planes  push  a  fine 
knitting- neddle,  as  shown  in  fig.  105. 

It  will  be  observed  that  the  three  needles  or  axes  lie  at  right 
angles  to  one  another,  and  that  the  distance  from  the  centre   or 
point  of  intersection  inside  the  crystal  to  each  plane  is  the  same. 
1  Gr.  isos  =  equal,  and  metron  =  &  measure. 


COMPOSITION    OF    EARTH'S    CRUST. 


179 


Thus,  in  the  cubic  system  we  have  three  axes  of  equal  length 
and  at  right  angles  to  one  another  ;  and  all  the  axes  being  equal, 
there  is  no  axis  that  can  be  regarded  as  the  principal  axis  in  pre- 
ference to  the  others. 

If  now  we  truncate  the  solid  angles  of  the  cube  down  to  the 
points  where  the  needles  emerge,  we  shall  get  an  eight-sided  figure 
or  octahedron. 

By  truncating  various  solid  angles  and  edges  we  may  obtain 
many  modifications  of  the  cube,  all  of  which  can  be  referred  to  the 
three  axes  of  the  cubic  system. 


6 


FIG.  107. — Showing  prisms  of  dimetric  system. 

(a,  a)  Pinacoid  faces.  (p,  p)  Prism  faces. 

(6,  6)  Basal  planes.  (c,  c)    Pyramid  faces. 

Dimetric 1  System. — Take  another  piece  of  soap  and  cut  it  into  a 
prism  three  inches  long,  with  ends  an  inch  and  a  quarter  square. 
Mark  the  centre  of  each  face,  and  through  the  marks  in  each  pair 
of  opposite  faces  push  a  needle. 

Here  we  have  three  axes,  all  at  right  angles  ;  two  are  of  equal 
length  and  lie  in  the  same  plane,  while  the  third  is  either  longer  or 
shorter  than  the  others  and  is  called  the  principal  axis.  In  our 
example  shown  in  the  last  figure  we  have  made  it  longer  than  the 
others. 

Observe  that  the  pinacoid  2  faces  or  planes  a,  a,  are  parallel  to 
the  principal  axis,  and  perpendicular  to  the  lateral  axes. 

1  Gr.  dis  =  double,  and  metron  =  a,  measure. 

2  Gr.  pinax  =  &  plank,  and 


180 


A    TEXT-BOOK    OF    GEOLOGY. 


The  top  and  bottom  planes  of  the  prism  are  marked  6,  6,  and 
are  crystallographically  called  basal  planes,  notwithstanding  their 
position  at  the  top  and  bottom  of  the  prism. 

If  now  we  truncate  or  pare  away  the  vertical  edges  of  the  prism 
until  the  new  planes  meet  at  the  points  where  the  lateral  axes 
emerge,  as  in  B  of  fig.  107,  we  shall  obtain  a  new  prism,  bounded 
at  the  ends  by  basal  planes  6,  6,  and  at  the  sides  with  four  new 
vertical  planes  lying  parallel  with  the  principal  axis  and  touching 
two  of  the  lateral  axes.  These  planes  are  marked  p,  p,  to  distinguish 
them  from  the  pinacoids,  each  of  which,  as  we  have  seen  above,  is 
perpendicular  to  a  lateral  axis. 


FIG.  108. — Showing  prism  of  trimetric  system, 
(a,  a)  Pinacoids.  (6,  6)  Basal  planes. 

If  we  take  another  prism  similar  to  the  first  used  to  illustrate 
this  system,  as  shown  in  A  of  fig.  107,  and  truncate  the  eight  solid 
angles,  we  shall  obtain  an  octahedron  bounded  with  pyramid 
faces  c,  c. 

Trimetric  x  System. — Cut  a  prism  with  oblong  ends,  and,  as  before, 
insert  the  needles  through  the  centres  of  the  opposite  faces. 

It  will  be  seen  that  the  three  axes  are  still  at  right  angles  to  one 
another,  but  that  all  are  of  different  length.  Here  also  the  principal 
axis  may  be  longer  or  shorter  than  either  of  the  lateral  axes. 

By  truncating  the  solid  angles  we  obtain  an  octahedron  bounded 
by  pyramid  faces  ;  and  by  truncating  the  vertical  edges  of  another 
prism  similar  to  the  one  we  started  with,  as  shown  in  fig.  108,  we 
obtain  a  prism  bounded  by  prism  faces. 

1  Gr.  treis= three,  and  metron=a,  measure. 


COMPOSITION    OF    EARTH'S    CRUST. 


181 


Monoclinic l  or  Oblique  System. — In  this  system  there  are  three 
unequal  axes,  two  at  right  angles,  the  third  inclined. 

To  illustrate  this  system,  first  cut  a  prism  three  inches  long, 
with  ends,  say,  one  inch  by  an  inch  and  a  half.  Insert  a  needle 
through  the  centre  of  the  top  and  bottom,  i.e.  basal  planes,  and 


FIG.  109. — Showing  prism  of  monoclinic  system, 
(a,  a)  Pinacoids.  (6,  6)  Basal  planes. 

another  through  the  centre  of  the  pinacoid  planes  lying  parallel 
with  the  longer  axis.  These  two  needles  or  axes  will  be  at  right 
angles,  but  are  of  different  lengths. 


FIG.  110. — Doubly  oblique  prism  of  copper  sulphate. 

Now  pare  away  the  basal  planes  so  that  the  model  will  not  lie 
vertical  when  placed  on  the  table.  Make  both  basal  planes  parallel 
to  one  another,  and  through  their  centres  push  the  third  needle. 
The  third  axis  will  be  seen  to  be  inclined  to  the  other  two. 

Triclinic  2  or  doubly  Oblique  System. — In  this  system  there  are 
three  unequal  axes,  and  all  inclined  at  different  angles. 

1  Gr.  monos  =  single,  and  klino=I  bend  or  incline. 

2  Gr.  treis  =  three,  and  klino  =  1  bend  or  incline. 


182 


A    TEXT-BOOK    OF    GEOLOGY. 


Take  the  prism  of  soap  used  in  the  last  experiment  and  pare  the 
basal  planes  away  in  a  direction  at  right  angles  to  the  first  paring 
which  caused  the  inclination  of  the  prism.  The  prism  will  now  be 
inclined  in  two  directions. 

Hexagonal  System. — This  system  differs  from  all  the  others  in 
having  four  axes,  of  which  three  are  equal,  lie  in  the  same  plane,  and 
are  inclined  to  one  another  at  an  angle  of  60°.  The  fourth,  called 
the  principal  axis,  is  at  right  angles  to  the  others,  and  may  be  of 
any  length. 

Cut  a  hexagonal  prism  three  inches  long,  and  through  the  centres 
of  the  opposite  pairs  of  faces  insert  the  needles.  By  truncating 
the  solid  angles  a  hexagonal  pyramid  will  be  obtained. 


FIG.  111. — Hexagonal  crystals. 

(a)  Hexagonal  dodecahedron.          (6)  Rhombohedron.          (c)  Hexagonal  prism 
(d)  Crystal  of  quartz  (combination  of  hexagonal  prism  and  pyramid). 

RECAPITULATION   OF   CRYSTALLOGRAPHIC   SYSTEMS. 

(1)  Cubic — 3  axes,  all  equal,  all  at  right  angles. 

(2)  Dimetric — 3  axes,  two  equal,  all  at  right  angles. 

(3)  Trimetric — 3  axes,  all  unequal,  all  at  right  angles. 

(4)  Monoclinic — 3  axes,  all  unequal,  two  at   right  angles,  the 

other  inclined. 

(5)  Triclinic — 3  axes,  all  unequal,  all  inclined. 

(6)  Hexagonal — 4  axes,  three  equal,  lying  in  the  same  plane, 

the  fourth  at  right  angles  to  others. 

In  all  the  systems  there  may  be  prisms  and  pyramids.  When 
crystals  are  very  narrow  and  long,  they  are  termed  acicular  or 
needle-shaped ;  and  when  broad,  they  are  said  to  be  tabular. 

Pseudomorphs.1 — These  are  crystals  which  have  the  form  of  one 
mineral  and  the  composition  of  another.  For  example,  crystals 
of  quartz  are  frequently  found  in  the  form  of  calcite,  and  orthoclase 
is  sometimes  partly  or  entirely  replaced  with  cassiterite,  tin  oxide. 

1  Gr.  psendos  =  false,  and  morphe 


To  face  page  182.] 


FIG.  112. — Showing  geode  of  calcite.     (After  Bassler.) 


COMPOSITION    OF    EARTH'S    CRUST.  183 

Fossil  organisms  are  frequently  found  replaced  with  pyrites  or 
silica. 

Pseudomorphism  is  the  result  of  mineral  replacement. 

Dimorphism.1 — A  mineral  substance  that  is  capable  of  crystal- 
lising in  two  different  systems  is  said  to  be  dimorphous.  Carbonate 
of  lime  is  a  notable  example  of  dimorphism.  In  the  form  of  calcite 
it  crystallises  in  the  hexagonal  system,  and  as  aragonite  in  the 
trimetric  system. 

Macles  or  Twin  Crystals. — These  are  groups  of  two  or  more 
crystals  that  appear  as  if  mutually  intersecting  one  another,  and 
sometimes  as  if  a  crystal  had  been  cut  in  two  and  one  part  turned 
round  on  the  other. 

Macles  are  common  in  alum,  albite,  spinel,  quartz,  orthoclase, 
magnetite,  pyrites,  rutile,  and  many  other  minerals. 

Geodes. — These   are   concretion-like   masses,   hollow   and   lined 


FIG.  113. — Showing  simple  goniometer. 

with  crystals  pointing  inwards.  They  are  common  in  all  kinds  of 
rocks  and  in  mineral  veins.  The  cavities  which  they  filled  are 
called  vughs. 

Measurement  of  Angles  of  Crystals. — The  angles  which  similar 
planes  make  with  one  another  are  constant ;  and  since  minerals 
always  crystallise  in  the  same  forms,  the  measurement  of  the  angles 
affords  an  important  aid  in  their  identification. 

The  angles  of  crystals  are  measured  with  a  goniometer,2  of  which 
there  are  many  mechanical  and  optical  forms.  A  simple  form  of 
goniometer  is  shown  in  fig.  113. 

Cleavage. — This  is  the  tendency  possessed  by  many  crystalline 
minerals  to  split  or  cleave  in  a  certain  direction.  The  cleavages 
are  usually  parallel  with  the  faces  of  one  of  the  simple  forms  of  the 
mineral.  They  are  spoken  of  as  perfect  when  smooth  and  easily 
obtained,  and  imperfect  when  rough  or  obtained  with  difficulty. 

Many  minerals  possess  no  cleavage-planes  ;    while  others  may 

1  Gr.  dis—  double,  and  morphe  =  shape. 

2  Gr.  gone  =  &n  angle,  and  metron  =  a  measure. 


184  A  TEXT-BOOK  OF  GEOLOGY. 

cleave  in  one,  two,  or  more  directions.  When  a  mineral  possesses 
two  or  more  cleavage-planes,  one  cleavage  is  generally  more  easily 
obtained  than  the  others. 

Cleavage  is  always  in  the  same  direction  in  the  same  mineral  ; 
hence  it  is  of  great  use  in  the  identification  of  crystallised  minerals. 

Quartz  possesses  no  cleavage  ;  mica  has  one  perfect  cleavage 
parallel  with  the  basal  plane  ;  and  orthoclase  has  two  cleavages,  viz. 
parallel  with  the  basal  plane,  and  with  one  pinacoid.  Calcite  has 
a  perfect  cleavage  parallel  to  all  the  faces  of  the  rhombohedron  ; 
hence,  if  a  large  crystal  of  that  mineral  be  broken,  it  will  fall  into  a 
number  of  small  rhombohedrons,  each  of  which  may  be  broken 
into  still  smaller  crystals  of  the  same  form. 

Crystal-cleavage  is  a  character  in  some  way  connected  with  the 
molecular  structure  and  building  up  of  the  crystal.  It  has  no  rela- 
tionship to  the  slaty  cleavage  of  rock-masses,  which,  as  we  have 
found,  is  a  structure  induced  by  enormous  lateral  pressure. 

Fracture.  —  This  relates  to  any  broken  surface  other  than  a 
cleavage-plane.  According  to  its  form  it  may  be  :  — 

(a)  Conchoidal  or  shell-like,  as  in  flint. 

(6)  Even  or  free  from  roughness. 

(c)  Uneven  or  rough,  as  in  cassiterite. 

(d)  Splintery,  as  in  serpentine  and  nephrite. 

(e)  Hackly  or  wiry,  as  in  native  copper. 

Hardness.  —  This  is  a  character  of  great  importance  in  determina- 
tive mineralogy.  It  varies  greatly  in  different  minerals  and 
slightly  according  to  the  face  taken,  and  is  generally  expressed  in 
terms  of  Moh's  scale,  which  ranges  from  1  to  10. 

Moh's  Scale  of  Hardness. 

(1)  Talc,    easily    scratched    with     (6)   Felspar,  difficult  to  scratch 

finger-nail.  with  knife. 

(2)  Gypsum,   difficult  to   scratch     (7)    Quartz, 

with  finger-nail. 

(3)  Calcite,  easily  scratched  with     (8)   Topaz,  A  ,     -, 

knife.  [scratched 

(4)  Fluor  Spar,         ~]  not     easily     (9)   Corundum, 

Scratched 

(5)  Apatite,  j  with  knife.    (10)  Diamond, 

Quartz  is  harder  than  steel  ;  therefore  it  is  not  scratched  with 
a  knife. 

A  mineral  is  tested  for  hardness  by  finding  a  test-mineral  which 
will  just  scratch  it,  and  one  below  which  will  not  scratch  it.  Its 
hardness  lies  between  these. 


kmfe- 


COMPOSITION    OF    EARTH'S    CRUST.  185 

Tenacity. — Minerals  may  be  : — 

(a)  Tough,  like  nephrite  or  jade. 

(b)  Brittle,  like  tourmaline. 

(c)  Pulverulent,  easily  reduced  to  powder. 

(d)  Sectile,  may  be  cut  with  a  knife,  like  kerate. 

(e)  Malleable,   may   be   flattened  by   hammering,    like    native 

copper. 
(/)    Elastic,  like  mica,  which  may  be  bent,  but  regains  its  original 

form  when  pressure  is  removed. 
(g)  Flexible,  like  asbestos,  which  may  be  bent,  but  is  not  elastic. 


FIG.  114. — Showing  specific  gravity  balance. 

Specific  Gravity  =S.G. — This  is  the  weight  of  a  mineral  compared 
with  the  weight  of  an  equal  bulk  of  water.  The  specific  gravity  of 
water  is  taken  as  1,  or  unity.  The  specific  gravity  of  quartz 
is  2-6,  which  means  that  a  cubic  foot  of  quartz,  or  any  given 
volume,  weighs  2-6  times  heavier  than  the  same  volume  of  water. 

A  cubic  foot  of  water  weighs  nearly  62*5  Ibs. ;  therefore  a  cubic 
foot  of  quartz  -2-6  x  62-5  =162-5  Ibs. 

Specific  gravity  is  a  character  that  is  frequently  of  great  use  in 
distinguishing  minerals. 

To  determine  S.G.  of  a  Mineral  Substance  heavier  than  Water : — 
First  method — 


(1)  Weigh  the  substance  carefully  in  a  pair  of  scales, 
the  weight  in  air  =a. 


Call  this 


186  A  TEXT-BOOK  OF  GEOLOGY. 

(2)  Suspend  the  mineral  from  one  of  the  pans  by  a  silk  thread, 

and  weigh  it  in  a  vessel  nearly  full  of  water.  Be  careful  to 
displace  any  air  bubbles  that  may  adhere  to  the  surface  of 
the  mineral.  Call  this  the  weight  in  water  =b. 

(3)  The  weight  in  water  will  be  less  than  the  weight  in  air  ;  that 

is,  b  will  be  less  than  a. 

Subtract  b  from  a,  and  the  difference  will  be  the  weight 
of  water  displaced  by  the  mineral. 

(4)  Divide  the  weight  in  air  by  the  difference  just  found,  and 

the  quotient  will  be  the  S.G.  required. 

This  may  be  expressed  in  the  form  of  a  simple  equation  : 


a 
Second  method  — 

A  more  exact  determination  is  made  by  means  of  a  specific  gravity 
bottle,  which  is  a  light  glass  bottle  provided  with  a  perforated 
stopper,  arranged  to  hold,  when  full,  a  known  quantity  of  water, 
say  500  grains. 

(1)  Fill  the  bottle  with  water,  insert  stopper,  and  wipe  dry. 

(2)  Make  a  counterweight  of  lead-foil  exactly  equal  to  the  weight 

of  the  full  bottle. 

(3)  Keduce  a  portion  of  the  mineral  to  be  tested  to  a  coarse  sand. 

Remove  all  the  fine  dust,  and  weigh  out  a  portion  of  the 
sand  less  than  the  capacity  of  the  bottle,  say  200  grains. 
Call  this  weight  a. 

(4)  Put  the  weighed  sand  into  the  bottle,  taking  care  to  lose  none. 

Some  water  will  be  displaced  in  doing  so.  The  water  so 
displaced  will  obviously  equal  the  bulk  of  the  sand  intro- 
duced into  the  bottle. 

(5)  Again  insert  the  stopper,  wipe  dry,  and  weigh,  using  the 

counterweight.  It  will  be  found  that  the  counterweight 
plus  a  less  weight  than  a  will  produce  equilibrium.  Call 
this  weight  6.  Then,  as  before  : 

S.G.  =^=-6. 
a 

Lustre  and  Feel.  —  Many  minerals  possess  a  characteristic  lustre, 
which  may  be  :  — 

(a)  Metallic,  like  galena  ; 

(b)  Brassy,  like  pyrites  ; 

(c)  Resinous,  like  blende  ; 

(d)  Vitreous  or  glassy,  like  calcite  ; 

(e)  Silky,  like  satin-spar  and  many  fibrous  minerals. 


COMPOSITION    OF    EARTH'S    CRUST.  187 

All  the  hydrous  silicates  of  magnesia,  as,  for  example,  talc  and 
steatite,  feel  greasy  to  the  touch. 

Actinolite  and  some  other  minerals  feel  harsh. 

Colour  and  Streak. — -The  characteristic  colour  of  many  minerals 
is  a  valuable  aid  in  their  identification,  especially  in  the  case  of 
those  possessing  a  metallic  lustre.  The  colour  of  earthy  minerals 
is  liable  to  great  variation  owing  to  the  presence  of  impurities. 

The  green  colour  of  chlorite,1  malachite,  and  glauconite  ;  the  blue 
of  azurite ;  the  scarlet-red  of  cinnabar — are  nearly  always  distinctive. 

The  streak  refers  to  the  colour  of  the  powder  of  a  mineral,  and  is 
best  obtained  by  drawing  the  substance  to  be  tested  across  a  plate 
of  unglazed  porcelain. 

The  streak  of  metallic  minerals  is  generally  as  dark,  or  darker, 
than  the  colour  of  the  mineral ;  and  of  non-metallic  minerals,  as 
light,  or  lighter,  than  the  colour. 

Classification  of  Minerals. — The  two  systems  of  classification  in 
common  use  are  called  the  Chemical  and  Economic.  In  the 
Chemical  classification  the  minerals  are  arranged  according  to  their 
chemical  composition  ;  thus  the  carbonates,  sulphides,  oxides,  and 
silicates  are  brought  together  into  distinct  groups,  which  are  further 
subdivided  into  hydrous  2  and  anhydrous.3 

In  the  Economic  classification  all  the  ores  and  compounds  of  each 
metal  are  brought  together  in  one  group  ;  thus,  in  the  iron  group  we 
have  metallic  iron,  and  all  the  oxides,  sulphides,  etc.,  of  that  metal. 
This  arrangement  possesses  many  advantages  from  a  commercial 
and  technological  standpoint. 

1  Gr.  chloros  —  green.  2  Gr.  hydor  =  water. 

3  Gr.  a  =  without,  and  hydor  =  water. 


CHAPTER   XII. 
ROCK-FORMING   MINERALS. 

An  Account  of  the  more  Common  Minerals  composing  the 
Crust  of  the  Earth. 

ABOUT  three-quarters  of  the  surface  of  the  globe  are  occupied  by 
the  sea,  and  one-quarter  by  dry  land.  The  dry  land  is  mainly  com- 
posed of  such  massive  rocks  as  sandstones,  shales,  slates,  lime- 
stone, granite,  various  lavas,  etc.,  but  in  geology,  clay,  sand, 
gravel,  and  other  unconsolidated  rocky  materials  are  also  classed 
under  the  general  term  rock. 

Rocks  defined.— Many  rocks  are  aggregates  of  several  distinct 
minerals,  a  good  example  being  granite,  which  is  composed  of 
quartz,  felspar,  and  mica.  Some  rock-masses  are  composed  of 
some  one  mineral  alone  in  a  more  or  less  impure  state  ;  thus  marble 
is  an  impure  form  of  calcite,  and  dunite  an  impure  massive  form 
of  olivine. 

Examination  of  Rocks. — That  branch  of  geology  which  deals 
with  the  study  of  rock-masses  as  seen  in  the  field,  and  with  the 
minute  structure  of  rocks  as  determined  in  the  laboratory,  is  called 
Petrology.^ 

The  megascopic*  examination  of  a  rock  refers  to  the  results 
obtained  by  viewing  the  rock  with  the  naked  eye.  The  micro- 
scopic 3  examination  refers  to  the  study  of  the  minute  structure  as 
seen  in  thin  slices  placed  under  the  microscope. 

Minerals  occur  in  Two  Conditions. — A  mineral  may  occur  in 
Nature  in  two  conditions  or  forms,  namely : — 

(1)  Crystalline — that  is,  in  more  or  less  well-defined  crystals. 

(2)  Amorphous — that  is,  massive,  or  without  definite  crystalline 

structure  or  form. 

In  mineralogy  the  crystalline  form  of  a  mineral  is  frequently 
given  a  distinct  name  ;  thus  the  diamond  is  the  name  applied  to 

1  Gr.  petra  =  &  rock,  and  logos  =  description. 

2  Gr.  megas=  large,  and  sJcopein  =  to  view. 

3  Gr.  micros  =  small,  and  skopein  =  to  view. 

188 


ROCK-FORMING    MINERALS.  189 

the  crystalline  form  of  carbon,  corundum  of  alumina,  and  selenite 
of  gypsum. 

A  mineral  may  be  chemically  composed  of : — 

One  element,  as  the  diamond,  which  is  pure  carbon. 

Two  elements,  as  ordinary  table  salt,  composed  of  the  metal 
sodium,  and  the  gas  chlorine. 

Three  elements,  as  calcite,  the  principal  constituent  of  all  crystal- 
line limestones,  composed  of  the  metal  calcium,  carbon 
and  oxygen. 

Four  or  more  elements,  as  the  garnet  and  mica,  which  are  complex 
and  variable  silicates  of  many  bases. 

Gold,  silver,  platinum,  iron,  lead,  and  mercury,  and  all  the  metals 
that  occur  in  Nature  in  the  native  or  metallic  condition,  are  minerals. 
The  chemical  combinations  of  the  metals  with  oxygen,  sulphur, 
arsenic,  fluorine,  etc.,  are  commonly  spoken  of  as  ores.  For 
example  : — 

Zinc  +  sulphur  =ZnS=  zinc  blende. 
Lead  +  sulphur  =PbS  =  galena. 

All  the  ores  of  the  metals  are  classed  as  minerals,  and  their  study 
forms  an  important  branch  of  mineralogy. 

Thus  we  find  that  oxygen,  sulphur,  etc.,  possess  the  property 
of  combining  with  metals,  or  bases  as  they  are  then  called,  to 
form  a  group  of  minerals  known  as  ores.  Ores  commonly  occur 
aggregated  in  lodes  or  veins  and  in  irregular  deposits.  As  rock- 
forming  minerals  they  are  not  important,  with  the  exception  of  the 
compounds  of  iron,  which  are  abundant  and  widespread. 

Silicates  and  Carbonates. — Oxygen,  sulphur,  and  other  elementary 
substances  combine  with  the  metals  to  form  ores  ;  but  silica 
(Si02)  and  carbonic  acid  (C02)  possess  the  property  of  being  able 
to  combine  with  the  oxides  of  the  metals,  as  bases,  forming  large 
and  varied  groups  of  minerals  termed  silicates  and  carbonates 
respectively.  Both  are  important  as  rock-forming  constituents, 
the  former  occupying  the  dominant  place. 

Take  the  case  of  carbonic  acid  (carbon  dioxide). 

Acid.  Base. 

Carbonic  acid  +  lime. 

C02  +  CaO  —Calcium  carbonate. 

Carbonic  acid  +  magnesia. 

C02  +  MgO  =  magnesium  carbonate. 

Carbonic  acid  may  combine  with  one  base,  as  with  lime,  forming 
calcite;  or  with  two  bases,  forming  dolomite,  the  carbonate  of 
calcium  and  magnesium. 

Carbonate  of  lime  and  carbonate  of  magnesia,  in  both  their 


190  A  TEXT-BOOK  OF  GEOLOGY. 

crystalline  and  massive  forms,  compose  rock-masses  that  are 
frequently  of  great  extent. 

Silica  possesses  all  the  properties  of  an  acid,  and  is  hence  chemi- 
cally termed  silicic  acid.  Now  silica,  unlike  carbonic  acid,  can 
combine  not  only  with  one  but  with  two,  three,  or  more  bases  in 
the  same  compound,  giving  rise  to  an  exceedingly  varied  and 
numerous  class  of  minerals  of  homogeneous  structure  and  uniform 
composition. 

Thus  silica  may  be  combined  with — 

One  base,  as  in  talc,  the  silicate  of  magnesia  ; 
Two  bases,  as  in  olivine,  the  silicate  of  magnesia  and  iron  ; 
Three  bases,  as  in  epidote,  the  silicate  of  alumina,  lime,  and  iron  ; 
Four  or  more  bases,  as  in  mica  (muscovite),  a  silicate,  of  alumina, 
potash,  and  other  bases. 

From  what  has  been  said,  we  see  that  silica  may  occur  in 
Nature  as — 

(1)  Free  or  uncombined,  as  in  quartz,  which  is  the  principal  con- 

stituent of  beach  sand  and  sandstones. 

(2)  Combined  with  bases  such  as  alumina,  lime,  magnesia,  soda, 

potash,  etc.,  forming  the  vast  group  of  minerals  termed 
silicates. 

PRINCIPAL  HOCK-FORMING  MINERALS. 

A  great  many  minerals  enter  into  the  constitution  of  the  crust 
of  the  Earth»  but  the  main  mass  is  composed  of  a  few  predominating 
compounds  of  these  :  silica,  Si02,  in  its  free  and  combined  condi- 
tions constitutes  more  than  half  of  the  known  crust. 

Alumina,  nearly  all  of  which  occurs  combined  with  silica,  is 
present  to  the  extent  of  15  per  cent. 

After  alumina  follow  iron  oxides,  7-5  per  cent. ;  lime,  5-5  per 
cent. ;  magnesia,  4-5  per  cent. ;  soda  and  potash,  each  3  per  cent. 
All  of  these,  except  a  portion  of  the  iron,  exist  in  the  condition 
of  carbonates  and  silicates. 

The  principal  rock-forming  minerals  are  as  follow  : — 

(1)  Quartz.  (10)  Nepheline. 

(2)  Felspar.  (11)  Tourmaline. 

(3)  Mica.  (12)  Calcite. 

(4)  Olivine.  (13)  Aragonite. 

(5)  Serpentine.  (14)  Dolomite. 

(6)  Chlorite.  (15)  Fluorite. 

(7)  Hornblende.  (16)  Apatite. 

(8)  Augite.  (17)  Iron  ores. 

(9)  Rhombic  pyroxene. 


ROCK-FORMING    MINERALS.  191 

Primary  and  Secondary  Minerals. — A  primary  mineral  or  rock 
constituent  is  one  that  is  developed  during  the  cooling  of  the 
molten  magma,  or,  in  the  case  of  a  sedimentary  rock,  that  appeared 
among  the  original  constituents. 

A  secondary  mineral  is  one  that  appeared  after  the  rock- mass 
was  formed.  It  is  generally  a  product  of  the  alteration  or  decom- 
position of  one  of  the  original  or  primary  minerals. 

Essential  Mineral.  —  Many  kinds  of  rock  are  recognised  by 
geologists  as  being  composed  of  an  aggregate  of  certain  minerals. 
Thus  granite,  as  previously  stated,  is  an  aggregate  of  quartz,  felspar, 
and  mica.  If  any  one  of  these  be  absent,  the  rock  would  not  be 
recognised  as  a  granite  ;  hence  these  three  are  spoken  of  as  essential 
minerals. 

Accessory  Minerals.— These  are  minerals  that  may  or  may  not 
be  present  in  a  rock.  They  are  accessory  because  their  presence 
or  absence  does  not  alter  the  constitution  of  the  rock,  though, 
if  abundant,  they  may  modify  it  to  some  extent.  Thus,  when 
tourmaline  is  present  in  granite  it  is  merely  accessory. 

Quartz. — This  occurs  in  both  the  crystalline  and  amorphous 
or  chalcedonic  forms.  It  is  harder  than  steel,  and  therefore  cannot 
be  scratched  with  a  knife  or  file.  On  account  of  its  great  hardness 
it  is  frequently  the  last  or  ultimate  residue  of  the  detritus  derived 
from  the  denudation  of  a  land  area  ;  for  while  the  softer  materials 
are  reduced  by  attrition  to  the  condition  of  mud  either  during 
their  transport  to  the  sea  or  after  they  reach  the  sea,  the  quartz 
particles  manage  to  survive,  although  doubtless  greatly  reduced 
in  size. 

These  surviving  quartz  grains,  sometimes  angular,  sometimes 
semiangular,  and  frequently  rounded  in  shape,  are  piled  up  on  sea 
and  lake  beaches,  forming  the  familiar  sea-sands  found  on  nearly 
every  strand. 

When  free  from  impurities,  quartz  is  clear  and  transparent, 
but  it  is  frequently  pale-grey,  pale-yellow,  golden- yellow,  or  reddish- 
brown  in  colour  owing  to  the  presence  of  iron  oxides.  The  intensity 
of  colour  becomes  greater  as  the  percentage  of  iron  oxide  increases. 

Quartz  is  the  principal  constituent  of  sandstones,  and  is  an 
essential  constituent  of  mica-schist,  gneiss,  quartzite,  rhyolite, 
and  quartz-porphyry.  As  a  secondary  mineral  deposited  from 
slowly  moving  siliceous  waters  it  occurs,  filling  cracks,  fissures,  and 
cavities.  It  is  frequently  developed  in  igneous  rocks  as  a  secondary 
product  resulting  from  the  alteration  or  decomposition  of  silicates. 

Large  bodies  of  quartz  in  the  form  of  siliceous  sinter  are  deposited 
by  thermal  springs  in  many  volcanic  regions. 

Siliceous  sinter  is  deposited  in  successive  layers,  and  for  that 
reason  frequently  possesses  a  banded  or  laminated  structure. 


192  A  TEXT-BOOK  OF  GEOLOGY. 

When  newly  formed  it  is  massive  or  amorphous,  but  in  course 
of  time  it  develops  a  finely  crystalline  structure. 

The  principal  varieties  of  crystalline  quartz  are  as  follows  : — 

Rock  crystal  is  a  colourless  transparent  variety  much   used   for 

spectacle-glasses,  lenses,  etc. 
Amethyst,  which  is  a  purple  or  violet  variety  often  of  great  beauty. 

The  colour  is  mainly  due  to  the  presence  of  manganese 

oxide. 

Cairngorm  has  a  fine  smoky-yellow  or  brown  colour. 
Ferruginous  quartz  possesses  a  yellow  or  reddish-brown  colour 

due  to  the  presence  of  iron  peroxide.     Abundant  in  many 

lands. 

Among  the  numerous  varieties  of  massive  or  chalcedonic  quartz 
are  : — 

Chalcedony,  found  lining  cavities  in  rocks  and  as  stalactites. 
The  colour  is  often  milk-white,  yellow,  brown,  or  lavender- 
blue. 

Carnelian,  red  or  reddish-brown. 

Flint,  of  various  shades  from  grey  to  black.  Occurs  as  nodules 
in  chalk,  and  as  beds,  forming  rock-masses. 

Agate  is  a  variegated  and  banded  chalcedony. 

Plasma  is  a  leek- green  variety  speckled  with  white. 

Heliotrope  or  bloodstone  is  a  leek-green  variety  speckled  with 
red. 

Onyx,  a  banded  variety  of  chalcedony. 

Chert,  a  calcareous  form  of  massive  quartz,  occurs  in  nodules 
and  beds,  and  is  a  rock  rather  than  a  mineral. 

Jasper,  a  massive  or  very  finely  crystalline  quartz  coloured  red, 
reddish-brown,  or  yellow  by  iron  oxides.  In  some  of  the 
older  formations  there  occur  beds  of  hard,  fine-grained, 
red,  or  purple  siliceous  shales  and  slates,  which  are  generally 
spoken  of  as  jasperoid  shales  or  jasperoid  slates. 

Among  the  different  forms  of  hydrous  silica  are  : — 

Opal,  which  occurs  in  great  variety  ranging  from  wood-opal  to 
the  gem  noble  opal.  Wood-opal  is  what  is  familiarly 
termed  silicified  or  petrified  wood.  It  is  merely  a  replace- 
ment of  wood  by  particles  of  hydrous  silica. 

Felspar. — This  important  family  consists  of  several  minerals, 
which  show  a  close  relationship  in  chemical  and  physical  properties, 
as  well  as  in  their  mode  of  occurrence. 

Chemically  considered,  the  felspars  are  silicates  of  alumina  and 
one  or  more  of  the  bases  potash,  soda,  and  lime. 

The  cleavage  of  the  felspars  is  specially  characteristic,  and  it 


ROCK-FORMING    MINERALS.  193 

enables  the  different  species  to  be  divided  into  two  natural  groups, 
namely : — 

I.   Orthoclase.1 
II.  Plagioclase.2 

This  *  subdivision  is  based  on  the  direction  of  the  cleavage- 
planes  ;  for,  whereas  all  the  felspar  minerals  show  good  cleavage  in 
two  directions,  in  orthoclase  felspar  these  two  directions  are  aft  right 
angles  to  one  another,  and  in  the  plagioclase  felspars  they  are 
slightly  oblique.  In  other  words,  orthoclase  crystallises  yin  the 
monoclinic  crystal-system,  and  plagioclase  in  the  triclinic. 

The  distinguishing  character  of  plagioclase  is  the  appearance  of 
fine  lamellce  (see  fig.  139),  arising  from  polysynthetic  twinning, 
which  is  never  exhibited  by  orthoclase. 

Orthoclase  usually  presents  dual  twinning  on  various  types,  the 
commonest  being  the  Carlsbad.  Twinning  may  often  be  detected 
with  a  hand  lens,  but  is  best  seen  in  thin  sections  viewed  in  polarised 
light. 

Orthoclase  (monoclinic)  is  typically  a  silicate  of  alumina  and 
potash,  consisting  of  silica  -64- 6,  alumina  =18*5,  and  potash  =16*9 
per  cent.  When  soda  replaces  the  potash  we  get  soda-orthoclase, 
which  is  a  triclinic  felspar.  Orthoclase  is  an  essential  constituent 
of  granite,  gneiss,  and  syenite,  in  which  it  occurs  as  tabular  crystals 
of  a  grey,  creamy,  or  pink  colour. 

Sanidine  is  a  clear  glassy  variety  of  orthoclase.  It  is  a  common 
constituent  of  modern  acidic  lavas,  as  rhyolite,  trachyte,  and 
obsidian. 

Plagioelases  or  Triclinic  felspars.  These  include  two  distinct 
species,  albite,  typically  a  silicate  of  alumina  and  soda;  and  anorthite, 
typically  a  silicate  of  alumina  and  lime.  Between  albite,  which 
represents  the  acidic  type  of  plagioclase  and  anorthite,  the  basic, 
there  are  various  mixtures  of  these  two,  producing  a  continuous 
series  of  closely  related  minerals. 

Let  Ab  =  albite  =  Na2Al2Si6016. 
An  =  anorthite  =  CaAl2Si208. 

Thus  the  series  of  plagioclases  includes — 

Albite3  (pure)  =Ab. 
Oligoclase  4       =Ab6An1  to 
Andesine  5         =Ab3An1  to 

1  Gr.  orikos— straight,  and  Uasis= breaking. 

2  Gr.  plagios  =  slanting,  and  Tdasis  =  breaking. 

3  Lat.  alba= white. 

4  Gr.  oligos  =  small,  and  klasls  —  breaking. 
6  From  Andes  in  South  America. 

13 


194  A  TEXT-BOOK  OF  GEOLOGY. 

Labradorite1  (most  acid)  =Ab1An1. 
Labradorite  (medium)      =Ab3  An4. 
Anorthite  2  (nearly  pure)  =  An. 

For  the  most  part  the  plagioclase  felspars  are  constituents  of  the 
basic  and  semi-basic  or  intermediate  types  of  igneous  rocks,  and 
orthoclase,  with  its  glassy  variety,  sanidine,  of  the  acidic. 

Acidic  and  Basic  defined. — An  acid  or  acidic  rock  or  mineral 
is  one  in  which  the  silica  or  silicic  acid,  Si02,  is  in  excess  of  the 
bases  ;  and  a  basic  rock  or  mineral  is  one  in  which  the  bases  pre- 
dominate. Take  the  case  of  orthoclase.  Its  composition  is — 

Silica,     ,        .         .         .       64-60  per  cent. 
Alumina,         .         .         .       18-45     .  ,, 
Potash,  .         .         .         „       16-95       „ 


100-00  per  cent. 

The  silica  exceeds  the  sum  of  the  bases,  alumina  and  potash ; 
therefore  this  mineral  is  acid  or  acidic. 

Mica.3 — The  mica  family  comprises  a  great  many  species,  all  of 
which  occur  in  thin  flexible  plates.  The  micas  are  silicates  of 
alumina  and  other  bases.  The  most  important  as  rock-forming 
minerals  are  as  follow  : — 

Muscovite*  (Potash-mica)  occurs  in  thin  transparent  plates,  and 
is  an  essential  constituent  of  granite,  gneiss,  mica-schist, 
and  many  crystalline  rocks.  It  is  the  white  mica  of 
commerce,  and  in  plates  over  two  inches  square  is  of 
considerable  value. 

Biotite5  (Magnesia-mica)  is  a  black  mica  which  is  abundant  in 
some  granites,  gneisses,  and  schists. 

Lepidolite  6  (Lithia-mica)  possesses  a  ruby-red  or  peach-blossom 
colour.  It  is  found  in  some  granites  and  schists. 

Sericite  is  a  colourless  hydrous  mica,  produced  by  the  alteration 
of  alkali-felspar.  It  is  also  developed  by  the  action  of  great 
pressure,  and  hence  is  abundant  in  schists  that  have  been  altered 
by  dynamo-metamorphism. 

Olivine. — This  is  a  silicate  of  magnesia  and  iron.  It  is  an  essen- 
tial constituent  of  basalt,  and  forms  the  main  mass  of  olivine 

1  From  Labrador  in  North  America. 

2  Gr.  a  =  without  or  not,  and  orthos  =  straight. 

3  Lat.  mico  =  I  glisten. 

4  From  Muscovy. 

5  From  Biot,  the  French  mineralogist. 

6  Gr.  lepis  =  &  scale,  and  Uthos  =  &  stone. 


ROCK-FORMING    MINERALS.  195 

or  peridotite ;  a  rock  which  in  some  places  occurs  in  masses  of  great 
extent. 

Serpentine. — This  is  the  hydrous  silicate  of  magnesia  and  iron. 
It  frequently  forms  rock-masses  and  also  occurs  in  veins  and  nests 
in  basic  igneous  rocks.  It  is  an  alteration  product  of  olivine  or 
other  basic  minerals. 

Chlorite. — This  is  a  hydrated  silicate  of  magnesia,  alumina,  and 
iron  which  occurs  in  small  dark  olive-green  scales,  or  in  green 
earthy  aggregates.  It  is  an  essential  constituent  of  chlorite- 
schist,  and  is  quite  common  as  an  alteration  product  of  hornblende 
in  igneous  and  metamorphic  rocks. 

Hornblende. — This  is  a  silicate  of  magnesia,  lime,  alumina,  and 
other  bases.  It  includes  a  great  many  varieties,  which  are  arranged 
in  two  groups  : — 

(1)  Aluminous  =  brown  or  black  varieties. 

(2)  Non-aluminous  =  pale-green  and  grey  fibrous  varieties. 

In  a  general  way  it  may  be  said  that  the  dark  hornblendes  affect 
semi-basic  rocks,  such  as  diorite  and  andesite ;  and  the  pale  green 
fibrous  varieties  acidic  rocks,  such  as  gneiss. 

The  dark  varieties  also  form  rock-masses,  as  in  the  case  of  horn- 
blende-schist and  amphibolite. 

Hornblende  is  an  essential  constituent  of  syenite,  diorite,  and 
hornblende-andesite,  but  it  occurs  abundantly  as  an  alteration 
product  of  augite. 

The  name  amphibole  is  frequently  used  as  a  family  name  to 
include  all  the  varieties  of  hornblende. 

Augite.1 — A  variable  silicate  of  lime,  magnesia,  alumina,  iron, 
and  manganese.  It  includes  many  varieties,  which  are  generally 
grouped  under  the  family  name  monoclinic  pyroxene. 

Like  hornblende,  the  augites  fall  into  two  natural  groups, 
namely  : — 

(1)  Aluminous  =  dark  varieties,  including  common  augite. 

(2)  Non-aluminous—green  varieties. 

The  green  varieties  are  found  abundantly  in  metamorphic  rocks, 
as  gneiss,  crystalline  limestone,  and  various  schists  ;  and  the 
dark  or  aluminous  varieties,  in  rocks  of  a  basic  type,  as  basalt, 
diabase,  and  andesite.  The  clear-green  variety  diallage  is  found 
in  serpentine  and  gabbro. 

Rhombic  Pyroxenes.2 — These  are  variable  silicates  that  occur 
abundantly  in  many  basic  igneous  rocks.  The  most  common 

1  Gr.  a uge= lustre. 

2  Gr.  pur  =  &ce,  and  xenos  =  a,  stranger. 


196  A  TEXT-BOOK  OF  GEOLOGY. 

varieties  are  enstatite,  bronzite,  and  hypersthene,  the  former  being 
plentiful  in  serpentine  and  olivine  rocks. 

Nepheline.1 — A  silicate  of  alumina  and  soda  with  some  potash. 
This  is  an  important  constituent  of  alkali  volcanic  rocks.  It  is 
always  present  in  phonolite,  and  is  also  found  in  some  basalts. 

Some  greenish  and  reddish  massive  varieties  of  nepheline, 
known  as  elceolite,  occur  in  some  syenites  and  ancient  crystalline 
rocks. 

Tourmaline. — A  silicate  of  alumina,  iron,  and  other  bases.  Colour 
generally  black,  but  green  and  red  varieties  are  not  uncommon. 
Frequently  occurs  in  long  well-developed  hexagonal  prisms. 

Tourmaline  commonly  occurs  in  granites,  gneisses,  schists,  and 
crystalline  limestones.  With  quartz  it  forms  tourmaline-rock. 
This  mineral  is  the  nearly  constant  associate  of  tin  ore. 

Calcite  (CaC03). —  This  is  the  principal  constituent  of  all 
limestones.  It  is  present  in  many  shales  and  sandstones.  As  a 
secondary  product  resulting  from  the  alteration  of  silicates  con- 
taining lime,  calcite  is  found  filling  cracks.,  fissures,  and  cavities  in 
many  igneous  and  crystalline  rocks. 

It  is  deposited  by  water  in  caves,  forming  stalactites  which  hang 
from  the  roof,  and  stalagmites  which  grow  up  from  the  floor. 

The  soft,  spongy,  or  porous  variety  deposited  by  water  at  the 
foot  of  limestone  cliffs  and  in  rock-shelters  is  a  calcareous  sinter 
known  as  travertine. 

Aragonite  (CaC03). — This  is  the  rhombic  form  of  carbonate  of 
lime.  It  composes  the  shells  of  many  molluscs,  but  is  a  less  stable 
compound  than  calcite.  It  is  not  abundant,  being  generally  found 
in  thin  veins  in  basalt  and  other  igneous  rocks.  The  fibrous  variety 
is  often  very  beautiful. 

Dolomite  (Carbonate  of  lime  and  magnesia).  — This  forms 
extensive  beds  of  massive  magnesian  limestone  belonging  to  many 
different  geological  formations.  It  also  occurs  in  small  quantity 
as  an  alteration  product  of  ordinary  limestone  and  aragonite. 

Fluorite  (Fluoride  of  calcium  =fluor  spar,  CaF2). — This  generally 
occurs  in  limestone  formation  as  the  gangue  or  matrix  of  lead  and 
zinc  ores. 

Apatite. — This  is  the  phosphate  of  lime  with  a  little  fluoride  or 
chloride  of  calcium.  It  occurs  in  large  crystals  and  as  massive 
deposits  in  metamorphic  rocks.  Minute  needles  are  common  in 
many  granites,  schists,  and  basalts. 

Iron  Ores. — Iron  in  its  various  forms  is  one  of  the  most  widely 

distributed  of  all  the  substances  that  enter  into  the  structure  of  the 

Earth's  crust,  being  found  in  rocks  of  all  kinds  and  all  ages.     It 

occurs   combined   with   silica   in   many   rocks   and   rock-forming 

1  Gr.  nephele=&  cloud. 


ROCK-FORMING    MINERALS.  197 

minerals,  and  also  as  separate  compounds  of  oxygen,  sulphur,  etc., 
forming  what  are  termed  ores  of  iron. 

Silica  combines  with  the  protoxide  of  iron  and  other  bases, 
forming  silicates.  The  indistinct  green  or  bluish-green  colour 
which  is  so  prevalent  in  all  classes  of  rock  is  commonly  due  to  the 
presence  of  iron.  When  such  rocks  weather  or  become  decomposed, 
the  silicates  are  frequently  broken  up  owing  to  the  removal  of  one 
or  more  of  the  bases.  The  iron  protoxide,  FeO,  being  liberated, 
changes  or  oxidises  into  the  peroxide,  Fe203,  which  possesses  a  red 
or  rusty-brown  colour.  Thus  it  comes  about,  as  we  so  frequently 
find,  that  rocks  which  possess  a  pale-green  colour  in  the  fresh 
undecomposed  portions,  become  red  or  rusty-brown  on  weathered 
surfaces,  or  even  produce  brick-clays  that  are  yellow  or  reddish- 
brown. 

The  most  abundant  natural  compounds  of  iron  are  as  follow  : — 

Native  iron,  found  in  meteorites  and   serpentine  alloyed  with 

nickel. 
Iron  protoxide,  FeO,  not  in  free  state,  but  combined  with  silica 

in  many  silicates. 

Magnetite,  Fe304,  the  black  magnetic  oxide. 
Hcematite,  Fe203,  the  red  peroxide,  i.e.  highest  oxide. 
Limonite  or  brown  hydrous  peroxide. 
Pyrite,  FeS2,  the  yellow  sulphide. 
Marcasile,  FeS2,  the  white  sulphide. 
Pyrrhotite,  Fe7S8,  the  magnetic  sulphide. 

Titanite  (Titaniferous  iron),  a  black,  feebly  magnetic  mineral. 
Glauconite,  a  dark-green  hydrous  silicate  of  magnesia  and  iron. 

Magnetite. — This  mineral  is  commonly  found  in  igneous  and 
crystalline  rocks.  It  occurs  in  thick  beds,  irregular  masses  frequently 
of  great  extent,  and  as  small  grains  disseminated  throughout  many 
igneous  and  altered  rocks.  In  rocks  subject  to  weathering  it 
changes  first  to  the  carbonate  and  then  to  the  brown  or  red  peroxide. 
Hence  sands,  gravels,  and  compact  rocks  containing  magnetite 
soon  assume  a  rusty-brown  colour  on  the  surface  when  exposed  to 
the  action  of  air  and  water. 

HcBmatite. — This  valuable  ore  of  iron  occurs  as  beds  interstratified 
with  sedimentary  and  schistose  rocks,  and  as  a  constituent  of  many 
mineral  veins. 

Limonite. — This  ore  occurs  in  beds  and  irregular  deposits  in 
stratified  formations,  and  as  the  gossan  or  cap  of  sulphide  lodes. 
In  the  form  of  bog-iron  it  is  frequently  found  as  irregular  sheets  on 
the  lake-bottoms  and  in  marsh  lands  where  it  has  been  deposited 
by  the  action  of  organic  acids. 


198  A  TEXT-BOOK  OF  GEOLOGY. 

This  is  the  oxide  of  iron  which  gives  the  prevailing  yellow  or 
rusty-brown  colour  to  soils,  clays,  sands,  and  many  sandstones. 

Pyrite. — This  mineral  is  present  in  the  majority  of  gold,  silver, 
copper,  and  other  mineral  veins.  It  also  occurs  as  disseminated 
crystalline  grains  in  slates,  and  many  varieties  of  schistose  rock. 
As  a  secondary  product  it  is  frequently  abundant  in  altered  ande- 
sites  and  other  igneous  rocks.  It  is  also  common  as  nodules  and 
pseudomorphs  in  clays  and  shales.  Pyrite  in  its  crystalline  form 
is  a  very  stable  compound,  being  hardly  affected  by  atmospheric 
oxidising  agents  even  after  long  exposure. 

Marcasite. — This  is  the  rhombic  form  of  iron  disulphide.  It  is 
quite  common  in  clays,  shales,  coal,  and  all  stratified  formations, 
also  in  mineral  veins.  It  decomposes  very  rapidly  when  exposed 
to  moist  air,  liberating  free  sulphuric  acid  which  at  once  attacks 
the  minerals  with  which  it  comes  in  contact,  forming  alum,  gypsum, 
or  other  sulphates. 

Pyrrhotite. — This  mineral  is  not  so  widely  distributed  as  pyrite 
and  marcasite.  It  occurs  mostly  as  grains  and  masses,  impreg- 
nating metamorphic  or  crystalline  rocks. 

Titanite. — This  composes  the  black  titanic  iron-sand  found  on 
many  sea-beaches.  It  occurs  as  scattered  grains  and  plates  in 
many  igneous  and  metamorphic  rocks.  It  is  a  very  stable  com- 
pound, and  for  that  reason  is  able  to  resist  weathering  for  a  long 
time  without  alteration  or  oxidation. 

Glauconite. — This  is  an  important  constituent  of  many  sand- 
stones and  limestones  to  which  it  frequently  imparts  a  characteristic- 
green  colour.  It  is  found  filling  and  coating  foraminifera  and  other 
minute  organisms,  and  is  generally  believed  to  have  an  organic 
origin.  Glauconitic  greensands  are  prevalent  in  Cretaceous  and 
Lower  Tertiary  marine  rocks  in  all  parts  of  the  globe,  but  are 
unknown  now  among  the  Palaeozoic  formations.  It  is  probable 
that  most  of  the  valuable  aggregations  of  iron-ore  associated  with 
the  more  ancient  altered  sedimentary  rocks  are  composed  of  iron 
segregated  from  Palaeozoic  glauconitic  sandstones  and  limestones. 


CHAPTER  XIII. 
SEDIMENTARY  ROCKS. 

A  ROCK  may  be  composed  of  one  or  more  simple  minerals,  or  it  may 
be  a  mechanical  aggregate  of  particles  derived  from  pre-existing 
rocks. 

Classification  of  Rocks. — Rocks  may  be  grouped  in  two  great 
natural  classes,  namely  : — 

I.  Sedimentary  or  Aqueous. 
II.  Igneous. 

The  altered  forms  of  sedimentary  and  igneous  rocks  constitute  a 
third  class : — 

III.  Metamorphic. 

The  grouping  of  rocks  as  Sedimentary  and  Igneous  is  purely 
genetic  and  therefore  based  on  a  scientific  principle.  The  class 
Metamorphic  does  not  possess  the  same  value,  as  it  merely  comprises 
altered  forms  of  rocks  that,  in  their  unaltered  condition,  are 
included  in  the  other  two  classes.  Its  use,  however,  may  be 
defended  on  the  grounds  of  expediency  and  convenience. 

Sedimentary  Rocks. 

Sedimentary  rocks,  as  the  name  implies,  are  composed  of  sedi- 
ments that  were  laid  down  by  the  agency  of  water;  hence  the 
equivalent  name  Aqueous  so  frequently  applied  to  them.  They 
are  also  called  Clastic  or  Fragmentary,  but  the  second  of  these  is 
open  to  the  objection  that  many  masses  of  igneous  rocks  are  frag- 
mentary, but  in  no  sense  sedimentary  or  aqueous. 

Sedimentary  rocks  that  are  composed  of  material  derived  from 
the  denudation  of  pre-existing  rocks  are  said  to  be  detrital ;  that 
is,  mechanically  formed.  Those  formed  by  the  accumulation  of 
organisms,  either  calcareous,  siliceous,  or  carbonaceous,  are  termed 
organic  ;  while  the  minerals  that  accumulate  on  the  floor  of  lakes 
and  inland  seas  as  the  result  of  chemical  precipitation  or  evapora- 
tion are  called  chemical. 

199 


200  A  TEXT-BOOK  OF  GEOLOGY. 

Here  we  have  a  basis  for  a  threefold  subdivision  of  sedimentary 
rocks  :— 

1.  Detrital. 

2.  Organic. 

3.  Chemical. 

These  three  groups  are  further  subdivided  as  under  :  — 

.    ,     J~(a)  Arenaceous1  —  Sandy  and  pebbly  rocks. 
\(6)  Argillaceous  2—  Clays  and  shales. 

{(a)  Calcareous  3  —  Limestones. 
(6)  Siliceous  4  —  Cherts  and  flints. 
(c)  Carbonaceous-Coals. 
(d)  Ferruginous  5  —  Ironstones. 


C(a)  Carbonates  —  Limesto 

Q    Chemical  J  (6>  Sulphates—  Gypsum. 

1  (c)  Chlorides—  Rock-salt. 

[_(d)  Silica  —  Siliceous  sinte 


Detrital  Group. 

ARENACEOUS  ROCKS. 
The  main  types  of  rock  included  in  this  group  are  :  — 

1.  Breccia. 

2.  Conglomerate. 

3.  Sandstones  and  gritstones. 

Breccia.6  —  This  is  a  rock  composed  of  angular  fragments  of  stone 
cemented  in  a  paste  of  sand  or  mud,  or  set  in  a  matrix  of  carbonate 
of  lime,  silica,  or  oxide  of  iron  (Plate  XV.). 

Breccias  were  formed  in  places  where  frost-formed  screes  or  talus- 
slides  descended  into  sheltered  bays  or  lake-basins  in  which  the 
material  was  spread  out  near  the  shore  without  being  subjected  to 
the  wear  and  tear  or  sorting  action  of  rapidly-moving  currents. 

Breccias,  from  the  nature  of  their  formation,  may  sometimes 
attain  a  great  thickness,  but  they  seldom  cover  an  area  of  large 
extent. 

Some  breccias  exhibit  a  rude  stratification,  and  in  places  where 
they  have  accumulated  slowly  they  may  even  contain  fossils.  As 
a  rule,  however,  they  are  not  fossiliferous. 


1  Lat.  arma=  sand.  2  Lat.  argilla  =  G\a,y.  3  Lat.  calx  =  ]ime. 

4  Lat.  silex  =  flint.  5  Lat.  ferrum  =  iron. 

6  It.  breccia  =  a,  crumb  (pronounced  brechia). 


To  face  page  200.] 


[PLATE    XV. 


QUARTZITE  AND  CHERT  BRECCIA — UTAH.     (U.S.  Geol.  Survey.) 


SEDIMENTARY   ROCKS.  201 

Many  breccias  contain  a  variable  proportion  of  water-worn 
material,  and  some  are  known  to  pass  in  the  same  plane  into 
coarse  conglomerate. 

A  breccia  composed  principally  of  angular  slaty  fragments  is 
called  a  slaty-breccia  ;  of  sandstone,  a  sandstone-breccia  ;  of  mica- 
schist,  a  mica- schist-breccia  ;  of  quartz,  a  guartzose-breccia,  and 
so  on. 

The  fragments  composing  a  breccia  may  range  from  less  than 
an  inch  to  many  feet  in  diameter. 

Not  infrequently  a  bed  of  very  coarse  breccia  riding  hard  on 
an  old  shore-line  is  found  at  the  base  of  a  conglomerate.  Such 
a  breccia  may  contain  angular  masses  of  rock  ten  feet  or  more  in 
diameter,  torn  from  the  bed-rock  on  which  it  rests.  Such  a  deposit 
would  appear  to  have  been  formed  by  the  undercutting  of  steep 


FIG.  115. — Showing  breccia.     (After  Davis,  U.S.  GeoL  Survey.) 

sea-cliffs,  at  the  foot  of  which  the  water  was  too  deep  for  the  fallen 
blocks  to  be  subjected  to  the  pounding  and  rounding  effects  of 
wave-action. 

Moraine-breccias  have  been  formed  where  the  angular  ice-borne 
material  was  shot  into  a  lake,  estuary,  or  fiord ;  or  left  when  the 
ice  melted. 

Fault-  or  Friction-breccias  are  frequently  found  on  the  walls  of 
great  faults  where  they  were  formed  by  the  crushing  and  breaking 
up  of  the  wall-rock  during  fault-movements. 

Friction-breccias  also  occur  on  the  walls  of  many  large  lodes. 
They  are  an  evidence  that  movement  has  taken  place  since  the 
filling  and  consolidation  of  the  lode-matter.  Breccias  of  this 
kind  are  generally  lens-shaped.  Only  in  rare  cases  are  they  co- 
extensive with  the  lode  itself.  Not  infrequently  the  lode-matter 
itself  is  found  to  be  brecciated,  showing  (a)  that  the  movement 
took  place  before  the  lode-filling  had  become  hardened,  or  (b)  that 
the  wall-rock  was  stronger  than  the  filling. 


202  A  TEXT-BOOK  OF  GEOLOGY. 

Shear-breccias  occur  along  the  course  of  shear-zones.  They 
have  arisen  from  the  crushing  and  shattering  of  the  county-rock 
traversed  by  a  shear-plane,  and  the  subsequent  cementing  of  the 
fragments  by  the  infiltration  of  silica,  oxide  of  iron,  or  carbonate 
of  lime. 

Wedges  of  hard  rock  that  have  become  entangled  in  great 
overturned  folds  are  frequently  found  to  be  crushed  and  brecciated. 

Friction  and  shear-breccias  are  sometimes  called  crush-breccias. 
They  are  purely  dynamical  in  origin,  and  hence  fundamentally 
different  from  ordinary  sedimentary  breccias  from  which  they  are 
not  always  easily  distinguished. 

Sedimentary  breccias  afford  valuable  evidence  of  former  terres- 
trial conditions.  They  tell  us,  for  example,  of  the  existence  of 
high  land  near  the  ancient  strands,  of  frost  and  glacier  action. 
Crush-breccias,  on  the  other  hand,  help  us  to  distinguish  the  zones 
of  rock  that  have  been  subjected  to  intense  folding,  shearing,  and 
faulting. 

A  certain  class  of  fragmentary  volcanic  rock  is  called  a  volcanic- 
breccia,  a  description  of  which  will  be  found  in  the  chapter  dealing 
with  igneous  rocks. 

Conglomerate. — This  is  a  rock  composed  of  consolidated  gravel  or 
shingle.  The  material  comprising  a  conglomerate  is  water-worn  and 
well-rounded,  and  has  given  rise  to  the  popular  name  pudding-stone. 

The  constituent  pebbles  usually  represent  the  hardest  rocks 
in  the  region,  or  those  hard  enough  to  survive  the  wear  and  tear 
of  fluviatile  or  marine  transport.  But  conglomerates  formed  at 
the  foot  of  sea-cliffs  in  sheltered  bays  may  be  composed  of  lime- 
stone pebbles,  or  of  other  rocks  not  noted  for  their  hardness.  Such 
conglomerates  are  not  common. 

A  conglomerate  that  contains  a  considerable  proportion  of 
angular  rock-fragments  may  be  called  a  breccia-conglomerate. 

When  the  pebbles  in  a  conglomerate  are  mainly  granite,  the  rock 
is  called  a  granite-conglomerate  ;  when  sandstone,  a  sandstone- 
conglomerate',  when  quartz,  a  quartzose  conglomerate',  and  when 
schist,  a  schist-conglomerate. 

The  cementing  medium  may  be  a  matrix  of  fine  sand  or  mud, 
carbonate  of  lime,  silica,  or  oxide  of  iron.  The  descriptive  name  of 
the  conglomerate  may  be  qualified  by  an  adjective  denoting  the 
nature  of  the  cementing  matrix.  Thus,  if  the  cement  of  a  quartzose- 
conglomerate  is  oxide  of  iron,  the  rock  might  very  well  be  called 
&  ferruginous  quartzose-conglomerate  ;  or  if  silica,  as  we  find  in  the 
gold-bearing  banket  of  the  Transvaal,  we  may  call  the  rock  a 
siliceous  quartzose-conglomerate.  These  names  for  all  purposes, 
except  perhaps  the  exact  petrological  description,  would  be 
shortened  to  ferruginous-conglomerate  and  siliceous-conglomerate. 


SEDIMENTARY    ROCKS.  203 

Conglomerates  are  essentially  shore-deposits,  and  where  they 
were  formed  slowly  may  contain  fossils  mixed  with  the  constituent 
pebbles  and  sands.  Some  of  the  boulders  and  pebbles  may  be 
fossiliferous,  but  obviously  such  fossils  were  derived  from  an  older 
rock-formation,  and  therefore  do  not  indicate  the  age  of  the  con- 
glomerate. It  is  quite  possible,  for  example,  for  a  Tertiary  con- 
glomerate to  contain  fossiliferous  boulders  derived  from  a  Silurian 
formation.  Moreover,  a  conglomerate  may  contain  rocks  that  have 
appeared  as  constituents  of  different  formations.  Thus,  in  the 
King  County  of  New  Zealand  there  is  a  coarse  conglomerate  at  the 
base  of  the  Lower  Tertiaries  mainly  composed  of  granite,  gneiss, 
and  crystalline  schists  derived  from  a  still  coarser  conglomerate 
interbedded  with  the  neighbouring  Triassic  rocks. 

Conglomerates,  as  might  be  gathered  from  the  manner  in  which 
the  original  gravels  were  formed,  thin  out  rapidly  when  traced 
seaward  from  the  old  strand.  They  are  frequently  intercalated 
with  tapering  beds  of  sandstone  and  shale. 

What  are  called  crush-conglomerates  are  sometimes  found  on  the 
walls  of  powerful  faults.  They  consist  of  large  fragments  of  wall- 
rock  that  have  become  more  or  less  rounded,  polished,  and  some- 
times striated  with  the  rolling  and  kneading  action  to  which  they 
have  been  subjected  during  the  fault-movements.  The  boulders 
are  usually  embedded  in  a  matrix  of  stiff  clay  composed  of 
crushed  rock.  The  origin  of  a  crush-conglomerate  is  purely 
dynamical. 

Sandstones  and  Grits. — Sandstones  are  merely  consolidated  sands. 

River  and  sea-sands  are  principally  composed  of  quartz  grains, 
but  the  composition  of  the  sand  in  any  given  locality  depends 
principally  on  the  nature  of  the  country  from  which  it  is  derived. 

Sands  derived  from  the  denudation  of  granite,  gneiss,  mica- 
schist,  or  sandstone  consist  mainly  of  quartz  frequently  associated 
with  a  small  amount  of  magnetite,  rutile,  zircon,  garnet,  and 
tourmaline.  If  the  sands  occur  in  a  situation  where  they  have 
not  been  subjected  to  much  attrition  by  wave-action  or  sea-currents, 
they  may  contain  a  small  percentage  of  mica  and  orthoclase.  The 
so-called  black  sands  of  New  Zealand  are  principally  composed  of 
magnetite  and  quartz  grains,  the  prevalence  of  the  magnetite 
in  places  being  due  to  its  concentration  by  the  laving  action  of  the 
advancing  and  retreating  tides. 

Sands  derived  from  volcanic  rocks  frequently  contain  a  con- 
siderable proportion  of  titanic  iron  and  magnetite,  and  in  some 
cases  olivine,  augite,  and  hornblende. 

In  a  general  way  it  may  be  said  that  the  sands  resulting  from 
denudation  are  the  residues  of  the  hardest  rock-components  in 
the  region.  Quartz  is  at  once  the  hardest  and  most  abundant 


204  A  TEXT-BOOK  OF  GEOLOGY. 

of  all  the  rock-forming  minerals,  and  for  these  reasons  it  is  the 
principal  constituent  of  nearly  all  sands. 

Sand  grains  are  not  always  quartz  or  other  simple  mineral. 
In  many  coarse  sands  they  are  found  to  consist  of  small  rock- 
particles.  This  is  particularly  the  case  in  desert  sands  which  often 
consist  mainly  of  comminuted  rock.  And  whereas  in  water- 
formed  sands,  quartz  is  the  principal  constituent  in  the  majority 
of  sands,  comprising  over  95  per  cent,  of  the  total  volume,  in  desert 
sands,  while  still  the  dominant  constituent,  it  is  frequently  accom- 
panied by  considerable  amounts  of  felspar,  olivine,  augite,  haematite, 
and  other  minerals  that  would  be  too  soft  to  survive  the  wear  and 
tear  to  which  sea-borne  sands  are  exposed. 

The  cementing  medium  or  matrix  of  sandstones  may  be  car- 
bonate of  lime,  silica,  oxide  of  iron,  or  clay. 

Carbonate  of  lime  forms  calcareous  sandstones. 

Silica  forms  siliceous  sandstones. 

Oxide  of  iron  (limonite)  forms  ferr uginous  sandstones. 

Clay  as  a  matrix  forms  argillaceous  sandstones. 

When  the  iron-oxide  matrix  occurs  in  large  quantity,  the  rock 
is  sometimes  called  a  limonitic  sandstone  ;  and  in  places  where 
the  iron  oxide  is  present  in  large  excess,  the  rock  may  pass  into  an 
ironstone. 

In  the  majority  of  sandstones  the  grains  are  rounded,  but  in 
some  arenaceous  rocks  they  are  subangular  or  angular. 

The  colour  of  sandstones  is  generally  due  to  the  presence  of 
some  oxide  of  iron  which  may  impart  straw- yellow,  yellowish-brown, 
dark-brown,  red  and  green  hues  according  to  the  degree  of  oxida- 
tion and  hydration. 

Glauconitic  sandstones,  generally  called  green  sands,  are  com- 
posed of  quartz  grains  coated  with  the  mineral  glauconite,1  or 
of  glauconite  grains  that  are  frequently  the  internal  casts  of 
foraminifera. 

Glauconite,  which  is  a  hydrous  silicate  of  iron  with  potash  and 
other  bases,  is  found  filling  or  coating  foraminifera  and  other 
marine  organisms  on  the  sea-floor  off  the  coast  of  South  Carolina. 
It  possesses  an  olive  or  blackish-green  colour,  and  hence  imparts 
a  characteristic  green  colour  to  marls,  limestones,  and  sandstones, 
in  which  it  occurs. 

A  sandstone  containing  much  mica  may  be  described  as  a  mica- 
ceous sandstone,  and  one  charged  with  carbonaceous  matter,  a 
carbonaceous  sandstone. 

Sandstones  that  split  easily  into  thin  slabs  are  called  flagstones  ; 
while  those  that  possess  no  distinct  bedding  are  frequently  spoken 
of  as  freestones. 

1  Gr.  glaukos^  sea-green. 


SEDIMENTARY   ROCKS.  205 

Many  of  the  more  ancient  sandstones  contain  a  considerable 
amount  of  felspar,  and  are  called  felspathic  sandstones  or  greywacke, 
to  which  reference  is  made  further  on. 

Calcareous,  argillaceous,  and  ferruginous  sandstones  are  usually 
soft  and  easily  cut  into  blocks  ;  the  grey wackes  are  hard  and 
frequently  much  jointed  and  broken  ;  while  siliceous  sandstones, 
which  consist  of  quartzose  sand  set  in  a  siliceous  matrix,  are  in- 
tensely hard  and  brittle. 

The  sands  of  which  sandstones  are  composed  were  laid  down  in 
a  river-bed,  or  on  the  floor  of  some  estuary,  sea,  or  lake.  Hence 
the  character  of  the  contained  fossils  will  be  a  record  of  the 
local  conditions  of  deposition.  Thus  marine  shells  will  indicate 
deposition  in  the  open  sea  ;  estuarine  shells  and  the  remains  of 
land  plants,  estuarine  conditions ;  freshwater  shells  and  freshwater 
fishes  with  plant  remains,  lacustrine  conditions. 

Some  sandstones  exhibit  fine  examples  of  false-bedding,  while 
those  of  a  felspathic  character  sometimes  show  a  tendency  to 
weather  in  spheroidal  forms,  the  partings  of  the  different  layers 
being  marked  by  iron-stained  seams. 

Among  well-known  examples  of  sandstones  we  have  the  Colley 
Sandstone  (Surrey),  of  which  Windsor  Castle  is  built ;  the  Stanley 
Sandstone  of  Shropshire,  used  for  grindstones  and  bridge-building  ; 
the  Brunton  Sandstone  of  Yorkshire  ;  the  Craigleith  Sandstone 
of  Edinburgh  ;  and  the  Old  Red  Sandstone  of  Scotland.  Some  well- 
known  sandstones  in  the  oversea  dominions  are  : — 

The  Desert  Sandstone — Queensland. 

The  Grampian  Sandstone — Victoria. 

The  Hawkesbury  Sandstone — New  South  Wales. 

The  Cave  Sandstone — Cape  and  Orange  States. 

The  Forest  Sandstone — Rhodesia. 

The  Beacon  Sandstone — South  Victoria  Land,  Antarctica. 

Grits.,  or  gritstones  as  they  are  sometimes  called,  are  composed  of 
coarse  angular  grains  usually  cemented  in  a  matrix  of  silica  or 
limonite. 

Gritstones  composed  of  material  derived  from  disintegrated 
granite  are  frequently  difficult  to  distinguish  from  the  parent  rock, 
particularly  when  they  rest  directly  on  it. 

A  gritstone  composed  of  quartz  grains  is  called  a  quartzose  grit- 
stone ;  and  one  that  contains  besides  quartz  a  considerable  pro- 
portion of  felspar,  slate,  and  felspathic  material,  constitutes  a 
greywacke. 

Many  Palseozoic  and  Lower  Mesozoic  sandstones  are  grey  wackes. 
They  appear  to  be  formed  of  detritus  derived  from  the  denudation 


206  A  TEXT-BOOK  OF  GEOLOGY. 

of  land  surfaces  in  which  igneous  rocks  largely  prevailed.  When 
fine-grained  they  are  sometimes  difficult  to  distinguish  from  igneous 
rocks  as  seen  in  the  field. 

Greywackes  frequently  alternate  with  shales  and  conglomerates. 
They  are  found  of  all  degrees  of  texture  from  fine-grained  to  coarse 
gritty  rocks  that  sometimes  approach  a  breccia  in  texture.  The 
prevailing  colour  is  a  dark  greenish-grey,  but  pale-green  and  purple 
varieties  are  common  among  the  Palaeozoic  formations.  Some  of 
the  grey  and  green  varieties  are  in  places  brecciated  with  peculiar 
thin  angular  flakes  or  splinters  of  dark  slate. 

The  Silurian  and  Devonian  greywackes  of  some  regions  contain 
a  rich  and  varied  fauna. 

ARGILLACEOUS  ROCKS. 

The  fine  sediments  resulting  from  the  decomposition  of  silicate 
minerals  are  mainly  composed  of  hydrous  silicate  of  alumina, 
which  in  its  pure  state  is  known  as  Kaolin  or  China-clay.  The 
majority  of  clays  are  not  pure,  but  contain  more  or  less  admixture 
of  rock-flour,  resulting  from  the  mechanical  erosion  of  rocks  by 
glaciers,  running  water,  or  wave-action. 

The  fine  sediments  laid  down  on  the  sea-floor  and  in  estuaries 
and  deltas  is  generally  called  mud.  Hardened  mud  may  form 
massive  beds  of  mudstone  that  possess  no  lamination,  but  more 
commonly  it  is  finely  banded  with  thin  layers  or  laminae  that 
easily  split  apart,  forming  what  is  geologically  called  shale. 

The  muds,  of  which  shales  and  mudstones  are  composed,  were, 
as  a  rule,  laid  down  in  deeper  water  than  the  sands  of  sandstones. 
When  traced  landward,  muds  graduate  into  sands,  and  in  the 
seaward  direction  pass  into  limestone. 

Clay  rocks  when  hardened  by  compression  and  cleaved  by 
pressure  are  converted  into  slates. 

A  slate  in  which  mica  has  been  developed  by  pressure  and  mole- 
cular change  is  called  a  micaceous  slate  or  phyllite. 

Thus,  according  to  the  degree  of  hardening  and  alteration,  we 
get  a  series  of  argillaceous  rocks,  beginning  with  muds  and  clays, 
that  pass  progressively  into  shale,  slate,  and  phyllite. 

Slates,  shales,  and  marly  clays  that  have  been  invaded  by 
igneous  dykes  are  sometimes  changed  into  an  intensely  hard, 
brittle,  fine-grained  black  rock  called  Lydian  Stone  or  Hornstone. 

Muds  containing  from  5  to  20  per  cent,  of  carbonate  of  lime  form 
marls  or  marlstones.  A  sandy  shale  is  called  an  arenaceous  shale, 
while  one  in  which  there  is  present  sufficient  carbonaceous  matter 
to  be  easily  distinguishable  is  spoken  of  as  a  carbonaceous  shale. 
A  shale  containing  bituminous  matter  forms  an  oil-shale.  When 


SEDIMENTARY    BOCKS.  207 

a  shale  contains  easily  recognisable  scales  of  mica,  we  get  a  mica- 
ceous shale. 

Clays,  marls,  shales,  and  slates  frequently  contain  fossils  which, 
in  the  last  two,  may  be  flattened  and  distorted  by  pressure.  In 
many  shales  the  fossil  are  replaced  by  pyrite.  The  shales  associated 
with  coals  are  usually  of  estuarine  or  deltaic  origin,  and  hence 
frequently  contain  an  abundance  of  plant  remains,  impressions  of 
leaves  being  in  many  cases  beautifully  preserved  along  the  lamina- 
tion planes. 

Loam  is  a  mixture  of  sand  and  clay  with  usually  some  carbonate 
of  lime.  Most  loams  are  of  alluvial  origin,  and  for  that  reason  are 
mostly  found  on  the  floor  of  river- valleys. 

Boulder  Clay,  or  Till  as  it  is  called  in  Scotland,  is  a  more  or  less 
gritty,  subglacial  clay  frequently  crowded  with  angular  and  sub- 
angular  blocks  of  stone.  It  varies  greatly  in  composition,  even 
within  the  limits  of  a  small  area.  In  one  place  it  may  be  clayey, 
in  another  sandy  ;  or  again  it  may  pass  with  startling  suddenness 
into  gravelly  beds. 

Fuller's  Earth  is  a  greenish-brown,  greenish-grey,  bluish  or 
yellowish  soft  earthy  mineral  with  a  greasy  feel.  Like  kaolin,  it 
adheres  to  the  tongue,  and  when  placed  in  water  it  falls  into  powder, 
but  does  not  form  a  paste.  It  possesses  great  absorbent  properties 
which  enable  it  to  remove  grease  and  oily  matters  from  cloth ; 
hence  its  name  Fuller's  Earth. 

China-clay  or  kaolin  and  pipe-clays  are  generally  found  in  the 
neighbourhood  of  granitic  masses,  the  hydrous  silicate  of  alumina 
of  which  they  are  composed  having  been  liberated  by  the  decom- 
position of  the  felspar  (orthoclase).  They  are  concentrated  by  the 
rain  and  streams  into  layers  and  beds.  Occasionally  they  are 
found  as  veins  filling  rock-fissures. 

The  underclays  of  many  coal-seams  are  often  found  to  be  almost 
free  from  lime,  alkalies,  and  iron  and  other  fusible  bases.  Hence 
they  possess  great  fire-resisting  properties,  being  what  is  termed 
refractory.  Such  clays  are  called  fire-clays.  They  are  ancient  soils 
from  which  the  lime  and  alkalies  have  been  exhausted  by  the 
coal-vegetation. 

Gannister  is  a  close-grained,  highly  siliceous  variety  of  fire-clay 

found  in  the  Lower  Coal  Measures  of  North  England.     It  is  of 

great  value  for  the  manufacture  of  gas  retorts  and  furnace  linings. 

Brick-clays  are  impure  clays,  in  many  cases  resulting  from  the 

decomposition  of  rocks  in  situ. 

Loess  is  a  fine  wind-borne  dust,  in  some  places  of  glacial,  in  others 
of  desert  origin.  It  is  mainly  derived  from  the  mechanical  pulverisa- 
tion of  rocks  by  moving  ice  or  desert  winds.  It  covers  large  areas 
in  Northern  China,  and  in  the  valleys  of  the  Rhine  and  Mississippi 


208  A  TEXT-BOOK  OF  GEOLOGY. 

Laterite  is  a  reddish-coloured  ferruginous  clay  found  in  many 
tropical  and  subtropical  lands.  It  is  formed  by  the  subaerial 
decomposition  of  rocks  in  situ,  especially  in  flat,  low-lying  jungle 
lands  where  the  drainage  is  feeble.  The  decomposition  of  the 
rock  is  accomplished  by  the  removal  of  the  silica  and  the  concentra- 
tion and  oxidation  of  the  iron.  Considerable  deposits  of  laterite 
occur  in  the  basalt  covered  areas  of  the  Deccan.  When  dried,  it 
frequently  forms  hard  surface  layers  sometimes  called  clay-pans. 


ORGANICALLY  FORMED  ROCKS. 

(a)  Calcareous.  (c)  Carbonaceous. 

(b)  Siliceous.  (d)  Ferruginous. 

CALCAREOUS  KOCKS. 

The  rocks  of  the  Calcareous  group  are  essentially  composed  of 
carbonate  of  lime.  The  principal  varieties  of  limestone  are  as 
follow  : — 

1.  Chalk. 

2.  Coralline  limestone!  £  ,. 

3.  Shelly  limestone     )forming  «««™  limestones. 

4.  Crystalline  limestone  or  marble. 

5.  Argillaceous  limestone. 

6.  Siliceous  limestone. 

7.  Magnesian  limestone  or  dolomite. 

Chalk. — This  is  a  very  pure  form  of  earthy  limestone  mainly 
composed  of  F oraminifera  and  other  allied  calcareous  organisms 
that  lived  in  clear  water  beyond  the  reach  of  sandy  or  muddy 
sediments. 

Coralline  Limestone. — Some  coralline  limestones,  like  the  beautiful 
Oamaru  stone  of  New  Zealand,  are  so  soft  that  they  can  be  easily 
sawn  into  blocks  of  any  desired  size.  Others  have  been  hardened 
by  the  infiltration  of  calcareous  waters.  The  softer  varieties  are 
generally  found  to  be  composed  of  broken  corals,  bryozoans,  fora- 
mi  nif  era,  echinoderm  spines  and  plates.  In  some  of  the  harder 
varieties  the  organic  structure  has  become  obliterated,  thus  giving 
rise  to  what  may  be  called  a  massive  limestone.  In  many  massive 
limestones  the  only  fossils  that  can  be  distinguished  are  those  that 
appear  on  the  weatfiered  surfaces. 

The  so-called  Petit  Granit  of  Belgium  is  a  dense  black  crinoidal 
limestone  containing  fragments  of  shells,  corals,  and  crinoids.  It 
is  a  valuable  building-stone. 


SEDIMENTARY    ROCKS.  209 

Shelly  Limestone. — This  is  a  limestone  mainly  composed  of 
shells  of  molluscs  that  lived  in  comparatively  shallow  water  where 
they  were  liable  to  be  mixed  with  sandy  matter  and  pebbles  ;  hence 
such  limestones  are  frequently  impure.  Shelly  limestones  in  many 
places  occur  in  irregular  beds  or  lens-shaped  tabular  masses, 
and  they  frequently  alternate  with  sandy  beds  or  conglomerates. 

A  limestone  containing  sandy  matter  is  called  an  arenaceous 
limestone,  and  one  mixed  with  pebbles  a  pebbly  limestone. 

Some  Tertiary  limestones  are  composed  of  freshwater  and  land 
snails. 

Crystalline  Limestone  or  Marble. — In  this  rock  the  original 
organic  structure  has  been  completely  obliterated  by  the  develop- 
ment of  a  granular,  crystalline  structure.  Crystalline  limestones 
are  found  in  stratified  formations  of  nearly  all  ages,  but  are  particu- 
larly prevalent  in  the  older  Palaeozoic  systems.  They  vary  in 
colour  from  the  finest  white  statuary  marble  of  Carrara  in  Italy  to 
the  dark  mottled  and  veined  varieties  found  in  Ireland.  Some  of 
the  ancient  limestones  contain  grains  and  nests  of  graphite  occurring 
throughout  the  whole  mass  or  confined  to  certain  horizons  of  the 
rock. 

Magnesian  Limestone  or  Dolomite. — Almost  all  limestones  con- 
tain a  small  percentage  of  magnesium  carbonate.  By  a  process 
of  partial  replacement  of  the  calcium  carbonate  and  concentration 
of  magnesium  carbonate,  the  rock  is  converted  into  a  dolomite. 

In  the  bore-holes  put  down  at  Funafuti  in  the  Ellice  group  lying 
north  of  Fiji,  dolomitisation  of  the  coralline  and  foraminiferal  rock 
was  found  to  have  taken  place  from  600  feet  downward  to  1114-5 
feet,  the  greatest  depth  reached,  resulting  in  the  formation  of 
magnesian  limestone  with  as  much  as  40  per  cent,  of  magnesium 
carbonate.  This  magnesian  limestone  closely  resembled  the 
d6lomitic  limestone  of  the  Tyrol,  which  occurs  at  a  height  of  10,000 
feet  above  sea-level. 

The  dolomites  found  associated  with  rock-salt  and  gypsum  have 
not  been  formed  in  the  same  way  as  the  massive  dolomites  referred 
to  above.  They  have  been  deposited  as  chemical  precipitates  on 
the  floor  of  saline  lakes  that  had  reached  a  stage  of  decadence 
through  insufficient  supplies  of  inflowing  water. 

Argillaceous  Limestone. — Earthy  or  chalky  limestones  containing 
a  considerable  proportion  of  clayey  matter  constitute  what  is  termed 
an  Argillaceous  or  Hydraulic  Limestone.  The  constituents  of  this 
rock  are  such  that  when  it  is  calcined  and  pulverised,  the  result- 
ing powder  forms  a  natural  cement  which  possesses  the  property 
of  setting  under  water  ;  hence  the  name  hydraulic  cement. 

Siliceous  or  Cherty  Limestone. — Bands  of  siliceous  limestone  are 
frequently  met  with  among  the  older  stratified  formations.  In 

14 


210  A  TEXT-BOOK  OF  GEOLOGY. 

some  limestones  of  this  class,  the  carbonate  of  lime  has  been  re- 
placed by  iron  oxides,  that  are  in  many  places  of  great  commercial 
value. 

Carbonaceous  Limestone. — A  limestone  containing  a  considerable 
proportion  of  carbonaceous  matter  of  vegetable  or  animal  origin 
is  called  a  Carbonaceous  or  Bituminous  Limestone.  Such  rocks 
often  give  off  a  fetid  smell  when  struck  with  a  hammer,  or  when 
two  pieces  are  rubbed  together,  and  are  therefore  spoken  of  as 
Stinkstone. 

Oolitic  Structure. — Many  of  the  Mesozoic  limestones  possess  an 
oolitic  structure  ;  that  is,  they  are  made  up  of  minute  rounded 
grains  about  the  size  of  a  small  pin-head,  cemented  together  so 
closely  that  the  rock  presents  the  appearance  of  fish-roe  ;  hence 
the  name  oolite 1  or  roe-stone. 

The  origin  of  this  peculiar  structure  is  still  uncertain.  The 
grains  consist  of  calcite  possessing  a  radial  and  concentric  structure. 
In  many  cases  a  grain  of  sand  or  a  fragment  of  shell  appears  to  have 
formed  the  nucleus  around  which  the  calcite  formed.  The  carbon- 
ate of  lime  may  have  been  deposited  from  solution  on  the  floating 
earthy  nuclei,  just  as  moisture  in  a  saturated  atmosphere  will 
collect  on  particles  of  dust. 

The  oolitic  limestones  furnish  valuable  building  stones  in  almost 
every  quarter  of  the  globe.  The  Ham  Hill  Stone  of  Somerset ; 
the  Portland  Stone  of  Dorsetshire,  used  in  the  erection  of  St  Paul's 
Cathedral,  London  ;  the  Caen  Stone  of  Normandy ;  the  Swdbian 
Stone  of  Wiirtemburg  ;  the  Boticino  Stone  of  North  Italy  ;  the 
White  Stone  of  Kentucky — are  some  well-known  examples. 

The  oolitic  ironstones  of  Cleveland  and  Northampton,  in  which 
the  grains  consist  of  carbonate  and  oxide  of  iron,  have  been  shown 
to  result  from  the  alteration  of  ordinary  oolitic  limestone. 

Cone-in- Cone  Structure. — Concretions  of  limestone  in  Cretaceous 
formations  are  frequently  covered  with  an  outer  layer  of  limestone 
generally  from  two  to  four  inches  thick,  composed  of  a  mass  of 
radial,  fibrous,  funnel-shaped,  crystalline  forms  that  fit  into  each 
other,  producing  a  cone-in-cone  structure.  The  origin  of  this 
structure  is  not  yet  well  understood. 

SILICEOUS  BOCKS. 

Cherts  and  Flint. — Many  of  the  older  limestones  are  intercalated 
with  sheets  or  lens-shaped  masses  of  siliceous  rock  called  Chert, 
which  is  mainly  composed  of  the  tiny  siliceous  shells  of  Radio- 
laria,  the  siliceous  cases  of  Diatoms  (diminutive  plants  of  a  low 
type),  and  the  spicules  of  sponges.  The  silica  is  carried  in  solution 
1  Or.  ocw  =  egg,  and  lithos  =  stone. 


SEDIMENTARY   ROCKS.  211 

in  sea- water,  and  these  organisms  are  able  to  extract  it  for  the 
building  of  their  coverings  or  skeletons. 

Chert  is  generally  a  fine-grained  buff-grey,  dark-grey,  or  black 
rock.  It  is  brittle,  and  breaks  with  a  conchoidal  fracture.  The 
siliceous  organisms  of  which  it  is  composed  are  set  in  a  cement  of 
secondary  silica  deposited  by  infiltration. 

Beds,  lenticular  tabular  masses,  and  nodules  of  Flint  frequently 
occur  in  chalk  and  other  earthy  limestones.  The  nodules  are 
generally  arranged  in  layers  parallel  with  the  bedding  planes. 
Like  chert,  flint  is  composed  of  silica  extracted  from  sea-water  by 
radiolarians  and  diatoms. 

The  soft  incoherent  forms  of  diatomaceous  earth  are  called 
Infusorial  Earth  or  Tripoli.  They  are  commercially  valuable 
as  the  base  or  matrix  of  many  nitro-glycerine  compounds,  the  tiny 
siliceous  shells  possessing  great  absorbent  properties. 

CARBONACEOUS  EOCKS. 

These  rocks  include  the  different  varieties  of  coal  and  graphite. 

Coal  is  altered  vegetable  matter.  It  consists  essentially  of  carbon 
combined  with  oxygen,  hydrogen,  nitrogen,  and  a  certain  amount 
of  earthy  matter  which  is  left  as  a  residue,  or  ash,  when  the  coal 
is  burnt. 

The  progressive  changes  that  take  place  in  the  formation  of  coal 
are  seen  in  the  different  varieties  of  that  mineral,  ranging  from  peat 
to  anthracite. 

Peat. — Consisting  of  decomposing  vegetable  matter. 

Lignite. — Compressed  and  altered  peat  showing  woody  structure. 

Brown  Coal. — Altered  lignite  showing  no  woody  structure. 

True  Coal  or  Bituminous  Coal. — Cokes  or  cakes  when  burnt. 

Anthracite. — Consists  mainly  of  carbon. 

Peat  consists  of  stems,  roots,  leaves,  and  mossy  vegetation,  and 
may  be  seen  in  process  of  formation  at  the  present  day  on  the  sites 
of  ancient  forests,  and  on  moss  and  heath-covered  water-logged 
lands.  In  recently  formed  peats,  the  vegetable  matter  is  only 
slightly  altered ;  while  in  the  older  peats,  it  is  partially  carbonised 
owing  to  the  escape  of  some  of  the  oxygen  and  hydrogen. 

Lignite  l  is  a  peaty  accumulation  that  has  become  covered  with 
sediments.  It  represents  the  second  stage  in  the  formation  of 
coal ;  and  although  the  woody  structure  of  the  vegetation  is  still 
well  preserved,  there  has  been  a  considerable  elimination  of  water 
and  gaseous  products. 

Brown  Coal  is  the  next  phase.  It  represents  a  greater  degree 
1  Lat.  lignum  =  wood. 


212  A  TEXT-BOOK  OF  GEOLOGY. 

of  alteration  than  lignite.  Some  of  poorer  qualities  cannot  be 
distinguished  from  lignite,  while  many  of  the  better  grades 
approach  a  true  coal. 

True  Coal,  or  Bituminous  Coal  as  it  is  frequently  called,  represents 
a  still  higher  degree  of  alteration,  and  in  it  all  trace  of  the  original 
woody  structure  has  generally  been  obliterated.  Microscopic 
examination,  however,  shows  that  many  coals  are  composed  of  the 
spores  of  plants  allied  to  ferns,  club-mosses,  and  horse-tails. 
Others  consist  mainly  of  woody  fibre  and  bark. 

Anthracite  l  is  the  hardest  coal.  It  consists  almost  entirely  of 
carbon,  practically  all  the  gaseous  products  having  been  eliminated. 

The  anthracites  of  Wales  and  Pennsylvania  are  Carboniferous  ; 
and  the  semi-anthracites  of  New  Zealand,  Eocene.  At  Malvern, 
in  the  last-named  state,  the  brown  coal  has  been  converted  into 
anthracite  by  contact  with  a  sheet  of  basalt. 

Composition  of  Coal. — The  constitution  of  coal  can  be  very  well 
shown  by  a  simple  test  :— 

(1)  Weigh  out  100  grains  of  finely  powdered  coal ;  place  in  a 

platinum  dish  and  dry  carefully  at  a  temperature  not 
exceeding  212°  Fahr.  The  loss  of  weight  =the  water. 

(2)  Place  the  dish  over  a  Bunsen  burner  with  the  lid  of  the  dish 

tipped  slightly  to  one  side.  Apply  a  dull  red  heat  and 
burn  off  the  volatile  gases.  The  loss  =  volatile  hydro- 
carbons, and  the  residue  =fixed-carbon  plus  ash  =  coke. 

(3)  Remove  lid ;  tip  the  dish  slightly  to  one  side  and  burn  off 

the  carbon,  keeping  the  heat  going  until  only  a  grey  or 
reddish-grey  ash  remains.  The  residue  =ash]  and  the 
ash  subtracted  from  the  weight  obtained  in  (2)  gives  the 
fixed-carbon. 

If  a  fine  balance  is  available,  10  grains  of  coal  will  be  sufficient 
for  the  test.  Tabulating  the  results,  we  may  get,  for  example  : — 

Water,          .  ..  ...  .    ~     .  2-00 

Hydro-carbons,  .  .  ',               .  34-00 

Fixed-carbon,  .  .  *  .       ;_•;  62-50 

Ash,     .  1-50 


100-00 

Cannel  is  a  dull  earthy  shaly  variety  of  coal  often  possessing  a 
conchoidal  fracture.  It  contains  a  large  amount  of  coal  gas,  and 
for  that  reason  is  valuable  for  gas-making.  It  sometimes  contains 

1  Gr.  anthrax  =  carbon. 


SEDIMENTARY   ROCKS.  213 

shells  and  fossil  fish,  and  may  pass  at  its  edges  into  bituminous 
shale.  These  facts  would  indicate  that  cannel  is  not  formed  of 
vegetation  that  grew  in  place,  but  is  detrital ;  that  is,  composed  of 
drift-vegetation  that  settled  on  the  floor  of  shallow  lagoons. 

Jet  resembles  cannel  coal,  but  is  harder  and  blacker,  and  takes 
a  fine  polish.  It  is  found  at  Whitby  in  Yorkshire,  and  elsewhere. 
Its  lightness  renders  it  suitable  for  personal  ornaments. 

Conditions  of  Coal  Formation. — Coal  is  the  result  of  the  growth 
of  a  dense  jungle-like  vegetation  on  low-lying  swampy  areas  on 
the  sea-board  near  the  mouth  of  great  rivers.  The  deltas  of  the 
Mississippi  and  the  swampy  forests  of  the  Amazon  and  Orinoco 
probably  approach  the  conditions  in  which  the  coal  vegetation 
flourished. 

The  coal  is  generally  found  resting  on  an  under-day,  which  is 
the  soil  in  which  the  vegetation  grew.  In  the  coals  of  Westphalia 
and  Nova  Scotia,  there  have  been  found  the  remains  of  trees  still 
standing  in  the  position  in  which  they  grew,  with  their  rootlets 
penetrating  the  under-clay.  This  evidence  supports  the  contention 
that  many  coals  now  occupy  the  original  sites  on  which  the  forests 
grew. 

After  centuries  of  growth,  the  accumulation  of  decaying  vegetable 
matter  became  buried  under  a  covering  of  sands  when  the  coastal 
lands  subsided  and  became  submerged. 

The  existence  of  numerous  seams  of  coal  in  the  same  formation 
separated  by  beds  of  sandy  material  would  indicate  a  corresponding 
number  of  oscillations  of  the  land,  each  elevation  being  marked 
with  a  revival  of  jungle  or  forest  conditions. 

The  thickness  of  the  strata  between  the  different  seams  of  coal 
affords  some  evidence  of  the  duration  of  each  subsidence  ;  but  the 
clay  or  stone-partings  met  with  in  many  coal-seams  cannot  always 
be  taken  as  an  evidence  of  submergence.  They  may  mark  the 
encroachment  of  flood-waters  on  to  the  forest-lands  during  an 
abnormal  inundation  whereby  a  layer  of  mud  was  deposited 
among  the  vegetation,  whose  growth  would  be  retarded  but  not 
destroyed. 

Quality  of  Coals. — Coals  enclosed  in  porous  grits  or  sandstones 
are  usually  of  inferior  quality  ;  while  those  interbedded  with  close- 
grained  fireclays  and  compact  sandstones  are  nearly  always  high 
class.  The  Upper  Cretaceous  system  at  Kaitangata,  New  Zealand, 
contains  two  coal-bearing  horizons,  a  lower  and  an  upper.  In  the 
lower  horizon,  which  consists  of  loose  quartzose  sands  and  porous 
conglomerates,  the  coal  is  an  ordinary  lignite  ;  while  in  the  upper 
horizon,  in  which  the  seams  are  enclosed  in  thick  beds  of  compact 
sandstone  conglomerate,  the  coal  is  a  hard  bright  coal  of  superior 
quality. 


214  A  TEXT-BOOK  OF  GEOLOGY. 

The  quality  of  the  coal  is  not  dependent  on  the  age  of  the  en- 
closing rock. 

Age  of  Coals. — Lignites  are  generally  confined  to  the  younger 
Tertiary  formations.  Brown  coals  are  found  in  rocks  ranging  from 
the  Cretaceous  to  Pliocene  ;  while  true  coals  occur  in  all  formations 
from  the  Cambrian  to  the  Eocene. 

The  anthracite  of  County  Cavan  in  Ireland  is  Silurian  ;  the  true 
coals  and  anthracites  of  Great  Britain,  Continental  Europe,  and 
Pennsylvania,  Carboniferous  ;  the  coals  of  New  South  Wales 
and  China,  Carboniferous  and  Permo-Carboniferous  ;  and  the 
semi-anthracites  and  bituminous  coals  of  New  Zealand,  Eocene. 

The  brown  coals  of  South  Hungary.  Transylvania,  and  North 
Germany  are  Lower  Miocene  ;  of  New  Zealand,  Upper  Cretaceous 
and  Lower  Miocene :  the  lignites  of  Ireland  are  Pliocene. 

All  the  greatest  coal-deposits  in  the  globe  are  of  Carboniferous 
age,  which  would  indicate  that  plant-life  in  this  period  reached 
a  development  and  luxuriance  unrivalled  in  any  other  geological 
age.  The  ferns,  mosses,  equisetums,  lycopodiums,  and  lepido- 
dendrons,  which  constitute  the  bulk  of  the  Carboniferous  coals, 
grew  to  a  gigantic  size,  resembling  in  habit  the  forest  trees  of  the 
present  day. 

Graphite.1 — The  ultimate  phase  of  altered  coal  would  appear  to 
be  represented  by  graphite,  from  which  all  the  gases  have  been 
eliminated,  only  carbon  and  ash  being  left  behind. 

Lenticular  beds  of  graphite,  frequently  associated  with  crystal- 
line limestones,  are  found  in  Canada,  Bavaria,  Bohemia,  New 
South  Wales,  interbedded  with  gneissic  and  schistose  rocks  of 
pre-Cambrian,  Cambrian,  and  Silurian  age.  Graphite  of  fine 
quality  is  obtained  from  the  Ordovician  volcanic  series  at  Borrow- 
dale  in  Cumberland,  and  it  is  a  constituent  of  graphite-slate, 
graphite-schist,  and  graphite-gneiss.  Some  of  the  Laurentian 
limestones  of  Canada  are  so  charged  with  it  as  to  be  profitably 
mined  for  it. 

Masses  of  graphite  still  adhering  to  the  original  sandstone  are 
sometimes  found  among  the  detritus  on  the  slopes  of  Mount 
Egmont,  a  volcano  which  has  broken  through  the  brown  coal- 
measures  of  that  part  of  New  Zealand.  This  graphite  has  obviously 
arisen  from  the  alteration  of  pieces  of  coal  that  became  entangled 
or  engulfed  in  the  ascending  floods  of  andesitic  lava. 

Graphite  also  occurs  in  veins  and  filling  cavities,  as  well  as  in 
disseminated  scales  in  granitic  rocks  in  Ceylon,  from  which  a  large 
proportion  of  the  world's  supply  is  drawn.  Scales  of  graphite 
have  also  been  identified  in  basalt  and  diorite.  Such  graphite 
can  hardly  have  had  an  organic  origin. 

1  Gr.  grapho  =  I  write,  and  lithos  =  a,  rock, 


SEDIMENTARY    EOCKS.  215 

FERRUGINOUS  ROCKS. 

The  rocks  included  in  this  group  are  chiefly  important  for  their 
great  economic  value  as  ores  of  iron.  They  are  usually  limestones 
in  which  the  carbonate  of  lime  has  been  partly  or  wholly  replaced 
by  carbonate  of  iron.  The  oolitic  iron-ore  of  the  district  of  Cleve- 
land in  Yorkshire  is  a  good  example  of  this  class  of  replacement 
deposit. 

CHEMICALLY-FORMED  EOCKS. 

(a)  Carbonates — Limestones.       (c)  Chlorides — Rock-salt. 
(6)  Sulphates — Gypsum.  (d)  Silica — Siliceous  sinter. 

The  Carbonate,  Sulphate,  and  Chloride  deposits  of  this  group  are 


FIG.  116. — Deposit  of  travertine  at  a  cascade. 

composed  of  granular  or  crystalline  precipitates  that  frequently 
occur  as  lenticular  sheets  interbedded  with  sands,  clays,  and  shales. 
They  were  deposited  on  the  floor  of  saline  lakes  as  a  result  of  the 
evaporation  and  consequent  concentration  of  the  dissolved  salts 
carried  into  the  basin  by  the  drainage  of  the  surrounding  country. 

The  sediments  of  saline  inland  lakes  seldom  contain  fossils 
except  those  carried  into  the  basin  by  streams. 

Carbonates. — Waters  laden  with  the  bicarbonate  of  lime  or 
magnesia  when  they  reach  the  open  air  part  with  carbonic  acid, 
and  the  carbonates  are  at  once  deposited.  Travertine  or  Calcareous 
Sinter  is  a  soft  spongy-looking  carbonate  of  lime  frequently 
deposited  in  rock-shelters  and  on  hill-slopes  in  the  form  of  rocky- 
cascades  where  the  calcareous  waters  issue  at  the  surface.  These 
waters  are  popularly  called  petrifying  springs,  from  the  circum- 
stance that  the  carbonate  of  lime  is  frequently  deposited  on  ferns, 
crosses,  twigs,  and  leaves,  the  forms  of  which  are  thus  preserved. 


216 


A    TEXT-BOOK    OF    GEOLOGY. 


Many  travertines  become  hard  and  crystalline  in  structure 
through  the  deposit  of  secondary  calcite. 

Dolomitic  or  magnesian  limestones  formed  on  the  floor  of  saline 
lakes  are  not  uncommon.  The  rock  is  often  concretionary,  granu- 
lar, or  finely  crystalline  in  structure,  and  sometimes  exhibits  false- 
bedding.  The  precipitation  of  the  mixed  carbonates  of  lime  and 
magnesia  is  partly  due  to  the  presence  of  sodium  carbonate  and 
partly  to  evaporation. 

Sulphates  and  Chlorides. — In  inland  lakes  that  have  no  outlet 
to  the  sea,  situated  in  regions  where  the  evaporation  about  balances 
the  flow  of  the  incoming  streams,  the  water  in  the  course  of  time 


1       1 

1        1 

1      1 

1      1. 

Schists.  Massive  and  bedded  igneous  rocks. 

FIG.  117. — Showing  symbols  used  to  represent  different  kinds  of  rock. 

becomes  charged  with  soluble  salts.  When  a  certain  degree  of 
saturation  is  reached,  a  portion  of  the  salts  passes  out  of  solution 
and  is  deposited  on  the  floor  of  the  lake.  In  this  way  sheets  of 
gypsum  and  rock-salt  have  been  formed  on  the  floor  of  the  Dead 
Sea  and  Great  Salt  Lake.  The  basins  of  many  of  the  saline  lagoons 
in  Central  Australia  and  Utah  are  covered  with  a  thick  crust  of 
rock-salt  mixed  with  various  impurities. 

Deposits  of  gypsum  are  sometimes  formed  in  volcanic  regions. 
A  striking  example  may  be  seen  at  White  Island,  New  Zealand. 
Here  the  bed  of  the  crater-lake  is  covered  with  a  thick  layer  of 
gypsum  deposited  from  the  hot  acid  waters  which  fill  the  basin. 
The  evaporation  of  the  steaming  water  is  rapid,  but  the  loss  is 
compensated  by  the  mineral  laden  waters  that  issue  from  the 
steam-holes  and  hot  springs  around  the  margin  of  the  crater, 


SEDIMENTARY    ROCKS.  217 

Silica. — In  regions  of  expiring  volcanic  activity,  the  thermal 
waters  frequently  carry  a  considerable  amount  of  silica  in  solution 
in  the  form  of  soluble  alkaline  silicates  which  are  easily  decomposed 
by  atmospheric  carbonic  acid.  On  reaching  the  open  air  the 
silica  is  deposited  in  the  form  of  sheets  and  cascade-like  streams. 
Extensive  deposits  of  siliceous  sinter  occur  in  the  volcanic  region 
of  New  Zealand  and  in  the  Yellowstone  National  Park. 


CONVENTIONAL  SYMBOLS. 

The  symbols  used  by  geologists  to  represent  the  more  common 
rocks  on  maps  are  shown  in  fig.  117. 


SUMMARY. 

(1)  Sedimentary  rocks,  according  to  the  character  of  the  con- 
stituents, may  be  classified  as  Detrital,  Organic,  or  Chemical. 

(2)  Detrital  rocks  are  composed  of  sediments  of  various  degrees 
of   texture   derived   from   the    denudation   of   pre-existing  rock- 
masses. 

In  breccia  the  material  is  angular  ;  in  conglomerate,  water-worn 
and  pebbly  ;  in  sandstone,  sandy  ;  and  in  clays,  shales,  and  slates, 
very  fine  or  clayey. 

The  cementing  medium  may  be  carbonate  of  lime,  silica,  oxide 
of  iron,  or  a  paste  of  sand  and  clay.  The  colour  is  generally 
determined  by  the  degree  of  oxidation  and  hydration  of  the  iron 
which  is  nearly  always  present. 

(3)  Organic  rocks  may  be  composed  of  animal  or  plant  remains. 
They  may  be  divided  into  four  groups  according  to  their  com- 
position, viz.  Calcareous,  Siliceous,  Carbonaceous,  and  Ferruginous. 

The  Calcareous  division  comprises  the  limestones  which  consist 
of  the  calcareous  shells  and  organisms  of  molluscs,  corals,  crinoids, 
and  foraminifera.  Some  limestones,  like  chalk,  are  soft  and 
earthy ;  others  hard  and  massive,  like  the  Belgian  limestones  ; 
while  many  possess  a  granular  or  finely  crystalline  structure. 
Coralline  limestones  may  develop  a  crystalline  structure  through 
the  infiltration  of  calcareous  waters  ;  and  by  the  replacement  of  a 
portion  of  the*  carbonate  of  lime  with  magnesium  carbonate,  the 
rock  may  be  dolomitised  or  altered  into  a  magnesian  limestone  or 
dolomite  that  may  resemble  the  older  dolomitic  limestones. 

Limestones  may  contain  certain  impurities.  They  may  be 
clayey,  forming  an  argillaceous  limestone  from  which  hydraulic 
cement  is  made,  sandy,  pebbly,  or  siliceous. 

The  Siliceous  rocks  of  this  group  are  chert  and  flint,  mainly  com- 


218  A    TEXT-BOOK    OF    GEOLOGY. 

posed  of  diatoms — tiny  plants  that  possess  the  power  of  extracting 
silica  from  sea-water. 

The  Carbonaceous  rocks  include  all  the  known  varieties  of  coal 
ranging  from  peat  to  anthracite. 

The  Ferruginous  rocks  are  mostly  carbonate  of  iron  that  has 
replaced  the  carbonate  of  lime  in  oolitic  limestones. 

(4)  Chemically-formed  rocks  comprise  carbonates,  sulphates, 
chlorides,  and  silica.  The  last  is  deposited  by  hot  springs  in 
volcanic  regions  in  the  expiring  or  solfataric  stage  of  activity  ; 
the  others  are  deposited  as  precipitates  on  the  beds  of  inland  saline 
lakes.  When  the  evaporation  balances  the  inflow,  the  mineral 
matter  carried  into  the  lake  in  solution  in  time  reaches  a  point  of 
saturation,  when  precipitation  takes  place.  In  this  way  beds  of 
gypsum  and  rock-salt  have  been  deposited  on  the  floor  of  the  Dead 
Sea  and  Great  Salt  Lake. 


CHAPTER   XIV. 
VOLCANOES   AND   VOLCANIC   ACTION. 

Definition  of  Volcano. — A  volcano  may  be  defined  as  a  more  or 
less  conical  elevation  having  a  crater  at  its  summit,  from  which 
steam,  gases,  streams  of  lava,  dust,  and  scoriae  are  ejected. 
Volcanoes  classified.— Volcanoes  may  be  classified  as  :— 

(a)  Extinct. 

(b)  Dormant. 

(c)  Active. 

An  Extinct  volcano  is  one  which  is  not  known  to  have  been 
active  within  historic  or  traditional  times. 

A  Dormant  volcano  is  one  which  enjoys  intervals  of  complete 
quiescence  between  the  different  eruptions,  which  maybe  separated 
by  hundreds  or  even  thousands  of  years. 

Active  volcanoes  are  always  in  a  state  of  disturbance.  Their 
paroxysmal  outbursts  are,  however,  generally  intermittent  and 
maybe  separated  by  long  intervals  of  comparative  quietude,  during 
which  the  only  evidences  of  activity  are  the  emission  of  steam,  and 
minor  explosions  that  may  give  rise  occasionally  to  showers  of  dust 
and  lapilli.1 

Examples  : — 

Extinct  volcanoes,  .  The  craters  of  Auvergne  in  France,  and 
Eifel  in  Rhenish  Prussia. 

Dormant  volcanoes,    .  Ruapehu  and  Tarawera,  New  Zealand. 

Active  volcanoes,  .  Etna,  Stromboli,  Vesuvius,  Hekla,  Coto- 
paxi,  Kilauea,  Ngauruhoe. 

This  classification  is  not  altogether  satisfactory,  as  it  is  not 
always  easy  in  its  application.  Historic  time  is  so  short  relatively 
to  the  life  of  a  volcano  that  the  so-called  extinct  cone  of  to-day  may 
be  an  active  volcano  to-morrow.  Up  to  the  time  of  its  first  known 
eruption  in  79  A.D.,  Vesuvius  was  looked  upon  as  extinct.  Simi- 

1  Ital.  lapilli  =  little  stones  (mostly  from  the  size  of  a  pea  to  that  of  a 
small  walnut). 

219 


220  A  TEXT-BOOK  OF  GEOLOGY. 

larly  Tarawera,  which  burst  into  activity  with  such  startling  sud- 
denness in  1886,  was  never  known  to  have  shown  the  least  sign  of 
action  before  that  date.  It  had  suffered  considerably  from  denuda- 
tion and  possessed  no  visible  crater.  The  whole  aspect  of  the 
mountain  seemed  to  indicate  a  state  of  complete  extinction  of  long 
duration. 

Sites  of  Volcanoes. — The  vent  of  a  volcano  may  break  through 
any  geological  formation,  and  may  originate  on  the  sea-floor,  on 
dry  land  near  the  sea,  on  a  plateau,  or  mountain-chain.  The 
numerous  volcanic  cones  of  Auckland  are  piled  up  on  a  platform 
of  Tertiary  marine  sandstones  and  clays  not  much  above  sea-level  ; 
the  Miocene  volcanoes  of  Auvergne  burst  through  the  granitic  and 
gneissic  plateau  of  Central  France. 

Eruptions. — These  may  be  of  different  types,  as  : — 

(a)  Volcanic  eruptions  =  Vesuvian  type. 
(6)  Fissure  eruptions  =  Icelandic  type. 
(c)  Explosive  eruptions  =Krakatoan  type. 

Vesuvian  Type. —  Volcanic  eruptions  are  those  confined  to  one 
crater  or  volcano,  as  at  Vesuvius,  Cotopaxi,  and  Egmont. 

Icelandic  Type. — In  Fissure  eruptions  there  is  a  quiet  welling-up 
of  lava  along  a  line  of  fissure,  accompanied  with  little  explosive 
action  and  hence  expelling  little  or  no  ash  or  fragmentary  matter. 
The  great  volcanoes  Kilauea  and  Mauna  Loa  in  the  Sandwich 
Islands  are  fine  examples  of  the  fissure  type  of  volcanic  action. 
The  lava  floods  of  Idaho,  Victoria,  Oregon,  Washington,  and 
California  form  plateaux  over  200,000  square  miles  in  extent  ; 
and  great  basaltic  plateaux  also  occur  in  Victoria,  in  Australia,  and 
the  Deccan  in  India.  All  have  been  formed  by  vast  floods  of  lava 
that  issued  from  fissure-rents  of  great  magnitude. 

The  greatest  outpouring  of  lava  in  historic  times  took  place  in 
Iceland  in  1783.  Floods  of  lava  issued  from  a  fissure  twelve  miles 
long  and  poured  over  the  land,  diverting  streams  from  their  course, 
filling  up  river-gorges,  and  forming  lakes  of  molten  rock  on  the 
neighbouring  plains.  The  main  streams  travelled  over  forty  miles 
from  the  point  of  emission. 

Hekla  does  not  form  a  cone,  but  an  oblong  ridge  which  has  been 
split  by  a  fissure  along  its  whole  length  that  bears  a  row  of  craters. 

Krakatoan  Type. — These  take  place  through  the  sudden  ex- 
pansive force  of  subterranean  steam.  They  usually  happen  with 
appalling  suddenness  ;  and  frequently  their  explosive  force  is  so 
titanic  that  they  rend  and  shatter  the  rocks  at  the  point  where 
the  explosion  is  concentrated  into  fragments,  which  may  be  hurled 
far  and  wide  with  devastating  effect.  In  a  few  hours  forests 
may  be  destroyed,  towns  overwhelmed,  lakes  dried  up,  old  land- 


VOLCANOES    AND    VOLCANIC   ACTION. 


221 


marks  obliterated,  and  the  surrounding  country  converted  into  a 
lifeless  desert. 

The  four  most  stupendous  and  destructive  explosive  eruptions 
of  historic  times  are  those  of  Vesuvius  in  79  A. D.,  Krakatoa  in  1883. 
Tarawera  in  1886,  and  St  Vincent  in  1902. 

Before  79  A.D.,  there  was  no  tradition  or  record  of  former  volcanic 
activity  at  Vesuvius.  The  ancient  crater  was  overgrown  with 
wild  vines,  while  the  mountain  slopes  and  neighbouring  plains  were 
dotted  with  villages  surrounded  with  vineyards  and  well-cultivated 
fields.  At  the  base  of  the  mountain  stood  the  populous  and 
cultured  cities  of  Herculaneum  and  Pompeii. 

The  premonitory  evidence  of  coming  disturbance  began  with  a 


FIG.  118. — Vesuvius,  showing  the  older  crater-ring  of  Monte  Somma 
and  the  new  cone  within  it.     (After  Phillips.) 

series  of  earthquakes  in  63  A.D.,  which  caused  much  damage  to 
buildings  and  created  considerable  alarm  among  the  people  living 
in  the  neighbourhood  of  the  mountain.  The  earthquakes  increased 
in  frequency  and  violence,  and  in  79  A.D.  finally  culminated  in  a 
series  of  terrific  explosions  which  truncated  the  bulk  of  the  ancient 
cone  by  nearly  one  half,  forming  the  well-known  Monte  Somma 
ring  from  which  the  present  cone  rises.  The  immense  volumes  of 
steam  which  issued  from  the  crater  were  condensed  and  fell  in 
torrential  rains  which  swept  down  the  mountain  slopes  carrying 
before  them  ejected  dust  and  scoriae.  The  floods  of  mud  thus 
formed,  together  with  the  showers  of  falling  ejecta,  buried  the  cities  of 
Herculaneum  and  Pompeii  and  devastated  the  surrounding  country. 
The  history  of  Vesuvius,  until  the  eruption  of  1036,  was  a  long 
series  of  explosive  outbursts,  producing  only  fragmentary  material. 
In  that  year  there  was  an  overflow  of  lava  for  the  first  time  ;  and 


222  A  TEXT-BOOK  OF  GEOLOGY. 

from  then  onward  the  mountain  entered  on  a  new  phase  of  volcanic 
activity. 

Perhaps  the  most  stupendous  explosive  eruption  in  historic 
times  was  that  which  took  place  at  Krakatoa,  a  volcanic  island  in 
the  Straits  of  Sunda,  between  Java  and  Sumatra,  in  August  1883, 
when  a  large  portion  of  the  island  was  destroyed  and  the  remainder 
devastated  with  a  thick  covering  of  dust  and  other  fragmentary 
ejectamenta.  The  actual  sound  of  the  explosions  was  heard  at 
Ceylon,  more  than  2000  miles  away  ;  and  much  of  the  dust  was 
projected  so  high  into  the  air  and  was  so  excessively  fine  that  it 
was  caught  up  in  the  upper  currents  of  the  atmosphere  and  carried 
many  times  round  the  globe,  giving  rise  to  a  series  of  gorgeously 
coloured  sunsets  that  continued  for  several  months. 

Particles  of  Krakatoan  dust  fell  in  Japan  and  America,  and  some 
even  reached  as  far  as  Europe. 

The  disturbance  in  the  surrounding  sea  was  relatively  greater 
than  on  land.  The  waves  propagated  by  the  explosions  caused 
enormous  loss  of  life  on  the  adjacent  coasts  and  low-lying  islands. 
They  travelled  as  far  as  Cape  Horn,  7818  miles  away. 

The  Tarawera  eruption  in  New  Zealand  took  place  in  June  1886, 
and  was  preceded  by  little  or  no  warning.  In  a  few  hours  after 
the  first  terrific  outburst,  the  mountain  and  plateau  at  its  base  were 
rent  with  a  gaping  fissure  nearly  nine  miles  long.  The  volcanic 
energy  soon  became  concentrated  in  numerous  independent  centres 
of  explosive  activity  along  the  fissure  from  which  there  issued 
continuous  showers  of  fragmentary  matter  and  enormous  volumes 
of  steam.  The  dust  was  spread  over  10,000  square  miles,  over- 
whelming the  neighbouring  forests  and  native  villages,  and  con- 
verting the  country  into  a  weird  grey  wilderness.  Immense 
volumes  of  steam  were  condensed  into  rain  which,  uniting  with 
the  falling  dust,  formed  a  plastic  mud  that  broke  down  the 
forest-trees  and  buried  the  hapless  villages  lying  in  the  track  of 
the  powerful  winds  that  accompanied  the  eruption. 

The  sounds  of  the  explosions  were  heard  at  Christchurch,  over 
400  miles  away.  They  resembled  the  detonations  of  cannon  or 
violent  blows  on  the  side  of  an  empty  iron-tank.  It  was  during 
this  eruption  that  the  far-famed  Pink  and  White  Terraces  at 
Rotomahana  were  destroyed. 

The  disastrous  explosive  eruption  of  Mon  Pelee  in  Martinique 
is  still  fresh  in  the  memory  of  everyone.  In  April  1902,  the  volcano 
began  to  emit  steam,  ashes,  and  sulphurous  vapours.  The  latter 
were  so  abundant  that  horses  dropped  dead  in  the  streets  of  St 
Pierre,  situated  on  the  plain  bordering  the  mountain.  On  5th  May, 
floods  of  mud  descended  from  the  crater  where  it  had  been  accumu- 
lating for  some  time,  and  earthquakes  were  numerous.  On  8th  May, 


VOLCANOES    AND    VOLCANIC    ACTION.  223 

the  eruption  reached  its  climax.  On  that  day  a  black  cloud  of  dust, 
steam,  and  sulphurous  vapours  swept  down  through  the  breach 
in  the  crater  fronting  St  Pierre,  passed  over  the  plain,  and  in  two 
minutes  struck  the  city,  which  was  at  once  overwhelmed,  and  the 
inhabitants  to  the  number  of  30,000  were  killed.  The  detonations 
of  the  explosions  that  followed  were  heard  300  miles  away. 

The  great  crater  of  Mon  Pelee  is  now  occupied  by  a  fragmental 
cone  which  terminates  in  a  column  of  solid  lava  several  hundred 
feet  high.  This  column  is  believed  to  be  the  lava  which  solidified 
in  the  vent  and  was  pushed  up  by  the  expansive  force  of  the  steam 
and  vapour  below. 


FIG.  119. — Showing  plug  of  phonolite  lava,  Mato  Teepee, 
Missouri,  U.S.  Geol.  Surv.     (After  Jagger.) 

Craters. — The  crater  1  of  a  volcano  is  a  cup-shaped  depression 
at  the  summit  which  communicates  below  with  a  fissure  in  the 
Earth's  crust.  It  is  through  this  vent  or  pipe  that  the  ejecta  and 
gases  issue  from  below. 

Active  and  dormant  volcanoes  as  a  rule  possess  well-defined 
craters.  In  extinct  volcanoes,  older  than  Miocene,  the  craters  and 
vents  are  generally  obliterated  by  denudation  ;  but  scores  of  craters 
in  the  Auvergne  and  New  Zealand,  that  probably  date  back  to  the 
Miocene  or  early  Pliocene,  are  as  fresh  as  if  only  recently  formed. 

Volcanic  Cones. — The  fragmentary  material  and  lava  streams 
ejected  by  volcanoes  accumulate  around  the  neighbourhood  of  the 
vent,  and  thus  build  up  the  cone-shaped  elevations  that  are  so 
characteristic  of  volcanoes.  At  Auckland,  where  some  of  the  cones 
have  been  cut  down  for  the  extraction  of  building-stone,  the  different 

1  Gr.  krater  =  a  large  bowl. 


224 


A    TEXT-BOOK    OF    GEOLOGY. 


layers  of  material  are  seen  to  dip  away  from  the  central  vent  in  all 
directions,  as  shown  in  the  next  figure. 

Tuff- Cones. — These  are  volcanic  cones  composed  of  dust  and 
scoriae  piled  up  by  different  eruptions.  Frequently  the  material 
is  spread  out  in  distinct  layers  that  may  in  some  cases  present  the 
appearance  of  well-stratified  aqueous  strata.  This  arises  from  the 


W//////// 


FIG.  120. — Showing  structure  of  volcanic  cone. 

fine  and  coarse  material  being  arranged  in  alternating  layers.  The 
sorting  was  apparently  effected  by  the  varying  force  of  the  ex- 
plosions and  the  winnowing  action  of  the  wind. 

The  structure  of  a  tuff-  or  cinder-cone  is  shown  in  fig.  121. 


FIG.  121. — Showing  structure  of  a  tuff-cone  with 
alternations  of  fine  and  coarse  ash. 

Lateral  Cones. — In  some  of  the  larger  volcanoes,  the  eruptions 
of  lava  do  not  always  take  place  from  the  central  crater  at  the 
summit,  but  from  smaller  vents  on  the  sides.  The  uprising  lava 
in  the  central  fissure  exerts  enormous  pressure  on  its  walls  ;  and  in 
the  case  of  a  high  volcano,  rupture  may  take  place  at  some  weak 
point  before  the  lava  has  risen  high  enough  to  overflow  the  crater- 
lip. 

The  fluid  pressure  of  a  column  of  molten  rock  with  a  specific 
gravity  of  2-65  is  114-75  Ibs.  per  square  inch  for  every  hundred 


VOLCANOES  AND  VOLCANIC  ACTION. 


225 


feet  of  height,  or  72  tons  per  square  foot  for  every  thousand 
feet.  When  the  crater- walls  are  unable  to  withstand  this  pressure, 
rupture  takes  place  at  the  weakest  point. 

The  summit-crater  of  Etna  is  nearly  always  in  a  state  of  mild 
activity,  emitting  clouds  of  steam  and  dust,  but  the  eruptions  of 
lava  usually  take  place  from  lateral  vents  around  which  there 
have  been  built  small  parasitic  cones,  ranging  from  200  to  600  feet 
high. 

Lava  Streams. — When  a  stream  of  lava  issues  from  a  vent,  it 
glows  with  a  white  heat,  and  at  night  may  light  up  the  sky  overhead 
with  a  ruddy  glow,  as  of  a  forest  fire.  It  flows  with  the  motion 
common  to  all  viscous  fluids,  its  rate  of  flow  depending  on  the 
fluidity  and  depth  of  the  mass,  the  steepness  of  the  declivity,  and 
the  roughness  of  the  ground  over  which  it  descends. 

The  upper  surface,  as  it  cools,  assumes  a  red  heat,  and  finally 


FIG.  122. — Showing  mode  of  progression  of  lava  stream. 

becomes  black  and  cindery,  the  last  effect  being  due  to  the  escaping 
steam  and  gases  which  are  expelled  during  the  process  of  cooling. 

The  onward  motion  of  the  fluid  mass  below  breaks  up  the  solid 
surface  into  jagged,  cindery,  ragged  cakes  that  are  tilted  up  and 
frequently  half  engulfed  in  the  molten  flood  below.  Moreover,  the 
rolling  motion  of  the  stream  tends  to  pile  the  solid  jagged  masses 
into  irregular  mounds  that  are  slowly  carried  forward  on  the  moving 
tide  of  lava. 

The  front  of  the  lava  stream  progresses  by  a  rolling  motion,  by 
which  the  upper  surface  becomes  the  lower.  In  this  way,  befoxe 
the  stream  has  travelled  far,  the  bottom  has  become  a  confused 
tangle  of  angular  blocks  that  are  slowly  dragged  along  by  the  pasty 
mass. 

The  upper  surface  of  very  fluid  lavas  frequently  becomes  spongy 
and  frothy,  while  the  slowly  cooling  mass  below  assumes  the  curious 
ropy  and  streaky  appearance  of  boiled  sugar  that  has  been  poured 
down  an  inclined  plane.  The  plane  of  this  flow-structure  is  always 
parallel  to  the  surface  over  which  the  lava  travels,  but  accidental 


226 


A    TEXT-BOOK    OF    GEOLOGY. 


obstructions  may  cause  the  fluxion-planes  to  become  bent,  twisted, 
or  gnarled. 

In  many  volcanoes  the  lava  rises  up  and  fills  the  crater,  over  the 
rim  of  which  it  flows  in  a  gentle  stream  ;  but  when  any  portion  of 
the  crater- wall  is  weak,  it  may  be  carried  away.  The  craters  of 
Stromboli,  Vesuvius,  Kuapehu,  White  Island  (Plate  XVI.),  and 
many  other  volcanoes  have  been  breached  in  this  way. 

The  molten  lava,  as  it  pours  out  of  the  vents,  resembles  the  slag 
of  a  blast-furnace.  When  it  cools  rapidly,  as  it  frequently  does, 


FIG.  123. — Showing  corded  structure  of  lava,  Galtalaekr,  Hekla, 
Iceland.     (After  Tempest  Anderson.) 

at  the  thin  edges  of  the  stream,  or  where  it  flows  into  a  sheet  of 
water,  it  sometimes  solidifies  in  the  form  of  a  glass. 

The  vitreous  or  glassy  form  of  acid  lavas  is  called  obsidian  or 
simply  volcanic  glass  \  and  the  black,  glassy  form  of  basic  lavas, 
tachylite.  Pumice  is  the  light,  frothy,  fibrous,  spongy  glass  which 
forms  on  the  surface  of  acid  lavas.  In  other  words,  it  is  the  cinder y 
form  of  obsidian,  and  is  frequently  composed  of  a  matted  mass  of 
glassy  fibres. 

Columnar  Structure. — This  structure  is  frequently  seen  in 
effusive  rocks.  It  is  developed  by  the  stresses  that  arise  in  a  thick 
stream  of  lava  as  it  passes  from  the  liquid  to  the  solid  state.  As  the 
result  of  the  cooling  and  shrinking,  the  crust  of  the  lava  becomes 


VOLCANOES  AND  VOLCANIC  ACTION. 


227 


intersected  with  more  or  less  symmetrical  sets  of  cracks  that  divide 
the  surface  into  hexagons  that  fit  one  another  like  the  cells  of  honey- 


FIG.  124.— Fingal's  Cave,  Staffa. 

comb.  As  the  pasty  mass  under  the  crust  cools,  the  cracks  extend 
downwards  at  right  angles  to  the  surface.  In  a  few  cases  the  columns 
are  arranged  in  a  radial  or  fan-shaped  form. 


FIG.  125. — Showing  columnar  structure  of  basalt,  Giant's  Causeway, 
Antrim,  N.  Ireland.     (After  Tempest  Anderson.) 

Columnar  structure,  being  mainly  a  result  of  contraction  arising 
from  cooling,  is  usually  best  developed  in  those  portions  of  the 
magma  exposed  to  the  cooling  effects  of  the  atmosphere,  and  of  the 


228  A  TEXT-BOOK  OF  GEOLOGY. 

rocky  surface  on  which  the  lava  rests.  Hence,  in  thick  lava-flows 
this  characteristic  structure  may  be  only  developed  in  the  upper 
and  lower  portions. 

The  columnar  structure  is  found  in  effusive  igneous  rocks  of  all 
kinds,  but  is  most  frequently  seen  in  those  of  a  basic  type. 

Pillow- Structure. — When  a  very  fluid  basaltic  lava  is  chilled  by 
contact  with  water,  the  surface  sometimes  assumes  the  appearance 
of  a  number  of  large  pillows  packed  together.  Pillow-structure  is 
seen  in  lavas  of  all  ages.  Good  examples  may  be  seen  in  the  sea- 
cliffs  near  Ballantrae,1  in  south-west  Scotland,  where  a  flow  of 
basalt  is  intercalated  with  the  Silurian  rocks  ;  at  Cape  Oamaru,2 
New  Zealand,  where  a  stream  of  basalt  is  associated  with  Middle 
Tertiary  strata  ;  and  on  the  sea-coast  of  Savaii,  where  the  recent 
lavas  of  the  Matavanu  volcano  flow  into  the  sea. 

Dr  Tempest  Anderson,3  who  was  an  eye-witness  of  the  actual 
formation  of  this  exceptional  lava-structure,  states  that  the  lava, 
chilled  by  the  waves,  extended  itself  into  lobes  which,  reduced  to 
a  pasty  condition  by  cooling,  would  be  seen  to  swell  into  buds  with 
narrow  necks,  and  these,  being  still  in  communication  with  the 
source  of  supply,  would  rapidly  increase  in  heat,  mobility,  and  size 
until  they  became  lobes  as  large  as  a  sack  or  pillow,  or  perhaps 
stopped  short  at  the  size  of  an  Indian  club  or  large  Florence  flask. 

At  Ballantrae  and  Cape  Oamaru,  the  spaces  or  cracks  between 
the  pillows  are  filled  with  fossiliferous  limestone. 

Spheroidal  Weathering.— Rocks  are  frequently  divided  by  two 
systems  of  joints,  or  by  joints  and  the  bedding-planes,  into  roughly 
cuboidal  blocks.  When  water  percolates  along  these  planes,  it 
decomposes  the  rock,  changing  it  into  clay.  At  the  edges  where 
two  planes  meet  the  action  is  twice  as  rapid  as  on  the  sides  of  the 
block  ;  and  at  the  corners  where  three  planes  meet,  it  is  three 
times  as  rapid.  This  differential  rate  of  weathering  causes  the  edges 
and  corners  to  become  gradually  rounded,  and  in  time  the  block 
may  assume  a  spheroidal  form. 

When  the  decomposition  of  the  block  is  complete,  as  frequently 
happens  near  the  surface  of  the  ground,  the  rock  is  entirely  changed 
into  clay;  but  when  incomplete,  a  rounded,  boulder-like  core  of 
undecomposed  rock  may  remain  in  the  centre. 

The  rocks  that  are  the  most  prone  to  spheroidal  weathering  are 
basalts,  andesites,  phonolites,  granites,  tufaceous  sandstones,  and 

1  B.    N.    Peach  and   J.    Home,  The  Silurian   Rocks   of  Scotland,  vol.    i. ; 
London,  1903. 

2  J.  Park,  "  Marine  Tertiaries  of   Otago   and  Canterbury,"  Trans.  N.Z. 
Inst.,  vol.  xxxvii.  p.  513. 

3  Tempest  Anderson,  "  Volcano  of  Matavanu  in  Savaii,"  Quart.  Jour.  Geol. 
Soc.,  vol.  Ixvi.  p.  633. 


[To  face  page  228. 


FIG.  126. — Showing  pillow- structure  at  Cape  Oamaru,  New  Zealand. 


VOLCANOES    AND    VOLCANIC    ACTION.  229 

grey wackes ;  but  such  relatively  soft  rocks  as  mudstones  and 
marly  clays,  that  have  been  broken  into  small  cuboidal  blocks  by 
shrinkage  cracks,  frequently  exhibit  this  phenomenon  in  a  perfect 
manner. 

The  clays  resulting  from  the  decomposition  of  rocks  in  situ  are 
called  residual  clays  to  distinguish  them  from  glacial  clays,  and  the 
detrital  clays  that  often  accumulate  on  slopes  and  in  hollows. 

Spheroidal  weathering  is  not  always  the  result  of  aqueous  de- 
composition. In  regions  where  there  is  a  considerable  range  of 
daily  temperature,  as  in  alpine  valleys  and  arid  highlands,  angular 
blocks  exposed  at  the  surface  soon  assume  a  spheroidal  shape. 
In  this  case  the  rounding  is  due  to  the  stresses  arising  from  the 
alternating  expansion  and  contraction  of  the  surface  skin  of  the 
rock.  If  the  intensity  of  stress  on  the  sides  of  the  block  is  repre- 
sented by  1,  that  along  the  edges  will  equal  2,  and  at  the  corners  or 
solid  angles,  3.  Owing  to  the  action  of  these  unequal  stresses  the 
block  peels  off  in  wedge-shaped  flakes  and  curved  splinters,  and 
the  effect  of  this  is  to  cause  the  block  to  assume  a  rounded  shape, 
but  without  the  formation  of  residual  clays. 

Amygdaloidal  Structure. — A  lava  stream,  through  the  expansive 
force  of  the  escaping  steam  and  gases,  is  usually  made  vesicular 
or  scoriaceous  at  the  surface  and  bottom.  Later  these  bubble- 
cavities  may  become  filled  with  mineral  matter  deposited  from 
solution,  constituting  what  is  called  an  amygdaloidal  structure.  The 
amygdaloids  or  almond-shaped  blebs  are  obviously  of  secondary 
origin. 

Fragmentary  or  Pyroelastie1  Ejeeta. — These  rocks  include  the 
various  fragmentary  material  ejected  by  a  volcano,  in  the  form  of 
large  and  small  blocks,  sconce,  lapilli,  ash,  and  dust. 

The  blocks  are  mostly  angular  or  subangular  masses  of  lava  that 
are  often  of  enormous  size.  Rounded  blocks  are  not  at  all  rare. 
Among  the  blocks  that  are  projected  into  the  air  many  fall  back 
into  the  throat  of  the  vent,  where  they  are  churned  up  until  again 
expelled.  In  this  way  the  blocks  that  are  not  reduced  to  small 
fragments  may  be  worn  until  they  become  well-rounded. 

The  cindery  pieces  of  lava  are  called  sconce  ;  while  the  smaller 
scoriaceous  fragments  are  usually  spoken  of  as  ash.  The  grit  and 
dust  are  merely  comminuted  lava. 

The  small  pieces  of  lava  torn  from  the  pasty  lava  form  what  are 
called  lapilli,  which  range  from  the  size  of  peas  to  that  of  walnuts. 
The  larger  masses  of  liquid  lava  that  are  projected  from  the  vent 
solidify  while  swirling  through  the  air,  and  thereby  frequently 
assume  a  torpedo  or  bomb-like  form  with  curious  twisted  ends  ; 
hence  their  name  volcanic  bombs. 

1  Gr.  pur  =  fire,  and  klastos  =  broken, 


230  A  TEXT-BOOK  OF  GEOLOGY. 

The  coarse  angular  blocks,  when  consolidated,  form  what  is 
called  a  volcanic  breccia.  The  pell-mell  mixture  of  large  and  small 
blocks  set  in  a  matrix  of  grit  and  dust,  is  usually  called  a  volcanic 
agglomerate.  The  loose  material  that  blocked  up  the  throat  of  an 
expiring  or  spent  vent  forms,  when  consolidated,  an  agglomerate- 
neck. 

The  finer  material  ejected  by  volcanic  explosions  is  frequently 
subjected  to  a  certain  amount  of  wind  sorting,  and  hence  is  fre- 
quently deposited  in  layers  that  often  possess  a  well-stratified 
appearance.  The  successive  layers  of  material  ejected  by  different 


FIG.  127. — Showing  volcanic  bomb  from  Auckland,  New  Zealand. 

eruptions  form  parallel  sheets  that,  when  exposed  in  gorges  or  on 
the  banks  of  streams,  appear  to  be  stratified.  Such  bedded  material 
is  called  volcanic  tuff. 

Most  volcanoes  occur  near  the  sea,  and  many  are  actually  situated 
on  the  sea-floor.  Lavas  that  flow  into  the  sea  are  sometimes  broken 
up  into  blocks  and  sand  by  the  explosive  action  of  the  steam 
suddenly  generated  on  their  surfaces.  The  lavas  and  ashes  of 
submarine  volcanoes  become  interstratified  with  the  ordinary  sedi- 
ments of  denudation. 

The  ejecta  from  a  land  volcano  may  fall  into  the  sea,  or  it  may  be 
carried  into  the  sea  by  streams  and  rivers  draining  the  eruptive 
area.  When  sorted  and  laid  down  in  beds,  this  material  forms  what 
are  called  marine  tuffs.  Tuffs  formed  in  this  way  may  attain  a 


To  face  page  231.] 


[PLATE    XVII. 


A. 


C. 


I). 


CLOUD  FORMS — ERUPTION  OF  COLIMO  VOLCANO,  MEXICO,  1910. 
(Photographed  from  railway  train  during  eruption.) 

A.  Form  of  steam  cloud  five  minutes         C.  At  fifteen  minutes. 

after  first  explosion.  D.  At  twenty  minutes. 

B.  At  ten  minutes. 


VOLCANOES  AND  VOLCANIC  ACTION.         231 

considerable  thickness,  but  they  will  always  be  confined  to  the 
neighbourhood  of  the  eruptive  area.  At  their  edges  they  may  be 
interbedded  with  ordinary  sandstones,  mudstones,  or  limestones. 
Sandstones  containing  a  considerable  proportion  of  fine  volcanic 
material  are  called  tufaceous  sandstones.  Marine  tuffs  and 
tufaceous  sandstones  are  frequently  fossiliferous. 

The  fragmentary  material  ejected  by  a  volcano  frequently 
includes  blocks  of  granite,  schist,  slate,  or  other  rock  torn  from 
the  rocks  through  which  the  volcanic  fissure  passes.  Among  the 
debris  ejected  by  Vesuvius,  blocks  of  fossiliferous  limestone  are 
comparatively  abundant. 

Steam  and  Gaseous  Emanations. — The  principal  product  of  many 
volcanic  eruptions  is  steam.  In  the  early  stages  of  the  eruption 
the  steam  rises  from  the  crater  in  the  form  of  a  pillar,  which  is 
crowned  with  a  cloud  shaped  like  a  cauliflower  ;  in  the  later  stages 
it  spreads  out  into  the  well-shaped  pine-tree  form.  These  cloud- 
forms  are  well  seen  in  Plate  XVII.,  which  shows  an  eruption  of 
Colimo  volcano,  in  Mexico,  in  1910.  The  cauliflower  cloud  was 
seen  to  rapidly  develop  into  the  pine-tree  form. 

Besides  steam,  many  other  vapours,  including  those  of  sodium 
chloride,  iron  chloride,  chlorine,  sulphuretted  hydrogen,  and  sul- 
phurous acid,  that  were  dissolved  in  the  molten  magma,  escape 
from  fissures  and  cracks  in  the  cooling  lava. 

The  exhalations  in  the  first  stages  of  cooling  are  mainly  the 
chlorides  of  sodium  and  iron,  and  in  the  later  stages  sulphuretted 
hydrogen  and  sulphurous  acid.  In  volcanic  regions  the  character- 
istic smell  of  sulphurous  acid,  S02,  pervades  the  atmosphere  at  all 
times,  more  particularly  in  regions  where  volcanic  activity  is  on 
the  wane. 

The  fissures  from  which  gaseous  emanations  escape  are  called 
Fumaroles,  the  walls  of  which  are  frequently  lined  with  beautiful 
incrustations  of  sulphur  crystals  sublimed  by  the  interaction  of 
H2S  and  S02. 

Expiring  Volcanic  Activity. — A  volcano  in  the  waning  stage  of 
its  existence  may  emit  only  steam  and  various  gases.  The  well- 
known  decadent  volcano  Solfatara,  near  Naples,  emits  only  steam 
and  gases  ;  hence  any  volcano  in  this  phase  of  activity  is  said  to  be 
in  the  Solfataric  stage. 

Thermal  Springs. — In  some  regions  of  waning  volcanic  energy, 
the  old  craters  are  honeycombed  with  underground  passages  from 
which  openings  rise  to  the  surface,  where  they  generally  terminate 
in  basins  filled  with  clear  bluish-green  mineralised  waters,  the 
majority  of  which  are  hot  or  boiling  furiously. 

The  basins  are  generally  situated  on  the  summit  of  low  mounds 
"composed  of  siliceous  sinter  of  various  hues  deposited  by  the  over- 


232  A  TEXT-BOOK  OF  GEOLOGY. 

flowing  waters  when  they  come  in  contact  with  the  air.  The 
largest  groups  of  thermal  springs  are  those  of  the  Yellowstone 
National  Park,  U.S.A.,  and  Eotorua,  N.Z.  At  the  latter  region 
some  of  the  springs  are  strongly  alkaline,  and  others  strongly  acid. 
They  all  possess  valuable  therapeutic  properties. 

Geysers. — These  occur  in  volcanic  regions  where  the  rocks,  some 
little  distance  below  the  surface,  are  still  intensely  hot,  and  within 
the  reach  of  springs  or  streams  of  water.  They  are  connected  by  a 
fissure  with  a  pool  or  basin  from  which  the  water  at  certain 
intervals  is  ejected  with  great  force,  frequently  to  a  height  of 
several  hundred  feet. 

The  principle  underlying  the  intermittent  action  of  geysers1  is 
dependent  on  the  expansive  force  of  superheated  steam.  The 
fissure  becomes  filled  with  a  column  of  water  from  the  overflow  of 
some  neighbouring  spring  or  stream.  The  heat  is  greatest  in  the 
lower  part  of  the  fissure ;  consequently  the  water  in  that  part  be- 
comes hot,  and  soon  reaches  a  temperature  of  212°  Fahr.  The 
water  would  boil  at  that  temperature  at  the  surface,  but  owing  to 
the  hydraulic  pressure  of  the  column  of  water,  it  does  not  boil.  On 
the  contrary,  it  becomes  hotter  and  hotter  until  in  some  portion  of 
the  fissure  the  boiling-point  corresponding  to  the  pressure  is  reached. 
The  steam  thus  generated  expands  and  pushes  some  of  the  water 
out  at  the  surface.  This  relieves  some  of  the  pressure,  with  the 
result  that  the  water  below  boils  furiously.  Steam  is  now  generated 
with  great  rapidity,  and,  exerting  enormous  expansive  force  on  the 
column  of  water,  with  a  mighty  roar,  hurls  it  into  the  air  together 
with  any  loose  stones  that  may  lie  in  its  path. 

Some  geysers  play  at  short  intervals,  while  others  are  quiescent 
for  days  and  even  weeks.  A  geyser  may  be  made  to  play  before 
its  customary  time  by  the  application  of  soap,  which  reduces  the 
surface  tension  or  strength  of  the  resistant  skin  of  water. 

Distribution  of  Volcanoes. — If  we  look  at  a  map  of  the  globe,  we 
are  at  once  confronted  with  the  significant  fact  that  almost  all 
recent  indications  of  volcanic  activity  are  to  be  found  either  along 
certain  lines  of  coast  or  in  islands.  Most  noticeable  of  all  is  the 
ring  of  volcanoes  that  fringes  the  great  basin  of  the  Pacific  Ocean. 
This  encircling  girdle  extends  along  the  whole  length  of  the  Andes 
from  Tierra  del  Fuego  to  Central  America,  whence  it  follows  the 
coastal  Sierras  to  the  Aleutian  Islands ;  from  there  passing  south- 
ward to  Kamtschatka,  Kurile  Islands,  and  Japan ;  thence  stretching 
through  the  Philippines,  Sumatra,  Java,  and  adjacent  islands  to 
New  Zealand. 

Another  remarkable  zone  of  volcanoes  girdles  the  globe  from 
Central  America  eastward  to  the  Azores,  Canary  Islands,  Medi- 
1  Ice.  geyser  =  gusher  or  roarer. 


VOLCANOES    AND    VOLCANIC    ACTION.  233 

terranean,  Red  Sea,  and  through  a  chain  of  islands  to  the  mid- 
Pacific  and  New  Zealand  group. 

Origin  of  Volcanoes. — It  will  be  observed  (a)  that  the  distribution 
of  active  volcanoes  is  linear,  and  (6)  that  volcanoes  rise  either 
directly  from  the  floor  of  the  ocean  or  lie  within  a  moderate  distance 
from  its  coasts  or  of  large  sheets  of  water. 

The  linear  distribution  of  volcanoes  seems  to  warrant  the  in- 
ference that  some  connection  exists  between  volcanic  vents  and 
crustal  lines  of  weakness  ;  while  the  coastal  or  oceanic  situation 
of  the  vents  leads  to  the  further  inference  that  eruptions  are 
dependent  on  the  presence  of  water. 

Active  volcanoes  generally  occur  on  the  crests  of  terrestrial 
ridges,  which  supports  the  view  that  ridges  of  elevation  resulting 
from  folding  are  lines  of  crustal  weakness.  Further  support  of  this 
contention  is  obtained  from  the  gravity  observations  made  in  India 
and  the  United  States.  These  observations  show  that  mountain 
masses,  such  as  the  Himalayas,  do  not  produce  in  the  direction 
of  gravity  the  effect  that  their  visible  mass  should  produce.  In 
other  words,  they  are  deficient  in  density. 

If  the  elevation  of  great  earth  flexures  produces  the  initial  lines 
of  weakness,  we  cannot  doubt  that  the  immediate  cause  of  volcanic 
activity  is  the  expansive  force  of  steam  generated  at  enormous 
pressures  from  contact  with  heated  rocks  below.  The  origin  of 
this  magmatic  water  is  still  problematical.  According  to  some, 
it  finds  its  way  below  by  seepage  from  the  floor  of  the  sea  ;  accord- 
ing to  others,  it  is  juvenile — that  is,  an  original  constituent  of  the 
molten  magma.  Whatever  its  origin,  its  presence  will  render  the 
magma  more  fluid,  and  since  it  must  exist  under  enormous  stress, 
it  will  cause  the  magma  to  penetrate  every  plane  of  weakness  pro- 
duced by  the  folding.  Where  the  plane  of  weakness  coincides 
with  a  ridge  of  elevation,  we  shall  obtain  a  manifestation  of  the 
phenomena  known  as  volcanic  activity. 

The  great  mountain-building  folds  of  the  globe  are  meridional, 
e.g.  Andes  and  Rocky  Mountains  ;  or  equatorial,  e.g.  Alps, 
Caucasus,  and  Himalayas  ;  and  it  is  only  on  the  meridional  folds 
or  their  prolongations  that  we  find  the  evidences  of  crustal  weakness 
as  expressed  by  volcanic  activity.  The  north  and  south  chains  are 
everywhere  crowned  with  active  volcanoes,  while  the  east  and 
west  chains  are  singularly  free  from  volcanic  phenomena. 

Former  Volcanic  Activity. — Piles  of  volcanic  rocks  are  found 
interbedded  with  stratified  formations  of  nearly  all  ages,  and  so 
far  as  we  can  form  an  opinion,  the  various  types  of  eruption  and  the 
character  of  the  lavas,  dust,  scoriae,  and  other  solid  ejecta  did  not 
differ  from  those  of  the  present  day. 

Some  of  the  outbursts  that  took  place  in  the  Middle  and  Older 


234  A  TEXT-BOOK  OF  GEOLOGY. 

Palaeozoic  eras  were  on  a  stupendous  scale,  and  have  only  been 
paralleled  by  those  of  the  Middle  Tertiary. 

In  the  Mesozoic  era,  there  was  a  singular,  almost  world-wide, 
interval  of  quiescence ;  but  with  the  close  of  the  Cretaceous,  there 
came  a  remarkable  revival  of  activity  which  probably  attained  its 
greatest  intensity  in  the  Miocene.  Since  then  volcanic  outbursts 
have  become  less  and  less  violent,  and  narrower  in  their  radius 
of  disturbance.  It  would  almost  appear  as  if  we  were  now  living 
in  a  period  of  volcanic  decadence. 

The  Lower  and  Middle  Palaeozoic  and  the  Middle  Tertiary  were 
the  great  mountain-building  periods  of  the  globe,  and  also  of 
maximum  volcanic  activity.  The  coincidence  may  be  more  than 
accidental,  and  may  be  additional  evidence  of  the  relationship 
of  erogenic  folding  and  volcanic  activity. 

Intermittent  Volcanic  Activity. — The  piles  of  volcanic  material 
that  occupy  some  regions  were  not,  as  a  rule,  ejected  by  one  out- 
burst, but  represent  the  accumulations  of  many  eruptions. 

Take  the  Hauraki  Peninsula  in  New  Zealand  as  a  typical  example. 
There  we  have  a  region  over  100  miles  long  and  from  20  to  30  miles 
wide  occupied  by  a  vast  pile  of  lavas,  breccias,  and  tuffs  that  have 
now  been  eroded  into  a  complex  of  rugged  mountains  and  steep 
ridges. 

The  first  eruptions  took  place  in  the  Middle  Tertiary,  and  were 
characterised  by  the  emission  of  floods  of  andesitic  lavas.  Then 
followed  a  period  of  quiescence  during  which  the  surface  of  the 
lavas  became  decomposed  into  soils  on  which  a  forest  vegetation 
soon  established  itself.  Many  large  streams  drained  the  slopes  of 
the  volcanoes  and  spread  the  eroded  material  on  the  adjacent  low- 
lands, thereby  forming  swampy  flats  on  which  peat-bogs  flourished 
until  they  attained  a  great  depth. 

The  next  eruptions  were  mainly  characterised  by  the  ejection 
of  fragmentary  material.  The  forests  were  destroyed  and  the  peat- 
bogs covered  with  thick  sheets  of  ashes  intercalated  with  streams 
of  andesitic  lavas. 

During  the  period  of  cessation  that  followed,  the  land  again 
became  subject  to  denudation,  and  forests  once  more  covered  the 
land. 

In  the  Pliocene  there  took  place  a  vast  outpouring  of  acid  lavas 
which  destroyed  all  vegetation  and  filled  up  the  eroded  contours. 
Since  then  there  has  been  a  complete  cessation  of  all  volcanic 
activity  in  the  Hauraki  area. 

Old  Land- Surf  aces. — The  old  land-surfaces  that  existed  between 
the  different  periods  of  activity  can  easily  be  traced  in  sea-cliffs 
and  river-gorges.  The  layers  of  soil  are  usually  baked  and  oxidised 
into  reddish  brick-coloured  clays  in  which  the  remains  of  trees 


VOLCANOES    AND    VOLCANIC    ACTION.  235 

are  found,   in  places  converted  into   charcoal,   or  silicified  into 
wood-opal,  while  the  peat-bogs  have  been  converted  into  lignite. 

The  old  land-surfaces  that  mark  periods  of  quiescence  between 
successive  eruptions  can  be  detected  in  many  volcanic  regions. 
The  seams  of  lignite  and  beds  of  shale  interbedded  with  the  Tertiary 
volcanic  rocks  of  the  Isle  of  Mull  and  Hauraki  are  a  record  of 
denudation  and  vegetable  growth  during  a  considerable  cessation 
of  volcanic  activity. 

SUMMARY. 

(1)  Volcanoes  may  be  classed  as  Extinct,  Dormant,  or  Active. 

(2)  Volcanic  eruptions  according  to  their  character  constitute 
three  well-marked  types  :— 

Vesuvian  type,  i.e.  eruptions  from  a  central  cone. 
Icelandic  type,  i.e.  eruptions  from  a  fissure-rent. 
Krakatoan  type,  i.e.  explosive  eruptions. 

(3)  Active  volcanoes  are  distributed  along  the  coasts  of  the 
sea  or  large  sheets  of  water.     They  are  frequently  situated  on 
terrestrial  ridges. 

The  linear  distribution  of  volcanoes,  and  their  proximity  to  the 
sea,  would  lead  to  the  inferences  (a)  that  they  occur  along  lines 
of  crustal  weakness ;  and  (6)  that  water  in  the  form  of  steam  plays 
an  important  role  in  their  origin  and  eruptive  force. 

(4)  The  solid  material  ejected  by  volcanoes  is  mainly  lava  and 
such  fragmentary  matter  as  blocks,  scoriae,  ashes,  and  dust.     The 
gaseous   products,  in   addition  to   enormous   volumes   of   steam, 
are   sodium   and  iron   chlorides,  chlorine,  sulphurous   acid,    and 
sulphuretted  hydrogen. 

(5)  When  the  crater-walls  are  weak,  the  lava  may  escape  at 
points  where  in  time  lateral  cones  are  built  up. 

(6)  There  is  abundant  evidence  of  volcanic  activity  throughout 
the  whole  of  geological  time,  particularly  in  the  Early  and  Middle 
Palaeozoic  and  Tertiary  eras.     In  the  Mesozoic  there  was  an  almost 
complete  cessation  of  volcanic   activity  throughout  the  greater 
part  of  the  globe. 

(7)  Solfataras,  geysers,  and  hot  springs  are  evidences  of  waning 
volcanic  activity. 


CHAPTER   XV. 
IGNEOUS   ROCKS. 

OCCURRENCE,  PHYSICAL  CHARACTERS,  AND  COMPOSITION. 

AN  igneous  rock  is  one  that  has  cooled  from  a  molten  condition. 

The  study  of  igneous  rocks  includes  a  consideration  of  the  follow- 
ing points  :— - 

(a)  Mode  of  Occurrence. 

(6)  Texture. 

(c)  Composition. 

Mode  of  Occurrence. — An  uprising  molten  magma  that  issues 
from  a  vent  or  fissure  and  spreads  over  the  surface  is  said  to  be 
effusive,  and  the  portions  that  cool  and  solidify  below  the  surface 
are  called  intrusive. 

The  effusive  rocks  that  are  extruded  from. a  volcanic  vent  generally 
take  the  form  of  streams  ;  while  those  that  issue  from  fissure- 
rents  may  form  streams,  or  they  may  first  fill  up  the  inequalities 
of  the  ground  over  which  they  flow  and  eventually  spread  over  the 
country  as  wide  sheets.  A  succession  of  very  fluid  lavas  issuing 
from  a  fissure  may  build  up  a  plateau  as  large  as  a  State. 

The  intrusive  rocks  are  not  seen  until  laid  bare  by  denudation. 
Their  form  is  dependent  on  the  shape  of  the  pre-existing  fissures 
or  cavities  which  they  fill,  or  of  the  cavities  which  they  open  for 
themselves  by  their  eruptive  force.  Commonly  they  appear  (a) 
as  more  or  less  vertical  sheet-like  veins  called  dykes  ;  (b)  as  irregular 
sheets  called  sills  or  intrusive  sheets  that  have  been  intruded  along 
the  bedding  planes  of  stratified  rocks  ;  or  (c)  as  more  or  less  dome- 
shaped  masses  called  bosses  that  have  solidified  in  huge  caverns 
at  a  considerable  depth  below  the  surface. 

Thus,  in  the  various  forms  assumed  by  eruptive  rocks  we  have 
the  basis  of  a  convenient  morphological  classification  which  em- 
braces all  kinds  of  igneous  rocks  : — 

I.  Effusive.— Streams  and  floods  of  lava. 
II.  Intrusive.— Dykes,  Intrusive  Sheets  or  Sills,  and  Bosses. 

236 


IGNEOUS   ROCKS. 


237 


FIG.  128. — Showing  lava  streams  and  sills. 


FIG.  129. — Showing  dyke  penetrating  tuffs  at  Kiama,  N.S.W. 
(After  Jaquet.) 

Intrusive  rocks  are  sometimes  divided  into  two  groups,  namely  : — 

(a)  Plutonic  or  Abyssal)  i.e.  those  that  have  solidified  at  a  great 

depth. 
(6)   Hypabyssal,  i.e.  those  that  have  solidified  at  a  less  depth. 


238 


A    TEXT-BOOK    OF    GEOLOGY. 


The  plutonic  rocks  are  generally  more  coarsely  crystalline  in 
texture  than  the  hypabyssal,  but  the  two  groups  are  not  very  well 
marked,  except  in  the  extreme  types. 

In  fig.  128  we  have  two  streams  of  lava,  a,  a,  that  issued  from 
fissures,  6,  6, ;  and  three  sills,  c,  c,  c,  that  forced  themselves  along 
the  bedding  planes  of  a  calcareous  sandstone. 

It  is  obvious  that  if  denudation  were  to  remove  the  greater 
portion  of  the  cone,  6,  6  would  appear  as  dykes  penetrating  the 
tuffs  and  sandstone  ;  and  the  outcrop  of  the  main  dyke  would 
not  improbably  be  somewhat  like  that  of  the  dyke  shown  in  fig.  129. 

Dykes,  it  should  be  noted,  may  vary  from  an  inch  or  less  to 
many  thousands  of  feet  wide.  When  the  dyke-rock  is  harder  and 
more  resistant  than  the  rock  which  it  penetrates,  it  may  be  left 


FIG.  130. — Showing  dome-shaped  sills  or  laccoliths. 
(After  Gilbert.) 

standing  above  the  general  level  of  the  ground  as  a  conspicuous 
wall  ;  hence  the  origin  of  the  name  dyke.  On  the  other  hand,  if 
softer  than  the  enclosing  rock,  it  may  be  worn  away  so  as  to  expose 
the  dyke-fissure,  as  shown  in  fig.  129. 

When  the  magma  rises  through  a  pipe-like  fissure,  as  it  frequently 
does  in  volcanoes,  the  mass  which  solidifies  in  the  pipe  forms  what 
is  called  a  volcanic  neck. 

When  a  sill  expands  out  to  a  dome-shaped  mass  lying  between 
two  beds,  it  is  called  a  laccolite.1  This  form  of  intrusion  has  a 
limited  lateral  extension,  and  generally  arches  and  disrupts  the 
overlying  strata  in  making  room  for  itself. 

The  intrusive  boss,  sometimes  called  a  batholith,  occupies  a  deep- 
seated  cavity  of  irregular  shape,  and  usually  of  large  dimensions, 
frequently  many  miles  across.     The  plutonic  types  of  granite, 
syenite,  diorite,  and  gabbro  usually  occur  as  bosses. 
1  Gr.  lakkos= cistern,  and  lithos  =  stone. 


IGNEOUS    ROCKS.  239 

Obviously,  then,  the  self-same  magma,  according  to  the  situation 
in  which  it  cooled  and  consolidated,  may  be  an  effusive  lava,  or  an 
intrusive  dyke,  sill,  or  boss. 

Texture. — The  texture  of  an  igneous  rock  is  not  dependent  on 
the  composition  of  the  original  magma,  but  is  mainly  determined 
by  the  rate  and  conditions  of  cooling  and  consolidation. 

The  rate  of  cooling  will  mainly  depend  on  the  mode  of  occurrence 
of  the  eruptive  magma.  A  lava  stream,  for  example,  will  cool 
rapidly  on  the  surface,  at  the  selvages,  and  wherever  it  flows  into 
a  sheet  of  water.  Moreover,  a  thin  stream  will  cool  more  rapidly 
than  a  thick  one.  The  magma  that  forms  dykes  and  sills  will  cool 
more  slowly  than  effusive  lavas,  and  bosses  more  slowly  than  dykes. 
The  relationship  existing  between  mode  of  occurrence  and  texture 
is  so  intimate  and  well  ascertained  that  when  one  of  these  is  given, 
the  other  can  be  postulated  within  narrow  limits  of  error. 


FIG.  13 J. — Showing  intrusive  boss  exposed  by  denudation. 
(«)  Uranite.  (6)  Slates. 

Dykes  and  intrusive  sills  have  cooled  slower  than  lava  streams, 
and  more  rapidly  than  bosses  lying  far  below  the  surface.  More- 
over, dykes  and  bosses  have  not  only  cooled  slowly  but  also  under 
great  pressure,  the  latter  arising  mainly  from  (a)  the  weight  of  the 
enclosing  and  overlying  rocks,  and  (6)  the  internal  stress  of  the 
imprisoned  gases  and  steam  occluded  in  the  magma. 

A  molten  magma  does  not  differ  from  a  furnace  slag;  hence, 
when  it  cools  rapidly  it  forms  a  glass.  When  it  cools  slowly  a 
crystalline  structure  is  developed  ;  hence,  according  to  the  rate 
of  cooling,  a  given  eruptive  magma  may  present  every  variety  of 
texture  from  the  glassy  to  the  completely  crystalline. 

A  magma  may  become  : — 

(1)  Wholly  glass  when  it  cools  rapidly. 

(2)  A  glassy  matrix  with  a  few  small  imperfectly  formed  crystals 

when  it  cools  less  rapidly. 

(3)  A  glassy  matrix  crowded  with  large  crystals  when  it  cools 

slowly. 


240  A  TEXT-BOOK  OF  GEOLOGY. 

(4)  A  matrix  of  minute  crystals  with  large  crystals  when  it  cools 

more  slowly. 

(5)  Completely  crystalline,  i.e.  holocrystalline,  when  it  cools  still 

more  slowly. 

A  glass  is  a  vitreous l  body,  and  when  it  is  partially  crystallised, 
it  is  said  to  be  partially  devitrified.  Devitrification  is  merely  a 
process  of  crystallisation,  and  when  the  glass  has  been  entirely 
replaced  with  crystals,  it  is  completely  devitrified. 

The  slower  the  rate  of  cooling,  the  more  is  the  glassy  matrix 
replaced  by  a  crystalline  structure,  until  a  point  is  reached  at  which 
the  ground-mass  is  wholly  crystalline. 

Generally  speaking,  the  slower  the  cooling,  the  larger  will  be 
the  crystals. 

The  incipient  forms  of  crystals  that  first  appear  in  a  cooling 
glassy  magma  are  minute  rods  and  plates  called  crystallites  or 
microlites.2  These  form  the  framework  of  crystal-skeletons. 
If  the  cooling  is  rapid,  they  are  unable  to  arrange  themselves  in 
geometrical  forms,  but  if  the  cooling  is  slow,  they  build  up  large 
crystals. 

The  crystallites  frequently  form  beautiful  radiating  clusters. 
When  grouped  in  spheres,  they  form  what  are  called  spherulites. 

Frequently  in  glassy  rocks  the  crystallites  arrange  themselves 
with  their  longer  axis  parallel  with  the  flow-structure. 

A  glassy  lava  may  sometimes  be  crowded  with  small  enamel- 
like  globules  with  an  imperfectly  developed  concentric  structure. 
This  structure  is  called  perlitic,  and  is  not  infrequently  seen  in  acid 
lavas  that  have  cooled  more  slowly  than  obsidian. 

When  conspicuous  crystals  of  some  mineral  are  enclosed  in  a 
finely  crystalline  ground-mass,  the  rock  is  said  to  be  porphyritic, 
and  the  large  crystals  are  called  phenocrysts. 

When  a  phenocryst  is  bounded  by  crystal  faces,  it  is  said  to  be 
idiomorphic? 

A  rock  in  which  the  majority  of  the  constituent  minerals  are 
idiomorphic  is  said  to  possess  a  panidiomorphic  structure. 

Should  only  some  of  the  crystal  faces  appear,  which  is  frequently 
the  case  with  the  minerals  that  are  the  first  to  crystallise  after  the 
phenocrysts,  the  structure  is  called  hypidiomorphic. 

In  most  holocrystalline  rocks,  owing  to  crowding  and  mutual 
interference,  the  minerals  do  not  often  assume  geometrical  forms, 
and  are  then  said  to  be  anhedral*  or  allotriomorphic,  their  shape 
being  determined  by  their  surroundings. 

1  Lat.  vitrum= glass. 

2  Gr.  micros  =  small,  and  lithos=a,  stone. 

3  Gr.  idios= distinct,  and  morphe= shape. 

4  Gr.  a  =  without,  and  hedron  =  i&ce. 


[To  face  page  240. 


FIG.  132. — Showing  spherulites  in  a  rhyolite. 


FIG.  133. — Rhyolite  showing  flow-structure. 


[To  face  page  240. 


FIG.  134. — Showing  perlitie  structure. 


FIG.  135. — Showing  porphyritic  structure.     (After  Rumbold.) 

F,  Phenocryst  of  felspar  altered  to  muscovite. 

B,  Biotite  altered  to  cassiterite 

C,  Cassiterite.  Q,  Quartz. 


IGNEOUS   BOCKS.  241 

One  mineral  may  penetrate  another,  and  intergrowths  of  minerals 
are  common,  arising  from  simultaneous  crystallisation. 

Composition. — A  rock  magma  may  be  denned  as  a  solution  of 
silicate-minerals ;  that  is,  of  silica  or  silicic  acid,  Si02,  combined 
with  various  oxides  of  the  metals  called  bases.  The  bases  with 
which  silica  combines  most  abundantly  are  : — 

(1)  Alumina     =  aluminium  sesquioxide  =  A1203. 

(2)  Iron  oxide  =  iron  protoxide  =  FeO. 

(3)  Lime  =  calcium  oxide  =CaO. 

(4)  Magnesia    =  magnesium  oxide  =MgO. 

(5)  Soda  =  sodium  oxide  =Na20. 

(6)  Potash        —potassium  oxide  =K20. 

The  great  mass  of  igneous  rocks  is  made  up  of  a  few  minerals 
which  are  vastly  more  abundant  than  all  the  others  put  together. 
These  few  are  quartz,  the  felspar  minerals,  the  ferro-magnesian 
minerals,  and  the  iron  oxides,  as  under  : — 

Quartz,     .         .         .         .As  free  silica. 

w  7  f  Orthoclase,  Monoclinic. 

****  \Plagioclase,  Triclinic. 

Felspathoids,    .  .         Leucite    and    nepheline,   alkali 

minerals. 
F  err  o-Magnesian  Minerals,       Pyroxenes,     amphiboles,      and 

micas. 
Iron  Oxides,     .         .         .'       Magnetite  (Fe304)  and  haematite 

(Fe203).     These  occur  as  free 

or  uncombined  bases. 

The  leading  pyroxene  minerals  are  augite,  diallage,  hypersthene, 
and  enstatite. 

The  principal  member  of  the  amphibole  group  is  hornblende. 

The  most  abundant  micas  are  biotite  (a  black  mica),  and  mus- 
covite,  a  clear  transparent  variety. 

The  felspars  are  silicate  compounds  in  which  iron  is  absent. 
The  ferro-magnesian  minerals  are  silicate  compounds  in  which  iron 
plays  an  important  part. 

Chemical  analysis  shows  the  respective  amounts  of  silica  and 
bases  present,  and  enables  the  following  convenient  grouping  of 
igneous  rocks  to  be  made  on  a  silica  basis  : — 

I.  Acid  Group,  with  65-80  per  cent,  of  silica  ;  and  S.G.  below 

2* 75.     Typical  rocks — Granites,  rhyolites,  felsites. 
II.  Intermediate   Group,  with  55-70  per  cent,  of  silica  ;    and 
S.G.   between   2-70  and  2-80.     Typical   rocks— Diorites, 
andesites. 

16 


242  A  TEXT-BOOK  OF  GEOLOGY. 

III.  Basic   Group,  with   45-60   per   cent,   of   silica  ;    and   S.G. 

between  2-80  and  3'00.     Typical  rocks — Gabbros,  doler- 
ites,  basalts. 

IV.  Ultra-basic  Group,  with  35-50  per  cent,  of  silica  ;  and  S.G. 

between  2-85  and  3-40.     Typical  rocks — The  peridotites. 

In  each  group  the  balance  is  represented  by  bases. 

In  the  Acid  rocks  we  have  65  per  cent,  of  silica  and  35  per  cent, 
of  bases.  Thus  it  happens  that  there  is  more  silica  present  than 
what  is  required  to  combine  with  the  bases.  The  silica  remaining 
uncombined  after  satisfying  the  bases  is  termed  free  silica,  which 
appears  in  the  rock  in  the  form  of  quartz.  Hence  we  find  that  all 
acid  rocks  contain  a  proportion  of  free  silica  or  quartz. 

In  the  Intermediate  rocks  the  silica  present  about  balances  the 
bases ;  hence,  in  these,  free  silica  is  not  often  present ;  or,  if  present, 
it  is  generally  in  small  amount. 

In  the  Basic  and  Ultra-basic  rocks,  the  bases  predominate  and 
free  silica  is  absent. 

Alteration  of  Igneous  Rocks. — This  is  mainly  effected  by  moist 
air,  rain,  or  underground  water  containing  carbonic  acid  and  oxygen, 
or  by  thermal  waters  and  acid  vapours. 

The  crystalline  minerals  are  hydrated,  broken  up,  or  replaced 
with  other  compounds.  Among  the  first  to  break  up  are  the 
felspars,  which,  are  converted  into  kaolin  and  mica.  The  felspars, 
when  kaolinised,  become  cloudy,  milky,  and  opaque. 

Augite  and  hornblende  become  converted  into  calcite,  chlorite, 
and  magnetite ;  and  olivine  into  serpentine. 

The  hydration  and  alteration  of  the  felspars  also  gives  rise  to  a 
very  characteristic  family  of  secondary  crystalline  minerals  called 
Zeolites,  which  are  mainly  hydrous  silicates  of  alumina  and  alkalies. 
The  zeolites  occur  mostly  encrusting  or  coating  the  walls  of  cavities, 
cracks,  and  veins  in  basalt,  andesite,  phonolite,  and  other  volcanic 
rocks,  and  occasionally  in  granite  and  gneiss. 

The  more  common  zeolites  are  natrolite,  analcime,  stilbite,  and 
prehnite. 

The  minerals  that  separate  out  of  the  molten  magma  are  called 
primary,  and  those  that  result  from  the  alteration  or  decomposition 
of  the  consolidated  rock,  secondary, 

Secondary  minerals  are  also  introduced  into  a  rock  by  infiltration. 
For  example,  the  amygdaloids  that  fill  the  bubble-cavities  in  a 
vesicular  lava  are  secondary. 

Petrographical  Provinces. — A  petrographical  province  is  part  of  a 
larger  region  in  which  the  rocks  exhibit  certain  points  of  resemblance 
due  to  some  real  genetic  relationship  or  community  of  origin.  It  is 
easily  conceivable  that  the  lavas  extruded  from  a  common  reservoir 


IGNEOUS    ROCKS.  243 

should  be  related,  even  though  the  points  of  emission  might  be  far 
apart.  The  sympathetic  action  of  Etna  and  Vesuvius  would  tend 
to  strengthen  the  belief  in  the  community  of  origin  of  magmas. 

Magmatic  Differentiation. — At  one  time  it  was  believed  that 
lavas  were  extruded  in  an  orderly  succession,  beginning  with 
acid  rocks  and  ending  with  basic.  Acid  lavas  were  thought  to 
characterise  the  earlier  outbursts  ;  intermediate  lavas  the  maturer 
periods  of  activity  ;  and  basic  lavas  the  expiring  or  waning  phases. 

All  the  lavas  were  believed  to  originate  in  a  common  stock-magma, 
and  the  different  rock-types  were  supposed  to  be  due  to  a  process 
of  magmatic  differentiation.  The  idea  probably  originated  in 
the  supposition  that  the  molten  magma  arranged  itself  in  horizontal 
layers  or  zones  of  different  density,  the  lighter  acid  glass  forming 
the  top  layer  and  the  basic  the  lower,  like  the  charge  from  a  blast-, 
furnace,  which  separates  itself  into  slag  and  regulus  when  poured 
into  a  mould. 

Later  investigation  has  shown  that  while  this  orderly  succession 
of  acid,  intermediate,  and  basic  lavas  characterises  many  regions, 
the  exceptions  are  so  numerous  that  magmatic  differentiation 
cannot  be  regarded  as  a  law  of  effusion  of  general  application. 

The  Atlantic  and  Pacific  Types  of  Igneous  Rocks. — The  genesis  of 
igneous  magmas  is  still  so  obscure  that  all  attempts  to  formulate 
a  satisfactory  genetic  classification  have  ended  in  comparative 
failure,  and  the  solution  of  the  difficulty  seems  no  nearer  now 
than  half  a  century  ago. 

Among  the  attempts  in  this  direction,  the  Cretaceous  and  Tertiary 
igneous  rocks  have  been  divided  into  two  main  groups  representing 
two  great  petrographical  provinces,  namely,  alkalic  and  calcic,  the 
former  characterising  the  Atlantic  type  of  coast-line,  the  latter 
the  Pacific  type. 

Suess  has  advanced  the  postulate  that  alkalic  rocks  are  typically 
associated  with  subsidence  due  to  radial  contraction  of  the  globe  ; 
and  the  calcic  with  erogenic  x  folding  arising  from  lateral  com- 
pression. The  volcanic  islands  scattered  throughout  the  Atlantic 
are,  he  conceives,  merely  remnants  of  a  once  extensive  tract  of 
alkalic  rocks  now  occupied  by  the  Atlantic  depression.  On  the 
other  hand,  the  Pacific  is  fringed  with  a  remarkable  ring  of  andesitic 
rocks  of  the  calcic  type,  and  this  encircling  ring  would  seem  to  be 
connected  with  the  uplift  of  the  great  crustal  folds  that  girdle  the 
Pacific  coast-line. 

The  alkalic  and  calcic  grouping  has  been  extended  so  as  to 
embrace  the  older  plutonic  rocks,  the  Atlantic  group  including  the 
alkali-granites,  nepheline-syenites,  etc. ;  and  the  Pacific  group  the 
granites,  quartz-diorites,  gabbros,  and  norites. 

1  Gr.  oros=a,  mountain,  and  genesis  —  origin  or  creation. 


244  A  TEXT-BOOK  OF  GEOLOGY. 

In  the  main,  the  alkalic  or  Atlantic  type  of  rocks  occurs  towards 
the  Atlantic  sea-board  in  both  the  American  and  Eurasian  con- 
tinents;  and  the  calcic  or  Pacific  towards  the  Pacific  sea-board. 
Singularly  enough,  in  Patagonia,  where  the  Atlantic  and  Pacific 
come  near  one  another,  the  types  are  curiously  interwoven.  In 
New  Zealand,  where  we  should  look  for  the  Pacific  type  alone,  there 
is  a  curious  commingling  of  the  calcic  and  alkalic  types  in  the  small 
petrographical  province  of  Otago  Peninsula,  on  the  east  coast  of 
the  South  Island  ;  but  in  the  greater  petrographical  provinces  of 
Taranaki,  Taupo,  and  Hauraki,  in  the  North  Island,  the  rocks 
belong  to  the  Pacific  group. 

It  is  noteworthy  that  in  the  South  Pacific  Ocean,  which  is  thickly 
dotted  with  groups  of  small  islands  in  a  way  that  would  suggest 
a  continental  subsidence  of  the  Atlantic  type,  the  Hawaii,  Tahiti. 
Cook;  and  other  islands  are  largely  composed  of  alkalic  volcanic 
rocks. 

Dr  Flett  has  suggested  a  third  group,  the  spilitic  suite,  consisting 
of  the  pillow-lavas  and  their  associates  occurring  in  the  Dalradian 
schists  of  Scotland  and  in  the  Ordovician  of  the  Southern  Uplands 
of  that  country. 

The  alkali  and  the  calcic  types  are  chemico-dynamic  rather 
than  genetic.  They  are  dominant  elements  in  tectonics,  and 
related  to  the  development  of  certain  crustal  features,  the  origin 
of  which  does  not,  however,  offer  a  satisfactory  explanation  of  the 
genesis  of  the  eruptive  magmas.  Moreover,  they  are  conceived  on 
too  broad  a  basis  to  be  adaptable  for  the  requirements  of  a  general 
classification  of  igneous  rocks. 

Classification  of  Igneous  Rocks. — After  nearly  a  century  of 
investigation,  no  classification  has  been  formulated  that  is  generally 
accepted  as  a  recognised  standard,  or  that  is  satisfactory  alike  to 
the  field-geologist  and  the  laboratory  student.  The  first  attempts 
at  classification  were  based  on  megascopic  character — that  is,  on 
outward  appearance  of  the  rock,  as  seen  in  the  field  and  in  hand 
specimens.  About  the  middle  of  the  nineteenth  century  there 
came  a  better  knowledge  of  the  mineralogical  characters  of  rocks, 
which  soon  led  to  a  recognition  of  the  prominent  part  played  by  the 
felspars.  Since  then  the  felspars  have  been  an  important  factor 
in  all  modern  classifications  of  igneous  rocks. 

The  first  great  advance  in  petrography  was  the  introduction  of 
microscopical  methods  of  investigation,  the  value  of  which  was  fully 
recognised  before  the  advent  of  the  seventies.  The  importance 
attached  to  this  new  branch  of  investigation  was  so  overwhelming 
that  for  several  decades  there  was  a  tendency  among  writers  to 
place  all  other  rock-characteristics  in  the  background,  and  at  one 
time  there  was  a  fear  among  field-geologists  that  the  study  of  rocks 


IGNEOUS    ROCKS. 


245 


would  be  altogether  relegated  to  the  laboratory.  However,  a 
reaction  set  in,  and  at  the  present  time  the  importance  of  geological 
relationships,  mode  of  occurrence,  and  formation  are  now  recognised 
as  no  less  important  than  mineralogical  composition. 

The  classification  of  Rosenbusch,  now  widely  adopted,  divides 
igneous  rocks  into  :• — 

(1)  Deep-seated  or  abyssal. 

(2)  Dyke  rocks  or  hypabyssal. 

(3)  Volcanic  rocks  or  effusive. 

This  grouping  is  intended  to  express  a  relationship  between  mode 
of  occurrence  and  texture.  Genetically  all  three  types  of  rock 
may  originate  from  the  same  magma. 

Zirkel  bases  his  classification  of  igneous  rocks  on  (a)  mineral 
character,  (b)  texture,  and  (c)  age. 

English  writers  have  always  strongly  combated  the  age  dis- 
tinction of  igneous  rocks,  it  being  held  that  igneous  magmas  of 
similar  composition  are  alike  when  solidified  in  similar  conditions, 
regardless  of  the  epoch  in  which  they  were  erupted. 

The  mode  of  occurrence  of  igneous  rocks  is  dependent  on  local 
or  regional  crustal  weakness,  as  indicated  by  the  presence  of  frac- 
tures and  fissures,  and  on  certain  dynamic  conditions,  the  origin 
of  which  is  still  obscure. 

The  texture  is  merely  an  expression  of  the  mode  of  occurrence. 

The  composition  is  independent  of  both  mode  of  occurrence  and 
its  concomitant  texture.  For  example,  the  same  magma,  according 
to  some  accident  of  local  crustal  weakness,  may  occur  as  a  boss,  a 
dyke,  an  intrusive  sill,  or  an  effusive  lava — four  well-marked  mor- 
phological types  that  may  differ  in  texture,  but  not  in  composition. 

The  following  arrangement  of  igneous  rocks  is  based  on  texture 
and  mineralogical  character,  and  is  useful  as  showing  the  succession 
of  related  rock-families  as  we  pass  from  the  acid  to  the  basic. 


TEXTURE. 

ACID.              INTERMEDIATE. 

BASIC. 

ORTHOCLASE. 

PLAGIOCLASE. 

Quartz. 

Hornblende  or  Augite. 

Augite  or 
Olivine. 

Glassy 

Obsidian 

Trachyte 

glass 

Andesite 
glass 

Tachylyte 

Partly  crystalline 

Rhyolite 

Trachyte 

Andesite 

Basalt 

Holoorystalline   . 

Granite 

Syenite 

Diorite 

Dolerite 

246  A  TEXT-BOOK  OF  GEOLOGY. 

An  acid  magma  of  uniform  composition  may  give  us  a  granite, 
rhyolite,  or  obsidian,  according  to  the  position  in  which  it  cooled 
and  consolidated  ;  and  a  basic  magma,  a  dolerite,  basalt,  or  tachy- 
lyte.  The  acid  rocks  differ  in  mineralogical  character  and  texture, 
but  not  in  chemical  composition ;  this  is  also  true  of  the  basic  and 
intermediate  rocks. 

The  holocrystalline  types  of  rock  occur  mainly  as  plutonic 
bosses,  dykes,  and  laccoliths ;  the  partly-crystalline  principally  as 
lava  streams  and  sills  ;  and  the  glassy  generally  as  effusive  lavas. 

SUMMARY. 

(1)  The  points  that  should  be  specially  emphasised  in  connection 
with  igneous  rocks  are  : — 

(a)  Mode  of  occurrence. 

(b)  Texture  or  grain. 

(c)  Composition. 

(2)  The  same  uprising  molten  magma,  according  to  the  situation 
in  which  it  cools  and  consolidates,  may  form  : — 

(a)  An  intrusive  boss  that   penetrates   the  crust,  crushing,  dis- 

rupting, displacing,  and  perhaps  even  dissolving  the  rocks 
with  which  it  comes  in  contact,  but  in  no  case  reaching 
the  outer  surface. 

(b)  Vein-like  sheets,  called   dykes,  that  fill   fissures   and  cracks 

in  the  rocky  crust. 

(c)  Intrusive  sills  or  sheets  that  have  forced  their  way  along  the 

bedding  planes  of  stratified  rocks. 

A  sill  that  has  swelled  out  to  a  dome-shaped  mass 
is  called  a  laccolith.  A  large  laccolith  that  is  only  partially 
uncovered  is  sometimes  difficult  to  distinguish  from  an 
intrusive  boss. 

(d)  An    effusive   stream   of   lava   that   issued   from   a   vent   or 

fissure. 

(3)  A  molten  magma  is  merely  a  natural  slag  or  glass.     When  it 
cools  rapidly  it  retains  the   glassy   structure,  but   when  it   cools 
very    slowly    it    develops    a    completely    crystalline    structure. 
Between  the  glassy  and  holocrystalline  forms,  we  get  an  endless 
variety   of    rock-textures,   varying   according    to    the    rate    and 
conditions  of  cooling. 

(4)  A  molten  magma  may  be  regarded  as  a  solution  of  rock- 
silicates,  and  as  it  cools,  the  silicates  separate  out  in  crystalline 
forms.    The    principal    constituents    of    igneous     rocks    are     as 
under  : — 


IGNEOUS    ROCKS.  247 

(a)  Quartz,  occurring  free  or  uncombined. 

(b)  Felspars — Orthoclase  and  Plagioclase. 

(c)  Felspathoids — Leucite  and  Nepheline,  both  alkali  minerals. 

(d)  Ferro-magnesian    minerals,   a   group  of    great    importance, 

including  the  augites,  hornblendes,  micas,  etc. 

(e)  Iron  oxides,  mostly  magnetite  and  haematite. 

(5)  The  alteration  of  igneous  rocks  is  mainly  effected  by  water 
containing  C02  and  0,  and  by  thermal  waters  and  various  gases. 

The  products  of  the  alteration  of  primary  minerals  are  called 
secondary  minerals. 

(6)  Petrographical    Provinces    are    regions  in  which  the  rocks 
exhibit  certain  points  of  resemblance. 

(7)  Magmatic  Differentiation  refers  to  the   succession   of   acid, 
intermediate,    and   basic   lavas   that  was   at   one   time   believed 
to    represent  the  normal   sequence   of  magmatic  effusions  in   a 
given  volcanic  region. 

(8)  The  Atlantic  or  Alkalic  and  the  Pacific  or  Calcic  types  of 
igneous   rock  represent  two  great  petrographical  provinces,  the 
former  characterising  the   Atlantic   type  of  coast-line,  the  latter 
the  Pacific  type.      These  two  groups  of  rock-types  are  believed 
to  be  related  to  the  genesis  of  certain  crustal  features,  the  Atlantic 
group  with  depression,  and  the  Pacific  with  orogenic  folding. 

Where  the  Atlantic  and  Pacific  meet  south  of  Patagonia,  there 
is  a  singular  commingling  of  the  alkalic  and  calcic  types  of  rock. 

(9)  No  satisfactory  basis  has,  so  far,  been  discovered  for  a  genetic 
classification  of  igneous  rocks.      Most  modern  classifications  are 
based  on  mode  of  occurrence)  texture,  and  composition. 

Based  on  mode  of  occurrence  alone,  we  get 

(a)  Deep-seated  or  abyssal. 

(b)  Dyke  rocks  or  hypabyssal. 

(c)  Volcanic  rocks  or  effusive. 

The  Deep-seated  or  plutonic  rocks  are  holocrystalline,  the  dyke 
rocks  mainly  holocrystalline,  and  the  volcanic  rocks  partly 
crystalline  and  glassy. 

According  to  their  composition,  igneous  rocks  are  divided  into 
four  main  groups  as  follows  : — 

(a)  Acid.  (c)  Basic. 

(b)  Intermediate.  (d)  Ultra-basic. 

It  must,  however,  be  remembered  that  the  study  of  rocks  is 
more  important  than  the  formulating  of  a  classification,  the  latter 
being  of  value  only  so  far  as  it  assists  us  in  the  systematic  investi- 
gation of  rock-masses. 


CHAPTER   XVI. 
PLUTONIC,   HYPABYSSAL,   AND   VOLCANIC   ROCKS. 

A. — Plutonic  Rocks. 

THE  rocks  of  this  type  generally  occur  in  bosses  or  boss-like  pro- 
trusions that  have  evidently  cooled  at  a  considerable  depth  below 
the  surface.  Their  presence  is  in  every  case  revealed  by  the 
denudation  of  the  overlying  rocks  ;  and  their  actual  extent  is 
never  known.  They  are  holocrystalline,  and  in  general  the  crystals 
are  imperfectly  formed  owing  to  mutual  interference.  The 
presence  of  water  and  gases  in  the  crystals  would  tend  to  show  that 
the  process  of  crystallisation  took  place  under  great  pressure. 

The  distinctive  feature  of  these  abyssal  rocks  is  their  coarsely 
crystalline  texture. 

Sequence  of  Crystallisation. — In  plutonic  rocks  which  from  their 
deep-seated  character  have  necessarily  cooled  very  slowly,  there 
has  been  recognised  a  normal  order  of  separation  for  the  crystalline 
constituents  which,  although  not  invariable,  is  sufficiently  general 
to  warrant  the  broad  generalisation  of  Rosenbusch  that  the  order 
of  crystallisation  follows  a  law  of  decreasing  basicity,  as  follows  : — 

I.  (a)  Minor  accessory  minerals  =  Apatite,  zircon,  sphene, 
garnet;  (b)  Iron  ores  =  Magnetite,  haematite,  pyrite. 

II.  Ferro-magnesian  minerals  =  Olivine,  hypersthene,  enstatite, 
augite,  hornblende,  biotite,  muscovite. 

III.  Felspar  minerals  =  (a)  Plagioclase  ;  (6)  Orthoclase. 

IV.  (a)  Quartz;  (b}  Microcline. 

It  will  be  observed  that  the  basic  accessory  minerals  and  iron- 
ores  appeared  before  the  ferro-magnesian  minerals  which  play 
so  conspicuous  a  part  in  plutonic  rocks.  After  the  complex 
silicates  follow  the  great  group  of  felspar  minerals  which  are 
devoid  of  iron ;  and  after  these  come  quartz  and,  finally,  microcline. 

Varieties  of  Structure. — The  principal  textures  met  with  in 
plutonic  rocks  are  as  under  : — 

(a]  Granitoid,  i.e.  like  that  of  an  ordinary  granite. 

(b)  Granulitic,  i.e.  consisting  of  small  grains  of  even  size,  impart- 

ing a  granular  appearance. 

248 


PLUTONIC,    HYPABYSSAL,    AND    VOLCANIC   ROCKS.       249 

(c)  Pegmatitic,  i.e.  strikingly  coarse-grained,  as  found  in  typical 

pegmatite. 

(d)  Porphyritic,  i.e.  where  conspicuous  phenocrysts  occur  in  a 

ground-mass  of  small  crystals. 

(e)  Graphic,1  i.e.  a  structure  depending  on  the  intergrowth  of 

two  essential  minerals,  and  so  named  from  its  resemblance 
to  Hebraic  writing.  It  commonly  arises  from  the  in- 
terpenetration  of  felspar  by  quartz.  This  structure  is 
usually  on  a  microscopic  scale,  and  hence  is  often  called 
micrographic. 

(/)  Gneissic,  i.e.  when  the  constituent  minerals  exhibit  a  tendency 
to  aggregation  in  parallel  bands,  arising  partly  from  the 
flowing  movement  of  the  parent  magma,  and  partly  from 
subsequent  rearrangement  due  to  pressure  and  heat. 


Principal  Plutonic  Rock  Types. 

1.  Granites    1    A  ., 

2.  Syenites    / 
pl             i    3.  Diorites    ^ 

4.  Gabbros     V  Intermediate. 

5.  Norites     J 

6.  Peridotites — Basic. 

ACID  PLUTONICS. 

Granites. — These  consist  of  rocks  in  boss-like  masses  from  which 
veins  or  dykes  may  extend  into  the  adjacent  rocks. 

The  colour  of  granite  is  mainly  dependent  on  the  hue  of  the 
felspar,  and  is  usually  light  or  dark  grey,  pink,  or  reddish.  The 
felspar  usually  shows  cleavage-planes  or  crystal  faces,  and  is 
thereby  easily  distinguished  from  the  quartz,  which  possesses  no 
cleavage,  and  usually  occurs  in  irregular  grains.  The  mica  occurs 
in  thin  flexible  scales.  Orthoclase  is  the  essential  felspar,  but  a 
little  accessory  plagioclase  is  nearly  always  present. 

The  leading  types  of  granite  are  : — 

(a)  Granite  =  quartz,  orthoclase,  and  two  micas ;  one  muscovite 

mica,  the  other  biotite. 

(b)  Biotite-granite  =  a  granite  in  which  the  grey  mica  is  replaced 

by  the  brown  mica,  biotite. 

(c)  Hornblende-granite  =  a  granite  in  which  the  mica  is  wholly 

or  partly  replaced  by  hornblende. 

1  Or.  Orapho  =  l  write. 


250 


A    TEXT-BOOK    OF    GEOLOGY. 


(d)  Pegmatite  =  a     strikingly     coarse-textured    granite    which 

commonly  occurs  in  veins  in  ordinary  granite,  or  on  the 
borders  of  granite  bosses. 

(e)  Greisen=&  granite  with  little  or  no  felspar;  occurs  in  veins 

in  ordinary  granite. 


o 


rw 


FIG.  136. — Photomicrograph  of  granite,  near  Dublin,      x  12. 
(After  Grenville  A.  J.  Cole.) 

(b)  Brown  mica  (biotite). 

(ra)  Muscovite,  a  hexagonal  section  of  this  mineral  occurs 

near  top  of  section, 
(o)  Orthoclase.  (q)  Quartz. 

A  normal  granite  containing  tourmaline  may  be  called  a  tourma- 
line-granite. 


FIG.  137.— Showing  Bear  Butte  Laccolith,  Black  Hill,  S.  Dakota, 
U.S.  Geol.  Surv.     (After  Jagger.) 

When  a  granite  contains  conspicuous  phenocrysts  of  felspar,  it 
is  said  to  be  porphyritic,  and  in  this  case  the  rock  may  be  called  a 
granite- porphyry. 


PLUTONIC,    HYPAB^SSAL,    AND    VOLCANIC    ROCKS.       251 

Many  granitic  rocks  contain  irregularly  rounded  or  ovoid  patches 
of  a  darker  colour  and  finer  grain  than  the  enclosing  rock.  These 
patches  are  called  basic  secretions.  They  contain  the  same  minerals 
as  the  parent  rock,  and  are  supposed  to  be  the  earlier  products  of 
crystallisation. 

Syenites. — These  are  coarse  to  fairly  fine-grained  holocrystalline 
rocks  with  a  granitoid  structure.  In  a  general  way  they  may  be 
defined  as  granites  without  free  quartz.  They  occur  in  the  form 
of  bosses  and  dykes,  but  are  much  less  common  than  granite.  The 
leading  types  are  as  follow  : — 


FIG.  138. — Showing  syenite  from  Plauenscher 
Grund,  Dresden.      x8. 

(h)  Green  hornblende.  (q)  Accessory  interstitial  quartz. 

(o)  Orthoclase  fairly  prismatic  in  habit.        (sp)  Sphene. 

(a)  Hornblende-syenite  =  Syenite  proper,  composed  essentially  of 

orthoclase  and  hornblende. 

(b)  Mica-syenite,  in  which  biotite  more  or  less  replaces  the  horn- 

blende. 

(c)  Augite- syenite,  an  ordinary  syenite  in  which  augite  is  present. 

(d)  Alkali-syenites.     These   are   syenites   distinguished   by  the 

presence  of  nepheline  or  sodalite. 

Plagioclase  is  present  as  an  accessory  constituent  in  all  syenites. 
When  it  becomes  a  prominent  associate  of  the  orthoclase,  we  get 
types  that  show  a  relationship  to  the  diorites.  To  this  intermediate 
type  the  name  monzonite  has  been  applied. 

Syenites  containing  quartz  form  a  connecting  link  with  the 
hornblende  granites. 


252  A  TEXT-BOOK  OF  GEOLOGY. 

INTERMEDIATE   PLUTONICS. 

Diorites. — These  are  holocrystalline  rocks  of  fine  or  fairly  coarse 
texture.  They  consist  essentially  of  plagioclase  and  hornblende. 
Free  quartz  is  frequently  present  in  them,  but  the  influence  of  the 
basic  felspar,  oligoclase,  or  sometimes  labradorite  keeps  the  rock 
in  the  intermediate  group. 

The  diorites  generally  occur  as  dykes,  and  they  are  found  in  all 
parts  of  the  globe.  The  leading  types  are  as  under  : — 

(a)  Diorite l' = plagioclase  +  hornblende. 
(6)  Mica-diorite  =  plagioclase  +  hornblende  +biotite. 
(c)    Quartz-mica-diorite  =  plagioclase  +  hornblende  +biotite  + 
quartz. 

Gabbro  Type. — These  occur  as  dykes  and  large  boss-like  intrusions. 
They  show  a  close  relationship  to  the  last  group,  and  may  be  called 
pyroxene-diorites.  They  consist  essentially  of  a  plagioclase  felspai 
(usually  labradorite)  and  a  pyroxene.  When  the  pyroxene  is 
augite  or  diallage,  we  get  gabbro  proper,  and  when  hypersthene, 
norite. 

Almost  all  the  rocks  of  this  group  contain  olivine,  and  in  the 
more  basic  varieties  it  becomes  an  essential  constituent. 

Gabbro  =  plagioclase  +  augite  or  diallage. 

Norite  =  plagioclase  +  hypersthene. 

When  quartz  is  present,  the  rock  becomes  a  quartz-gabbro  or 
quartz-norite. 

Many  of  the  basic  gabbros  are  rich  in  magnetite  and  ilmenite,  and 
some  pass  into  iron-ore  rocks,  as  in  Minnesota. 

BASIC   PLUTONICS. 

Peridotites. — These  occur  mostly  as  dykes  in  other  basic  rocks. 
They  are  holocrystalline  in  texture,  and  basic  or  ultra-basic  in 
composition.  They  consist  essentially  of  olivine,  which  may  con- 
stitute 50  per  cent,  or  more  of  the  rock,  while  felspar  is  typically 
absent.  Enstatite  or  bronzite  is  nearly  always  present,  and,  in 
some  types,  augite  or  hornblende.  The  leading  types  are  : — 

(a)  Enstatite-peridotite  =  olivine  +  enstatite. 

(b)  Augite- per  idotite  =  olivine  +  augite. 

(c)  Hornblende-peridotite  =  olivine  +hornblende. 

An  oli vine-rock  containing  a  little  chromite  and  magnetite  forms 
mountain  masses  of  great  extent  in  New  Zealand.  It  has  received 
the  distinctive  name  Dunite,  so  called  after  Dun  Mt.,  where  it  is 
typically  developed. 

1  Gr.  diorizo  =  I  distinguish. 


To  face  page  252. 


[PLATE    XVIII. 


A.  DIORITE,  YELLOWSTONE  NATIONAL  PARK.     (U.S.  Geol.  Survey.) 


B.    DlORITE-PORPHYRY    WITH    PHENOCRYSTS    OF    PLAGIOCLASE,    YELLOWSTONE 

NATIONAL  PARK.     (After  Iddings,  U.S.  Geol.  Survey.) 


[To  face  page  252. 


FIG.  139. — Micro-drawing  of  diorite-porphyry, 
U.S.  Geol.  Surv.     (After  Pirsson.) 

P,  Phenocryst  of  plagioclase  with  well-developed  albite- 

twinning  and  distinct  pericline-twinning. 
O,  Orthoclase.  H,  Hornblende.  M,  Magnetite. 


FIG.  140. — Showing  gabbro  from  Yellowstone  National  Park. 
(After  Iddings.) 


PLUTONIC,    HYPABYSSAL,    AND    VOLCANIC    ROCKS.       253 

The  olivine  of  the  peridotites  and  dunite  alters  with  great  readi- 
ness into  the  hydrous  form,  serpentine. 

B.— Hypabyssal  Rocks. 

The  igneous  rocks  included  in  this  group  as  a  rule  occur  in  the 
form  of  dykes,  and  are  probably  protrusions  from  deep-seated 
bosses. 

Most  of  them  are  holocrystalline,  and  this  texture  is  so  general 
as  to  be  characteristic.  In  some,  however,  there  is  a  glassy  residue. 


FIG.  141.  —  Micro-drawing  of  peridotite,  deeply  serpentinised.      x70. 
(After  Hartog.) 

0,  Olivine.  S,  Serpentine  apparently  massive. 

A,  Antigorite,  scale  serpentine.          G,  Garnet.     Solid  black,  magnetite. 

Dyke  rocks,  like  the  plutonics,  are  typically  compact,  but  a  few 
are  known  that  possess  a  vesicular  structure  like  a  volcanic  lava. 

In  some  of  the  families  of  this  group,  the  porphyritic  structure 
is  so  constant  as  to  be  characteristic. 

I.       Quartz-porphyry  —  Acid. 


m    /  Lamprophyres  \    fi    . 
U'   \Dolerite  / 

ACID   GROUP. 

Quartz-Porphyries.  —  These   are   also   known   as  felsites,  quartz- 
fclsites,   or   elvans.      They   abound   as   dykes    and    veins   in    the 


254  A  TEXT-BOOK  OF  GEOLOGY. 

neighbourhood  of  granite  bosses,  with  which  they  are  doubtless 
genetically  related.  Their  colour  varies  from  white  to  buff. 

In  the  rocks  of  this  family  the  ground-mass  consists  of  a  micro- 
crystalline  or  crystalline  aggregate  of  quartz  and  felspar,  in  which 
are  embedded  crystals  of  quartz  and  orthoclase.  and  frequently 
biotite  or  some  other  ferro-magnesian  mineral.  Mineralogically 
and  chemically  many  of  the  varieties  of  this  family  are  merely  fine- 
grained granites  or  microgranites,  as  they  are  sometimes  called. 

When  the  ground-mass  is  so  fine  that  it  is  difficult  to  recognise 
the  various  constituents  even  under  high  powers,  it  is  called 
felsitic. 

The  fine-grained  and  compact  forms  of  granite  frequently  found 
traversing  ordinary  granites  as  narrow  dykes,  are  called  quartz- 
felsite  in  England,  microgranulite  or  microgranite  in  Continental 
Europe,  and  are  covered  by  the  name  eurite. 

The  fine-grained  granites  that  consist  only  of  quartz  and  ortho- 
clase in  a  felsitic  ground-mass,  are  sometimes  called  aplite. 

Many  of  the  so-called  felsites  and  quartz-felsites  are  ancient 
rhyolites  that  have  undergone  some  secondary  changes. 

INTERMEDIATE   GROUP. 

These  are  holocrystalline  dyke  rocks  with  a  porphyritic  structure, 
due  in  most  cases  to  the  presence  of  felspar  phenocrysts.  They 
are  divided  into  two  families,  the  porphyries  and  porphyrites  ; 1  the 
former  is  dominated  with  orthoclase,  the  latter  with  plagioclase. 
Thus,  while  the  porphyries  are  related  to  the  syenites,  the  porphy- 
rites approach  the  diorites. 

Porphyries. — The  leading  types  of  these  are  as  follow  : — 

(a)   Orthoclase-porphyry  =  orthoclase  +a  little  biotite,  hornblende, 

or  augite. 
(6)  Syenite-porphyry  =  ground-mass  of  quartz  and  felspar,  mostly 

orthoclase;  phenocrysts,  plagioclase  +  hornblende. 

Porphyrites. — The  leading  types  of  these  are  : — 

(a)  Hornblende-porphyrite  =  plagioclase  +hornblende  +  biotite. 

(b)  Mica-porphyrite  —plagioclase  +  biotite. 

BASIC  GROUP. 

Lamprophyres. — This  is  a  peculiar  family  of  basic  dykes  typically 
found  traversing  rocks  of  older  Palaeozoic  age.  They  are  fine- 
grained and  essentially  holocrystalline.  They  are  peculiarly  rich 
in  the  ferro-magnesian  minerals,  biotite,  hornblende,  or  augite. 

1  Gr.  porphura  =  purple,  and  litfios  =  a,  stone. 


[To  face  page  254. 


FIG.  142. — Micro-drawing  of  quartz-porphyry  from  Mariana  Mine. 
Tres  Cruces.     Ground-mass  felsitic.     (After  Rumbold.) 

O,  Orthoclase.        M,  Felspar  altered  to  muscovite. 
B,  Biotite.  Q,  Quartz. 


FIG.  144. — Micro-drawing  of  Negro  Pabellon  mica-andesite. 
(After  Rumbold.) 

Ground -mass,  Glass.     M,  Mica  altering  to  magnetite.     F,  Plagioclase  felspar. 


PLUTONIC,    HYPABYSSAL,    AND    VOLCANIC    ROCKS.       255 

The  felspars,  which  may  be  orthoclase  or  plagioclase,  occupy  a 
subordinate  place.  Olivine  is  absent  or  sparingly  represented.  In 
some  the  silica  is  as  low  as  40  per  cent. 

The  various  types  take  their  name  from  the  dominant  ferro- 
magnesian  mineral  Thus  we  have  : — 

(a)  Hornblende-lamprophyre,  with  dominant  plagioclase  =camp- 

tonite  type. 

(b)  Mica-lamprophyre. 

(c)  Augite-lamprophyre. 

In  the  monchique  type  of  lamprophyre,  the  characteristic  minerals 
are  olivine  4-augite,  or  sometimes  hornblende. 

Dolerites. — These  occur  as  laccoliths,  sills,  and  dykes.  They  are 
holocrystalline,  but  not  conspicuously  porphyritic.  The  leading 
types  are  as  under  : — 

(a)  Olivine-dolerite  =  plagioclase  +augite  +  olivine. 

(b)  Mica-dolerite  =  plagioclase  +  augite  +biotite. 

(c)  Hornblende-dolerite  =  plagioclase  +augite  +  hornblende. 

(d)  Enstatite-dolerite  =  plagioclase  +augite  +enstatite. 

The  more  ancient  dolerites  have  generally  undergone  considerable 
alteration  to  a  greenish-black  rock,  to  which  the  distinctive  name 
diabase  has  been  applied.  The  greenish  colour  is  due  to  the 
chloritic  decomposition  products.  The  principal  varieties  are  as 
follow  : — 

(a}  Diabase  proper  =  plagioclase,  mostly  labradorite  +augite. 

(6)  Olivine- diabase  =  plagioclase  +  augite  +  olivine. 

(c)  Quartz-diabase  -plagioclase  +augite  +  quartz. 

(d)  Hornblende-diabase  =  plagioclase  +  hornblende. 

Many  of  the  Palaeozoic  diabases  are  conspicuously  amygdaloidal, 
notable  examples  being  the  copper-bearing  diabases  of  Lake 
Superior,  and  the  great  sheet  of  diabase  overlying  the  gold-bearing 
banket  series  at  the  Eand. 

C. — Volcanic  Rocks. 

In  this  group  are  included  all  the  solid  effusive  lavas  as  well  as 
the  dykes  and  sills  that  are  directly  connected  with  lava  streams. 
They  range  from  the  glassy  to  the  holocrystalline  forms.  The 
glassy  structure  is  most  prevalent  among  the  acid  types  of  rock. 

Volcanic  rocks  are  frequently  vesicular,  and  usually  exhibit 
flow  phenomena  such  as  flow-lines,  parallel  orientation  of  crystals, 
elongation  of  bubble- vesicles,  and  banding.  The  majority  exhibit 
a  porphyritic  structure,  due  to  the  presence  of  two  generations  of 


256  A  TEXT-BOOK  OF  GEOLOGY. 

crystals.  The  large  felspars  separate  out  of  the  slowly  uprising 
glassy  magma,  and,  being  free  from  crowding  or  interference, 
generally  grow  to  a  large  size. 

At  a  later  stage  of  the  eruption,  probably  after  effusion  of  the 
magma  as  a  lava-stream,  the  separation  of  the  second  crop  of 
felspars  begins ;  and  the  rate  of  cooling  being  more  rapid  than 
before,  the  crystals  are  small  and  often  crowded. 

With  the  volcanic  rocks,  as  with  the  plutonic  and  hypabyssal, 


FIG.  143. — Showing  phenocrysts  of  plagioclase  in  glassy 
ground-mass,  U,8.  Geol.  Surv. 

we  have   the   threefold  division,  based  on  chemical  composition 
as  under : — 

I.       Rhyolites —     Acid. 

C  Trachytes    "| 

II.   <  Phonolites     >  Intermediate. 
^  Andesites    J 

.  III.       Basalts —        Basic. 

ACID  GROUP. 

Rhyolites. — These  include  all  the  more  acid  lavas  of  Recent  and 
Tertiary  date,  as  well  as  the  contemporaneous  lavas  and  dykes 
associated  with  the  Palaeozoic  and  Mesozoic  formations. 

The  ground-mass  may  be  wholly  or  partly  glass,  or  crypto- 
crystalline.  Fluxion  structure  is  generally  well  marked  by  alternat- 
ing bands  of  different  texture  and  colour,  or  by  alternating  glassy 
and  spherulitic  layers.  The  vitreous  or  glassy  form  is  found  in 


PLUTONIC,  HYPABYSSAL,  AND  VOLCANIC  ROCKS.     257 

obsidian,  which  is  a  natural  volcanic  glass  in  which  crystallites  or 
embryonic  crystals  are  frequently  developed. 

The  phenocrysts  of  rhyolites  are  orthoclase,  including  the  glassy 
form  saiiidine,  an  acid  plagioclase,  quartz,  biotite,  and  sometimes 
augite  or  hornblende. 

Tertiary  rhyolites  are  found  in  Antrim,  in  the  Lipari  group  of 
islands,  Nevada,  and  New  Zealand  ;  and  recent  rhyolites  in  New 
Zealand. 

INTERMEDIATE  GROUP. 

Trachytes. — These  are  in  some  respects  rhyolites  without  free 
quartz.  The  sanidine  form  of  orthoclase  is  the  chief  constituent 
of  the  ground-mass  and  also  the  dominant  phenocryst.  Horn- 
blende or  biotite  is  nearly  always  present  in  true  trachytes. 

Phonolites. — These  rocks  are  distinguished  by  the  presence  of 
nepheline  in  the  ground-mass.  Those  poor  in  that  alkali-mineral 
are  closely  related  to  the  trachytes,  and  are,  by  some  writers,  named 
trachytoid  phonolites.  The  varieties  in  which  nepheline  is  fairly 
abundant  are  sometimes  called  nephelinitoid  phonolites. 

Andesites.1 — In  most  typical  andesites  the  ground-mass  is  a  felted 
mass  of  felspar  laths,  and  a  residue  of  glassy  matter.  The  pheno- 
crysts are  plagioclase,  hornblende,  augite,  biotite,  or  hypersthene. 

The  different  varieties  of  andesite  are  named  from  the  dominant 
ferro-magnesian  mineral,  thus  : — 

(a)  Augite-andesite. 

(b)  Hypersthene-andesite. 

(c)  Hornblende-andesite. 

(d)  Quartz-andesite  or  dacite. 

(e)  Mica-andesite. 

Among  the  accessory  minerals,  magnetite,  ilmenite,  apatite,  and 
zircon  are  usually  present. 

The  andesites  that  have  been  altered  by  thermal  waters,  steam, 
or  gases  are  sometimes  called  propylite. 

The  andesites  of  United  States,  New  Zealand,  and  Transylvania 
are  of  great  importance  for  their  valuable  gold-  and  silver-bearing 
lodes. 

BASIC  GROUP. 

Basalts. — These  occur  as  lavas,  sills,  and  dykes.  The  essential 
minerals  are  a  plagioclase  felspar,  rich  in  lime,  augite,  and  olivine. 
They  exhibit  every  form  of  texture  from  the  glassy  to  the  holo- 
crystalline. 

In  the  glassy  form  we  have  tachylyte ;    and  in  those  basalts  in 

1  So  named  from  the  Andes  in  South  America. 

17 


258  A   TEXT-BOOK   OF 

which  the  phenocrysts  are  embedded  in  a  holocrystalline  ground- 
mass  we  get  rocks  that  are  essentially  dolerites. 
The  typical  rock  of  this  group  is  olivine-basalt. 

SUMMARY. 

1.  Plutonic  rocks  occur  in  boss-like  masses  that  have  been 
uncovered  by  denudation. 

2.  They  are  holocrystalline  in  structure,  and  evidently  cooled 
slowly  and  under  great  pressure. 

3.  The  order  of  crystallisation  in  plutonic  rocks  is  one  of  decreas- 
ing basicity  ;  that  is,  the  basic  minerals  separate  out  first,  and  the 
acid  last. 

4.  The  leading  types  of  the  acid  plutonics  are  Granite  and  Syenite  ; 
of  the  intermediate  plutonics,  Diorite,  Gabbro,  and  Norite  ;   and  of 
the  basic,  Peridotite. 

5.  Hypabyssal  igneous  rocks  occur  mostly  as  dykes,  and  their 
texture  is   generally  holocrystalline.     The  acid   types  are  closely 
related  to  the  granites,  and  the  intermediate  to  the  syenites  and 
diorites.     Some  of  the  intermediate  families  are  characteristically 
porphyritic. 

6.  The  volcanic  rocks  include  all  the  solid  lavas  and  the  dykes 
that  are  directly  connected  with  lava  streams.     They  range  from 
the  glassy  to  the  holocrystalline  forms  of  texture. 

The  rhyolites  are  typical  of  the  acid  group,  the  andesites  of  the 
intermediate,  and  the  basalts  of  the  basic. 


[To  face  page  258. 


FIG.  145. — Micro-drawing  of  biotite-dacite  from  Huanuni. 
(After  Rumbold.) 

Ground-mass,  Glass.         B,  Biotite.         P,  Plagioclase.         Q,  Quartz. 


Fro.  146. — Microphotograph  of  oli vine- basalt  from  North  Auckland,  N.Z. 
showing  phenocryst  of  plagioclase  and  ophitic  structure  of  ground-mass 
there  is  a  general  orientation  of  felspars. 


CHAPTER  XVII. 
METAMORPHISM  AND  METAMORPHIC   ROCKS. 

Metamorphism. 

IN  metamorphic l  rocks  the  original  constituents  of  the  rocks 
have  in  many  cases  formed  new  combinations  among  themselves. 
The  minerals  developed  by  this  process  of  alteration  invariably 
possess  a  crystalline  structure  ;  hence  metamorphic  rocks  are  fre- 
quently spoken  of  as  crystalline.  In  many  metamorphic  rocks, 
the  newly  developed  minerals  have  arranged  or  aggregated  them- 
selves in  more  or  less  parallel  layers  or  folia  that  give  rise  to  the 
structure  called  foliated.  Foliation  is  characteristic  of  many 
metamorphic  rocks.  Foliated  rocks  generally  split  readily  into 
thin  plates  or  flags  parallel  with  the  foliation  planes.  Meta- 
morphic rocks  that  split  in  this  way  are  called  schists.2 

Genesis  of  Metamorphism.— The  three  agencies  chiefly  concerned 
in  the  metamorphism  of  rock-masses  are  heat,  pressure,  and  water. 

The  heat  and  pressure  may  arise  from  three  possible  sources, 
namely — igneous  intrusions,  the  intense  folding  of  strata,  or  the 
subsidence  of  crustal  blocks  within  the  zone  of  high  subterranean 
temperatures. 

The  water  may  be  magmatic,  or  interstitial  in  the  sedimentary 
rocks  subjected  to  heat  and  pressure. 

According  to  the  source  of  the  heat,  the  alteration  of  rock-masses 
may  be  divided  into — 

(1)  Contact- Metamorphism. 

(2)  Regional- Metamorphism. 

Contact-Metamorphism. — All  igneous  intrusions  produce  a  cer- 
tain amount  of  alteration  in  the  rocks  which  they  invade.  In  the 
case  of  lavas  the  thermal  effects  are,  as  a  rule,  slight  and  unimpor- 
tant. Moreover,  the  alteration  caused  by  the  intrusion  of  small 
dykes  and  thin  sills  is  in  most  cases  remarkably  small,  and  is 
generally  confined  to  the  dehydration  and  baking  of  the  skin  of 

1  Gr.  meta= change,  and  morphe  =  shape. 

2  Gr.  schistos  =  easily  split. 

259 


260  A  TEXT-BOOK  OF  GEOLOGY. 

rock  at  the  actual  line  of  contact.  Coals,  however,  may  be  changed 
to  anthracite,  or  even  graphite  ;  and  pieces  of  clay  entangled  in 
the  magma  baked  into  an  impure  porcelain  called  porcellanite. 

Large  dykes  and  plutonic  bosses  that  have  cooled  slowly  and 
under  pressure,  frequently  effect  considerable  changes  in  the  rocks 
into  which  they  intrude.  Along  the  line  of  contact,  the  intruded 
rocks  are  generally  baked  and  hardened,  but  as  a  rule  the  mere 
thermal  effects  of  dry  heat  are  among  the  least  conspicuous  of  the 
changes  effected  by  the  intrusion.  In  some  cases  the  invaded  rock 
is  shattered  and  impregnated  with  new  minerals  for  many  hundreds 
or  even  thousands  of  yards  beyond  the  actual  contact ;  in  other 
cases  it  is  metamorphosed  into  a  foliated  crystalline  schist. 

The  metamorphic  effect  of  great  plutonic  intrusions  is  mainly 
thermal,  and  hence  is  greatest  at  the  contact,  gradually  diminishing 
as  the  distance  from  the  intruding  mass  increases.  The  amount  of 
change  arising  from  the  contact-action  will  depend  on  the  degree  of 
heat,  rate  of  cooling  of  the  igneous  mass,  the  thickness  of  the 
superincumbent  strata,  as  well  as  on  the  chemical  composition  and 
structure  of  the  invaded  rocks. 

The  dominant  sedimentary  rocks  in  the  rocky  crust  are  sand- 
stones, shales,  and  limestones.  The  sandstones  are  changed  into 
quartzites,  the  shales  into  slate  or  mica-schist,  and  the  limestones 
into  marbles. 

Pressure  alone  will  alter  argillites  and  shales  into  true  slates 
possessing  the  characteristic  slaty  cleavage,  but  heat,  pressure,  and 
water  acting  together  will  usually  lead  to  the  development  of 
muscovite  mica  on  a  grand  scale.  In  the  zone  of  greatest  pressure, 
the  shale  may  be  changed  completely  into  mica,  forming  a  phyllite  ; 
or  in  cases  where  the  shale  is  sandy,  into  a  mica-schist. 

Where  a  boss  of  granite  has  intruded  slates  there  is  frequently 
developed  in  the  slate  a  crop  of  what  are  called  contact  minerals. 
These  secondary  crystalline  minerals  are  mostly  simple  silicates  of 
alumina,  and  the  commonest  are  chiastolite  and  staurolite.  When 
the  intruded  rock  contains  sufficient  lime  and  alkali  to  combine 
with  the  free  silica  many  complete  silicates  may  be  developed, 
conspicuously  muscovite  and  actinolite. 

Pure  limestones  are  changed  to  granular  marbles,  while  impure 
limestones  give  rise  to  a  series  of  complex  lime-silicates,  of  which 
grossularite  (lime-garnet),  actinolite,  and  diopside  (lime-pyroxene) 
are  the  most  frequent. 

The  mere  fact  that  a  series  of  sedimentary  rocks  becomes  more 
and  more  altered  as  a  granitic  boss  is  approached  does  not,  of  itself, 
afford  conclusive  proof  that  the  granite  is  intrusive  in  the  sedimen- 
tary formation.  Many  granitic  massifs  are  fixed  blocks  of  great 
antiquity,  against  which  younger  formations  have  sometimes 


METAMORPHISM    AND    METAMOBPHIC    BOOKS.  261 

been  crushed  and  intensely  folded,  and  in  the  process  have  suffered 
a  high  degree  of  metamorphism.  Hence  when  an  altered  sedi- 
mentary formation  lies  against  a  granite  boss,  it  is  not  safe,  in  the 
absence  of  apophyses1  or  intrusive  veins  on  the  fringe  of  the  igneous 
mass,  to  conclude  that  the  granite  is  younger  than  the  altered 
clastic  rocks  with  which  it  is  in  contact. 

The  slower  the  magma  cools,  the  greater  are  the  changes  effected 
by  it.  Hence  we  usually  find  that  the  greatest  alteration  has 
been  effected  by  granites,  diorites,  and  other  plutonic  masses  of 
coarse  texture. 


FIG.  147. — Showing  aureole  or  zone  of  metamorphism,  N.S.W. 
(a)  Granite  boss.         (6)  Mica-schist.         (c)  Zone  of  tin  impregnation. 

Among  the  non-metallic  minerals  introduced  into  or  developed 
in  the  country-rock  by  the  igneous  intrusion  are  biotite,  tourma- 
line, hornblende,  epidote,  felspars,  garnets,  and  idocrase ;  and 
of  metallic  minerals,  ores  of  tin,  wolfram,  copper,  gold,  silver,  and 
iron.  The  impregnation  of  the  country-rock  within  the  aureole 
or  zone  of  metamorphism  with  tin  and  wolfram  is  a  characteristic 
feature  of  granite  intrusions. 

Obviously  such  extensive  alteration  and  impregnation  must  be 
due  to  some  other  agency  than  mere  dry  heat. 

Daubree's  experiments  on  silicates  and  rocks  have  shown  that  not 
dry  heat  alone,  nor  even  vapours  or  gases,  would  be  sufficient  to 
effect  changes  of  any  moment ;  but  that  superheated  water, 
1  Gr.  aohu#is  =  a,n  offshoot. 


262  A  TEXT-BOOK  OF  GEOLOGY. 

under  great  pressure,  was  the  most  important  agent  concerned  in 
metamorphism.  To  prove  this  he  partially  filled  a  glass-tube 
with  water,  sealed  both  ends,  and  placed  it  in  a  strong  iron  tube 
which  was  closed,  and  exposed  to  a  temperature  below  red  heat 
for  several  days.  The  glass-tube  was  attacked  by  the  water  and 
converted  into  a  zeolitic  mineral.  In  some  places  a  laminated, 
in  others  a  spherulitic,  structure  was  present.  With  superheated 
steam  he  obtained  orthoclase  and  a  micaceous  mineral. 

During  the  process  of  cooling,  the  intrusive  magma  will  prob- 
ably liberate  enormous  quantities  of  steam,  which  will  penetrate 
the  surrounding  rocks  for  considerable  distances.  At  a  certain 
point  the  steam  will  be  condensed  into  superheated  water,  which 
will  continue  the  work  of  metamorphism  in  the  outer  zone  of  the 
aureole.  When  the  igneous  mass  and  the  neighbouring  country- 
rock  have  sufficiently  cooled,  the  zone  of  steam  and  vapour  sur- 
rounding the  boss  will  be  invaded  by  superheated  water,  and  in 
this  way  we  may  get  the  metamorphic  effects  of  the  vapour  and 
gases  supplemented  by  those  of  superheated  water. 

The  intrusion  of  an  igneous  magma  is  thus  capable  of  performing 
an  important  role  in  the  processes  of  metamorphism  and  mineralisa- 
tion. By  its  intrusion  it  cracks  and  fissures  the  surrounding  rocks. 
It  is  a  source  of  great  heat,  which  is  slowly  transferred  to  the  country- 
rock  ;  and  is  a  carrier  of  steam  and  gases,  which  are  capable  of  alter- 
ing the  constitution  of  the  surrounding  rocks,  and  impregnating 
them  with  mineral  matter,  perhaps  mainly  derived  from  the  parent 
magma. 

Regional-Metamorphism. — Foliated  crystalline  rocks  frequently 
cover  thousands  of  square  miles  in  regions  where  they  have  no 
direct  association  with  known  plutonic  intrusions.  And,  singularly 
enough,  these  rocks  for  thousands  of  square  miles,  and  sometimes 
throughout  an  enormous  thickness,  may  exhibit  as  high  a  degree 
of  metamorphism  as  the  most  intense  alteration  produced'on  the 
borders  of  a  great  plutonic  boss. 

The  origin  of  this  widespread  regional-metamorphism  is  not 
well  understood.  By  some  it  is  believed  to  have  been  caused  by 
the  uprising  of  enormous  floods  of  plutonic  magmas  that  consoli- 
dated at  a  considerable  depth  and  have  never  been  uncovered  by 
denudation.  In  other  words,  this  view  supposes  that  regional- 
metamorphism  is  merely  an  exaggerated  kind  of  contact-meta- 
morphism. 

Another  hypothesis  postulates  that  great  crustal  blocks  lying 
under  piles  of  younger  strata  have  been  depressed  by  subsidence 
until  brought  within  the  influence  of  a  high  subterranean  tem- 
perature. This  view  is  merely  a  modification  of  the  Huttonian 
plutonic  theory,  according  to  which  blocks  of  rock  were  depressed 


METAMORPHISM   AND    METAMORPHIC    ROCKS.  263 

until  they  reached  a  zone  where  they  were  first  softened  and 
melted,  eventually  crystallising  as  they  cooled. 

The  temperature  of  the  Earth  increases  with  increasing  depth 
below  the  surface,  but  is  not  proportional  to  the  depth.  In  volcanic 
regions  the  zone  of  high  temperature  lies  close  to  the  surface, 
but  in  non- volcanic  regions  the  temperature-gradient  varies  enor- 
mously. In  some  regions  the  rate  of  increase  of  temperature  is 
as  high  as  1°  Fahr.  for  every  60  feet  of  depth ;  in  other  places  it  is 
not  more  than  1°  Fahr.  in  200  or  more  feet.  Moreover,  the  rate 
of  increase  of  temperature  is  not  uniform.  But  it  is  not  unreason- 
able to  suppose  that  with  considerable  subsidence  and  a  thick 
covering  of  strata  a  sufficient  heat  might  be  encountered  at  a  depth 
of  a  few  miles  to  effect  in  the  presence  of  superheated  water  great 
alterations  in  the  constitution  of  rock-masses,  without  actual 
softening  and  fusion,  as  required  by  the  Huttonian  theory. 

The  intensity  of  metamorphism  of  rocks  is  in  many  cases,  per- 
haps the  majority,  proportional  to  the  amount  of  crushing,  folding, 
and  plication  they  have  suffered.  The  metamorphism  induced  by 
intense  folding  and  other  crustal  movements  constitutes  what  is 
sometimes  called  dynamo-metamorphism.  In  this  case  we  are 
warranted  in  assuming  that  the  heat  and  pressure  of  crustal  move- 
ment in  conjunction  with  water  were  important,  but  not  necessarily 
the  sole,  agents  of  metamorphism.  For  it  is  obvious  that  the 
powerful  lateral  or  tangential  stresses  generated  by  crustal  move- 
ment, can  only  become  effective  in  the  production  of  intense  folding 
and  plication  when  they  are  strongly  resisted  by  the  vertical 
stress  of  a  pile  of  superincumbent  strata.  The  existence  of  such 
a  pile  of  strata  would  necessarily  imply  considerable  subsidence, 
sufficient  perhaps  to  bring  the  basement  rocks  within  the  influence 
of  a  high  subterranean  temperature,  not  sufficiently  high  to  cause 
fusion,  but  enough  to  supplement  the  heat  generated  by  the 
folding. 

But  intensely  folded  strata  are  not  always  altered  into  meta- 
morphic  rocks.  On  the  contrary,  they  frequently  exhibit  little  or 
no  evidence  of  internal  change.  And  crystalline  schists  are  not 
always  folded.  The  highly  altered  mica-schists  of  Central  Otago 
in  New  Zealand  lie  perfectly  horizontal,  or  are  gently  undulating, 
over  thousands  of  square  miles,  and  they  are  not  connected  with 
any  visible  plutonic  masses. 

The  genesis  of  regional-metamorphism  is  a  difficult  problem 
for  which  no  satisfactory  solution  has  been  formulated.  When 
we  review  the  available  evidence,  it  does  not  seem  unwarrantable 
to  assume  that  regional-metamorphism  may  be  caused  partly 
by  folding  and  partly  by  the  subsidence  of  crustal  blocks  till  they 
come  withinrthe  zone  of  considerable  subterranean  heat. 


264  A  TEXT-BOOK  OF  GEOLOGY. 

Metamorphic  Rocks. 

Metamorphic  rocks  may  be  schistose  or  massive.  In  the  schistose 
group,  the  original  matter  has  become  for  the  most  part  crystalline, 
and  a  foliated  or  schistose  structure  has  been  induced  by  the  arrange- 
ment of  the  newly-formed  crystalline  constituents  in  short  leaves 
or  folia  x  lying  more  or  less  parallel  with  one  another. 

The  separate  folia  may  consist  of  one  or  several  minerals.  They 
usually  occur  as  flat  lenses,  sometimes  even  and  parallel,  but  most 
frequently  overlapping,  uneven,  and  undulating,  puckered,  or 
plicated.  In  many  of  the  more  highly  altered  rocks  they  thin  out 
rapidly  in  all  directions,  again  increase  in  size,  and  once  more  thin 
out,  and  so  on  indefinitely. 

The  folia  may  vary  from  a  fraction  of  an  inch  to  several  inches 
or  even  many  feet  thick.  Fossils  present  in  the  original  sediments 
are  usually  completely  obliterated  by  the  development  of  the 
crystalline  structure. 

The  foliation  planes  may  be  parallel  to  the  original  bedding- 
planes  or  they  may  follow  any  direction.  The  foliation  is  doubtless 
developed  in  the  rock  when  under  the  influence  of  enormous 
pressure  ;  and  it  is  not  improbable  that  the  foliation  planes,  like 
slaty-cleavage  to  which  they  are  closely  related,  always  lie  at  right 
angles  to  the  direction  of  the  stress. 

Rocks  are  found  showing  all  degrees  of  metamorphism  from 
highly  contorted  granitoid  gneissic  schists  to  altered  sediments 
in  which  the  character  of  the  original  sediments  can  still  be  traced. 

Many  of  the  older  schists  are  believed  to  be  altered  igneous 
rocks  of  great  antiquity.  The  greenstones  (altered  andesites  and 
basalts)  forming  the  hanging-wall  of  the  Alaska-Treadwell  ore- 
body  on  Douglas  Island,  Alaska,  possess  a  well-developed  schistose 
structure,  as  also  do  some  of  the  greenstones  or  amphibolites 
associated  with  the  gold-bearing  lode-formations  at  Kalgoorlie. 

Among  the  massive  metamorphic  rocks  that  possess  a  crystalline 
structure,  but  are  not  foliated,  are  marble  and  quartzite. 

Foliated  Schists. 

The  leading  and  most  prevalent  types  of  these  rocks  are  as 
follow  : — 

Gneiss  is  a  schistose  aggregate  of  quartz,  felspar,  and  mica 
(muscovite  or  biotite).  Accessory  minerals  :  usually 
hornblende,  magnetite,  garnet,  rutile,  tourmaline,  and 
pyrite.  Abundant  in  Canada,  Highlands  of  Scotland, 
Scandinavia,  and  New  Zealand.  Usually  a  rock  of 
1  kat.  folia  =  leaves. 


To  face  page  265.] 


FIG.  148. — Photomicrograph  of  quartz-biotite-schist  from  Central  Otago,  N.Z. 


FIG.  149. — Microphotograph  of  phyllite  from  Central  Otago,  N.Z. 


METAMORPHISM    AND    METAMORPHIC    ROCKS.  265 

great   antiquity.     It  may  graduate   into   mica-schist  on 
the  one  hand,  or  become  granitoid  on  the  other. 

The  different  varieties  of  gneiss  are  named  after  the 
dominant  ferro-magnesian  mineral. 

Mica-Schist  consists  of  alternating  folia  of  mica  (mostly  mus- 
covite)  and  quartz.  Accessory  minerals :  magnetite, 
garnet,  rutile,  and  pyrite.  Abundant  in  Canada,  High- 
lands of  Scotland,  Scandinavia,  Alps,  New  South  Wales, 
Western  Australia,  and  New  Zealand. 

Chlorite- Schist  is  a  schistose  aggregate  of  scaly  chlorite,  usually 
with  quartz.  Accessory  minerals  :  magnetite,  specular 
iron,  felspar,  talc,  mica,  actinolite,  and  apatite.  Com- 
monly occurs  as  subordinate  bands  in  mica-schist.  In 
many  cases  appears  to  be  a  metamorphosed  basic  igneous 
rock.  The  characteristic  colour  is  a  pale  olive  green. 

Hornblende- Schist  is  generally  an  aggregate  of  hornblende, 
quartz,  felspar,  and  mica.  Accessory  minerals :  magnetite, 
garnet,  and  epidote.  This  schist  is  probably  an  altered 
igneous  rock.  It  commonly  occurs  in  association  with 
gneiss  and  mica-schist. 

Actinolite- Schist,  composed  mainly  of  light-  or  dark- 
green  actinolite,  often  in  clustering  or  radiating  sheaves, 
is  a  common  associate  of  mica-schist  and  gneiss. 

Quartz- Schist  is  a  flaggy  quartzite  that  breaks  readily  into  thin 
laminae.  Sometimes  the  splitting  is  facilitated  by  the 
presence  of  mica  along  the  foliation  planes.  In  this  case 
we  get  a  micaceous  quartz- schist,  which  may  graduate 
into  an  ordinary  mica-schist.  The  common  accessory 
minerals  are  actinolite,  garnet,  specular  iron,  and  mag- 
netite. Quartz-schist  forms  bands  associated  with  mica- 
schist  and  slate  in  the  older  Palaeozoic  formations. 
Found  in  all  the  continents. 

Talc-Schist  consists  of  scaly  talc,  often  with  some  quartz, 
chlorite,  or  mica.  Colour  pale-green  or  greenish-grey. 
Feels  greasy  and  is  quite  soft.  Accessory  minerals  : 
magnetite,  tourmaline,  felspar,  magnesite,  and  actinolite. 
Frequently  associated  with  mica-schist  as  small  sub- 
ordinate bands. 

Phyllite,  a  highly  altered  clay-shale  in  which  an  abundance  of 
mica  has  been  developed.  When  the  mica  forms  the 
dominant  constituent,  the  rock  possesses  a  silvery-grey 
colour  and  a  silky  lustre.  Quartz  is  frequently  present. 
Phyllite  is  intermediate  between  an  ordinary  clay-slate 
and  mica -schist,  into  either  of  which  it  may  pass 
insensibly. 


266  A  TEXT-BOOK  OF  GEOLOGY. 

Clay-Slate  is  a  compact  finely-granular  clay-rock.  Splits  readily 
into  thin  plates  in  a  direction  parallel  with  the  slaty- 
cleavage,  which  may  coincide  with  the  original  planes 
of  deposition,  or  lie  in  any  other  direction.  The  colour 
ranges  from  grey  to  green,  blue,  and  purple.  Clay-slate 
is  essentially  composed  of  hydrous  silicate  of  alumina 
and  various  other  silicates.  The  accessory  minerals 
are  quartz,  mica,  felspar,  rutile,  iron  oxides,  and  pyrite. 
Graphite-slate  contains  a  large  amount  of  graphite. 
Spotted-slate  is  a  slate  containing  little  knots  or  spots 
which  would  appear  in  some  cases  to  be  incipient  forms 
of  chiastolite  or  andalusite.  These  minerals  are  frequently 
developed  in  slates  near  igneous  contacts,  and  when  re- 
latively abundant  give  rise  to  the  varieties  of  slate  called 
chiastolite- slate  or  andalusite- slate. 
As  a  rule,  the  schistose  structure  is  best  developed  in  fine-grained 

rocks,  but  under  the  influence  of  great  pressure  even  conglomerates 

may  become  schistose. 

Massive  Crystalline  Rocks. 

Marble  is  a  granular  crystalline  aggregate  of  calcite  of  fairly 
uniform  texture.  The  accessory  minerals  may  be  mica 
(generally  muscovite),  talc  (or  more  rarely  graphite  scales), 
garnet,  actinolite,  tremolite,  or  molybdenite  scales.  A 
marble  is  merely  a  metamorphosed  limestone  ;  and  when 
the  original  limestone  was  pure  we  get  a  high-class  marble, 
and  when  impure  a  low-grade  marble,  the  impurities 
being  changed  into  the  accessory  minerals. 

Quartzite  is  a  rock  consisting  essentially  of  quartz  grains  cemented 
with  silica.  It  is  an  altered  sandstone  and  possesses 
a  crystalline  texture  induced  by  heat  in  the  presence  of 
water.  The  grains  frequently  present  a  semi -fused 
appearance.  Quartzite  can  be  formed  from  blocks  of 
sandstone  subjected  to  prolonged  heat.  The  meta- 
morphism  is  probably  accelerated  by  the  presence  of 
superheated  water.  Where  igneous  rocks  have  intruded 
into  sandstones,  a  zone  of  the  latter  surrounding  the 
intrusive'mass  is  frequently  altered  into  typical  quart zite. 

SUMMARY. 

(1)  Metamorphic  rocks  generally  possess  a  crystalline  structure. 
They  may  be  foliated  or  massive.  In  the  foliated  rocks,  the 
crystalline  mineral  constituents  are  arranged  in  more  or  less  parallel 


METAMOBPHISM    AND    METAMOBPHIC    BOCKS.  267 

or  overlapping  lenses.     The  foliated  rocks  split  readily  along  the 
foliation-planes  ;    and  are  therefore  called  schistose. 

(2)  The  massive  metamorphic  rocks  are  marble  and  quartzite. 

(3)  The  most  abundant  crystalline  schists  are  gneiss,  mica-schist, 
chlorite-schist,  quartz-schist,  talc-schist,  and  phyllite. 

(4)  The  alteration  or  metamorphism  of  rocks  is  mainly  due  to 
heat,  pressure,  and  superheated  water. 

(5)  The  metamorphism  may  be  what  is  called  contact-metamor- 
phism,  which  is  caused   by  igneous   intrusions   and   hence   quite 
local ;    or  regional-metamorphism,  which  affects  large  areas  of  rock. 

(6)  The  effects  of  contact-metamorphism  have  been  successfully 
imitated  by  Daubree  and  others  on  artificial  compounds. 

(7)  The  Qrigin  of  regional-metamorphism  is  still  obscure.     It 
may  be  due  (a)  to  the  uprising  of  floods  of  plutonic  magmas  that 
have  consolidated  at  a  considerable  depth  and  have  never  been 
exposed  by  denudation  ;    (6)  to  the  subsidence  of  large  crustal 
blocks  to  the  zone  of  subterranean  heat ;    or  (c)  to  the  intense 
folding  and  plication  of  rocks  subjected  to  the  load  of  a  pile  of 
superincumbent  strata. 

It  is  not  improbable  that  in  certain  situations,  one,  two,  or  all 
of  these  together,  may  have  been  concerned  in  the  process  of 
metamorphism. 


CHAPTER   XVIII. 

FOSSILS:    THEIR   OCCURRENCE,   PRESERVATION, 
CLASSIFICATION,   AND   USES. 

THE  remains  of  animals  and  plants  that  have  been  embedded  in 
rocks,  as  well  as  all  traces,  casts,  impressions,  and  trails  of  what 
were  at  one  time  living  organisms,  are  called  fossils. 

Fossils  are  found  in  the  majority  of  stratified  rocks  ;  and  since 
most  stratified  rocks  are  of  marine  origin,  it  is  not  surprising  to  find 
that  the  majority  of  fossils  belong  to  organisms  that  lived  in  the  sea. 

The  most  abundant  fossils  are  the  shells  of  marine  molluscs  ; 
and  after  these  come  corals  and  foraminifera. 

Preservation  of  Fossils. — Let  us  consider  the  case  of  the  molluscs. 
Most  molluscs  are  provided  with  a  calcareous  shell  or  covering. 
When  the  animal  dies  and  the  soft  parts  decay,  the  shell  usually 
becomes  filled  up  with  sand  or  mud,  and  is  eventually  buried  in 
the  sediments  that  are  continually  accumulating  on  the  sea-floor. 

Shells  buried  in  a  mud  or  fine  sediment  that  subsequently  be- 
comes hardened  into  an  impervious  rock  are  usually  perfectly 
preserved,  with  the  exception  perhaps  of  the  original  colouring. 
But  shells  embedded  in  a  sandstone  through  which  water  can 
percolate  freely  are  frequently  dissolved  and  removed  by  the  water, 
and  there  remain  only  external  casts,  or  impressions  of  the  exterior 
of  the  shells.  In  cases  where  the  shell  was  filled  with  sediment 
at  the  time  it  was  buried,  besides  the  external  mould,  there  will 
be  found,  when  the  shell  is  dissolved,  an  internal  cast  reproducing 
the  exact  shape  of  the  interior  of  the  shell. 

By  filling  with  plaster  the  space  from  which  the  shell  was  dissolved 
a  hollow  cast  is  obtained  that  is  in  all  respects  a  replica  of  the 
original  shell.  If,  on  the  other  hand,  we  remove  the  internal 
cast,  and  fill  the  whole  interior  of  the  mould  or  impression  from 
which  it  was  removed  with  plaster,  we  shall  get  a  solid  representa- 
tion of  the  outward  form  of  the  shell  before  its  burial. 

When  a  shell  is  gradually  replaced  by  mineral  matter  deposited 
from  water  percolating  through  the  rock,  we  frequently  get  a  com- 
plete reproduction  of  the  whole  organism  even  to  the  minutest 

268 


[To  face  page  268. 


FIG.  150. — Showing  fossil  casts  of  Turritella. 
(a)  Cast  of  exterior  of  shell.  (b)  Cast  of  interior. 


b. 


FIG.  151. — Showing  fossil  forarainifera. 
(a)  Amphistegina.       (b)  Clavulina.       (c)  Textilaria.       (d)  Polystomella. 


FOSSILS  :     OCCURRENCE,    CLASSIFICATION,    USES.       269 

detail.  The  carbonate  of  lime  of  the  shell  may  be  replaced  by 
carbonate  of  iron,  pyrite,  or  silica.  Siliceous  replacements  of  bones 
and  wood  are  quite  common,  and  often  they  preserve  the  internal 
structure  with  marvellous  exactness. 

Shells  which  live  in  sand  or  mud  become  buried,  as  a  rule,  in 
the  place  where  they  lived.  But  many  shells  are  cast  up  on  the 
beach,  where  they  are  broken  up  into  sand  by  the  pounding  action 
of  the  waves.  It  is  in  this  way  that  shelly  sands  are  formed. 
Thick  shells  that  are  not  easily  comminuted  soon  become  rounded 
and  water-worn. 

The  fossil-shells  that  occur  in  rocks  composed  of  littoral  deposits 
are  frequently  fragmentary  and  water- worn. 

Deltaic  and  estuarine  deposits  may  contain  the  remains  of  land 
animals  and  plants,  mingled  with  estuarine  and  marine  forms. 

jShelly  limestones  are  usually  composed  of  the  dead  shells  of 
gregarious  molluscs  that  grew  on  shell-banks  ;  and  coral-limestones 
are  formed  where  the  coral-builders  lived ;  but  deep-sea  shells  are 
not  infrequently  found  in  shallow- water  deposits,  and  littoral  shells 
in  deep-sea  sediments  where  they  have  been  carried  by  sea-currents. 

The  remains  of  land  animals  and  plants  are  sometimes  carried 
far  out  to  sea,  where  they  become  buried  among  marine  organisms  ; 
but  marine  deposits  are  typically  distinguished  by  the  presence  of 
marine  organisms,  and  terrestrial  deposits  by  terrestrial  organisms. 

Fossiliferous  Rocks. — As  a  rule  the  best-preserved  fossils  are 
found  in  rocks  composed  of  fine  sediments.  Clays,  marls,  shales, 
and  limestones  frequently  contain  a  rich  and  varied  assemblage 
of  fossils  in  a  beautiful  state  of  preservation. 

The  best  leaf-impressions  are  met  with  in  fissile  shales  and 
argillaceous  sandstones,  and  they  are  most  numerous  where  the 
rock  is  black  and  carbonaceous. 

Coarse  sandstones,  grits,  and  conglomerates  are  characteristic- 
ally poor  in  organic  remains  ;  and  when  shells  are  present  in  them, 
they  are  usually  broken  and  water- worn.  In  most  sandstones 
the  fossils  are  represented  by  internal  casts,  and  impressions  of 
the  exterior  of  the  shells  or  organisms. 

Volcanic  tuffs  intercalated  with  marine  strata  are  sometimes 
richly  fossiliferous,  but  igneous  rocks  are  devoid  of  all  organic 
remains  except  those  that  occur  in  blocks  derived  from  fossili- 
ferous sedimentaries  in  the  neighbourhood  of  the  volcanic  vent. 

Derived  Fossils  are  comparatively  common  in  the  pebbles  and 
boulders  of  pebbly  beds  and  conglomerates.  Conglomerates,  like 
all  other  sedimentary  rocks,  are  composed  of  material  derived  from 
older  rock-formations,  many  of  which  were  fossiliferous.  When  a 
fossiliferous  rock-formation  becomes  broken  up  by  denudation 
some  of  the  pebbles  may  contain  fossils,  and  in  this  way  the  con- 


270  A  TEXT-BOOK  OF  GEOLOGY. 

glomerate,  of  which  these  pebbles  eventually  become  a  constituent, 
may  contain  derived  fossils .  A.  Tertiary  conglomerate  may  contain 
Tertiary  shells  embedded  in  the  sandy  matrix,  and  derived  fossils 
of  Silurian  age  embedded  in  the  pebbles.  The  fossils  met  with 
in  the  matrix  are  contemporaneous,  and  belong  to  molluscs  that 
lived  in  the  sea  at  the  time  the  pebbles  and  sands  were  deposited. 
But  this  requires  some  qualification.  Derived  fossils  do  not 
always  occur  embedded  in  pebbles  or  blocks.  They  are  sometimes 
met  with  in  sandy  and  clayey  rocks  mingled  with  the  contem- 
poraneous shells  from  which  they  cannot  always  be  easily  distin- 
guished. In  places  where  the  sea-coast  is  fringed  with  low-sloping 
cliffs  composed  of  fossiliferous  sands,  clays,  marls,  or  shales,  it 
sometimes  happens  that  well-preserved  shells  become  liberated  by 
the  crumbling  away  of  the  rock  and  fall  on  to  the  beach,  where 
they  become  embedded  in  the  sands  or  mud  accumulating  on  the 
sea-floor. 

Classification  of  Living  Organisms. 

All  living  organisms  are  divided  into  two  kingdoms,  namely  : — 

I.  Animal  kingdom. 
II.  Vegetable  kingdom. 

THE  ANIMAL  KINGDOM. 

The  study  of  the  animals  that  now  inhabit  the  globe  belongs  to 
the  domain  of  the  science  known  as  Zoology.  The  branch  of 
Zoology  which  concerns  itself  with  fossil  organisms  is  called 
Palceontology.1 

For  convenience  of  study,  animal  life  has  been  subdivided  into 
Species,  Genera,  Families,  Orders,  Classes,  and  Sub-kingdoms, 
in  much  the  same  way  as  the  human  race  is  divided  into  Individuals, 
Families,  Tribes,  Nations,  and  Races.  Thus  the  related  members 
of  a  household  constitute  a  Family,  a  number  of  families  form  a 
Tribe  or  Clan,  a  number  of  tribes  form  a  Nation,  and  several 
related  nations  constitute  a  Race. 

The  individuals  of  any  kind  of  animal  are  called  species  ;  and  a 
species  may  be  defined  as  comprising  those  individuals  that  are 
the  same  in  all  essential  features,  and  reproduce  their  kind  true  to 
the  type. 

A  genus  includes  all  the  species  that  are  nearly  related  by  some 
prominent  structural  characteristic.  Thus  all  the  species  of  the 
cat-kind,  whether  domestic  or  wild,  are  included  in  the  genus 
Felis.  In  this  way  we  have  : — 

1  Gr.  palaios  =  ancient,  onta  =  beings,  and  logos  =  a  description  or  discourse. 


FOSSILS:    OCCURRENCE,  CLASSIFICATION,  USES.      271 


Felis  catus  =the  domestic  cat. 
Felis  tigris  =the  tiger. 
Felis  leo      =the  lion. 

Similarly  all  the  members  or  species  of  the  dog-kind  are  grouped 
in  the  genus  Canis.  Thus  we  have  : — 

Canis familiaris  =the  domestic  dog; 
Canis  lupus         =  the  wolf; 
Canis  vulpes       =the  fox. 

Related  genera  are  grouped  in  Families,  related  families  in 
Orders,  related  orders  in  Classes,  and  related  classes  in  Sub- 
kingdoms,  of  which  there  are  nine. 

The  groups  of  animals  which  are  known  to  occur  in  the  fossil 
state,  beginning  with  the  simplest  forms  and  ending  with  the  most 
highly  organised,  are  as  shown  in  the  following  table  : — 

OUTLINE   CLASSIFICATION   OF  ANIMAL   KINGDOM. 


Sub-kingdoms. 
I.  Protozoa 


II.  Porifera 
III.  Coelenterata 


Classes. 
Rhizopoda. 

Spongise. 

{(a)  Hydrozoa. 
(b)  Actinozoa. 
f(a)  Crinoidea. 

(b)  Asteroidea. 
IV.  Echinodermata  4  (c)  Echinoidea. 

(d)  Blastoidea. 

(e)  Cystoidea. 
Annelida. 


V.  Annulata 
VI.  Molluscoidea 


VII.  Mollusca 


VIII.  Arthropoda 


IX.  Vertebrata 


{(a)  Polyzoa. 
(b)  Brachiopoda. 

(a)  Lamellibran- 

chiata. 

(b)  Gasteropoda. 

(c)  Cephalopoda. 

(a)  Crustacea. 

(b)  Arachnoidea. 

(c)  Insecta. 

(a)  Pisces. 

(b)  Amphibia. 

(c)  Reptilia. 

(d)  Aves. 

(e)  Mammalia. 


Fossil  Types. 
Foraminifera,  Radio- 

laria. 
Sponges. 
Graptolites. 
Coral-reef  builders. 
Sea-lilies. 
Starfish. 
Sea-urchins. 


Worms. 
Sea-mats. 
Lamp-shells. 
Oysters  and  common 

bivalves. 
Univalve  shells. 
Nautilus,  ammonites. 
Crabs. 

Spiders,  scorpions. 
Insects. 
Fishes. 
Frogs. 
Reptiles. 
Birds. 
Mammoth,  seal,  whales. 


272  A  TEXT-BOOK  OF  GEOLOGY. 

Protozoa.1 — This  is  the  lowest  division  of  the  animal  kingdom. 
The  organisms  of  this  group  consist  of  a  single  cell  of  jelly-like 
matter  ;  and  some  protect  themselves  with  a  strong  covering 
secreted  from  the  sea- water. 

Only  those  possessing  a  hard  cover  are  preserved  as  fossils. 
Among  these  we  have  the  Foraminifera*  which  secrete  a  carbonate 
of  lime  covering,  and  the  Radiolaria,3  which  form  a  hard  case  of 
silica. 

The  shells  of  the  Foraminifera  are  shaped  like  flasks  or  flattened 
globes  with  a  biconvex  section,  or  like  globes  and  flasks  entwined. 

The  walls  of  the  shells  are  pierced  with  numerous  holes  through 
which  the  animal  extends  thread-like  organs.  The  Foraminifera 
form  important  deposits  on  the  floor  of  the  deep  seas  ;  and  they 
have  played  an  important  part  as  limestone  builders  in  the 


FIG.  152. — Nummulites,  Lower  Tertiary  Species. 

earlier  periods  of  the  Earth's  history,  and  as  chalk  builders  in  the 
Cretaceous. 

Among  the  best-known  genera  of  Foraminifera  are  Dentalina, 
Nodosaria,  Cristellaria,  Globigerina,  Rotalia  and  Nummulites. 

The  Radiolaria  secrete  siliceous  skeletons  that  are  often  a  geo- 
metrical framework  of  extreme  beauty.  They  form  deposits  of 
ooze  on  the  floor  of  the  deep  sea  ;  and  as  fossils  are  found  in  cherts 
and  other  siliceous  rocks. 

Porifera. — This  sub-kingdom  includes  the  sponges,  which  are 
somewhat  more  complex  organisms  than  the  protozoans.  The 
body  is  generally  supported  on  a  framework  or  skeleton  of  horny 
or  siliceous  fibres,  or  of  spicules  which  may  be  composed  of  silica 
or  carbonate  of  lime. 

The  majority  of  the  sponges  are  marine.  The  portions  found 
fossil  are  generally  the  siliceous  spicules  and  fibres. 

Ccelenterata. — This  group  contains  the  Hydrozoans  and  Actino- 

1  Gr.  protos  =  first,  and  zoow  =  an  animal. 

2  Lat.  f or amina  =  holes,  and/ero  =  I  bear. 

3  Lat.  radius  =a  ray. 


FOSSILS  :     OCCURRENCE,    CLASSIFICATION,    USES.        273 

zoans,  which  are  of  immense  geological  importance.  The  Hydro- 
zoans  l  include  the  graptolites,2  which  have  long  been  extinct,  but 
are  of  great  value  as  a  means  of  determining  the  age  of  the  rocks  in 
which  they  occur.  Graptolites  are  found  in  shales,  slates,  and 
argillaceous  sandstones,  in  which  they  occur  as  flattened  bodies  that 
are  usually  converted  into  graphite. 


FIG.  153. — (a)  Monograptus 

spiralis. 
(6)  M.  cyphus. 


FIG.  154.—  Diplo- 
graptus. 


FIG.  155.—  Didymo- 
graptus. 


FIG.  156.— Rastrites. 


FIG.  157. — Tetragraptus. 


The  Actinozoans  include  the  well-known  coral-builders  ;  they 
consist  of  a  soft  body  supported  in  a  cup  of  carbonate  of  lime. 
They  build  up  huge  coral  reefs  and  enormous  masses  of  limestone. 
They  are  perhaps  the  most  important  of  all  living  organisms 
considered  as  geological  agents. 

Eehinodermata.3 — These  are,  as  the  name  implies,  spiny- 
skinned  animals.  They  possess  a  calcareous  covering  made  up  of 
a  number  of  plates.  The  portions  found  fossil  are  the  spines  and 


1  Gr.  hudor  =  water,  and  zoow=an  animal. 

and  lithos  =  a,  stone, 
and  derma  =  skin. 


v^x.    rnwvf  —  Yvctcit/i.j   ctjivj.  <&t/i 

2  Gr.  graphein  =  to  write,  ai 

3  Gr.  echinos=a,  hedgehog, 


18 


274 


A   TEXT-BOOK   OF   GEOLOGY. 


plates.     The  Crinoidea,   Echinoidea,  and    Asteroidea  are  the  chief 
classes  of  this  sub-kingdom. 


FIG.  158. — Showing  corals, 
(a)  Oculina.  (6)  Trochocyathus. 


FIG.  159.— Fossil  sea-lily, 
Encrinus. 


FIG.  160.— Fossil  sea  lily, 
Pentacrinus. 


FOSSILS  I     OCCURRENCE,    CLASSIFICATION,    USES.        275 

The  Crinoidea,1  called  crinoids  or  sea-lilies,  usually  consist  of 
long  flexible  stalks  with  a  calyx  at  the  upper  end.  The  calyx 
contains  the  internal  organs  of  the  animal,  and  is  protected  with 
plates  symmetrically  arranged.  Bound  the  calyx  there  is  a 
number  of  flexible  arms  which,  like  the  stalk,  are  encased  in  cal- 
careous plates.  The  animal  is  attached  to  objects  on  the  sea-floor 
by  the  stalk,  which  is  jointed  and  flexible.  Broken  arms  and  stalks 
of  crinoids  are  sometimes  so  plentiful  as  to  compose  masses  of 
limestone. 


FIG.  161. — Fossil  sea-urchin, 
Palcechinus. 


FIG.  162.— Fossil  sea-urchin, 
Nucholites. 


The  Echinoidea 2  include  the  well-known  Sea-urchins  so  often 
cast  up  on  sandy  beaches  or  seen  in  rocky  pools  below  high-water 
mark.  They  are  usually  globular  or  heart-shaped  animals  enclosed 
in  a  spiny  case  or  shell  composed  of  closely-fitting  calcareous  plates. 
The  spines,  plates,  and  frequently  whole  shells,  are  found  fossil. 


FIG.  163. — The  fossil  starfish,  Palceaster,  from  the  Cambrian. 


The  Asteroidea  3  or  Starfishes  consist  of  a  central  flattened  disc 
with  several  radiating  arms. 

The  Ophiuroidea  4  are  related  to  the  Starfishes.  They  comprise 
a  remarkable  group  of  Brittle-stars  in  which  the  viscera  are  excluded 
from  the  arms.  They  consist  of  a  central  flattened  disc-like  body 
from  which  project  five  long  flexible  arms  used  by  the  animal  as 
a  means  of  locomotion. 

1  Gr.  krinon=&  lily,  and  Lat.  <nrfes=like. 

a  Gr.  echinos  =  a,  hedgehog,  and  Lat.  oides=like. 

3  Gr.  aster  =  a  star,  and  Lat.  aides  =like. 

4  Gr.  ophis  =  a,  snake,  and  Lat.  oides=like. 


276  A  TEXT-BOOK  OF  GEOLOGY. 

Annulata.1 — The  annelids  or  segmented  worms  are  the  only  ones 
found  fossil.  Some  of  the  annelids  secrete  calcareous  tubes  which 
have  been  preserved  in  shales  and  slates.  The  former  existence 
of  worms  is  also  known  by  the  fossil  trails  left  in  muds  now  hardened 
into  shales,  and  by  the  worm-burrows  made  in  sands  now  converted 
into  sandstones. 

Worm-burrows  and  trails  are  among  the  oldest  known  fossils. 
Many  of  the  so-called  fucoids  which  are  found  in  rocks  of  all  ages, 
but  are  particularly  abundant  in  the  Cretaceous,  are  probably 
the  remains  of  tube-building  Terebelloid  annelids.2 

Molluseoidea.3 — These  comprise  the  Polyzoa  4  and  Brackiopodaf 
which  are  soft-bodied  animals  provided  with  a  calcareous  shell  or 
covering. 


FIG.  164. — Showing  fossil  brittle-star,  Ophioderma. 

The  Polyzoa  or  Sea-mats,  sometimes  called  Bryozoans,  are  tiny 
animals  living  in  a  separate  cell ;  but  a  number  of  individuals  are 
united  in  a  colony  which  may  form  an  encrusting  mat  on  the  rocks 
on  the  seashore,  or  on  some  other  organism.  They  are  found  as 
fossils  in  rocks  of  all  geological  ages. 

The  Brachiopoda  comprise  one  of  the  most  important  classes  of 
fossil  shells.  They  are  poorly  represented  by  living  species,  but 
occur  in  great  abundance  in  the  Palaeozoic  and  Mesozoic  formations. 

Brachiopods  are  soft  animals  enclosed  in  symmetrical  bivalve 
shells,  the  valves  of  which  are  typically  unequal  in  size.  The  larger 
valve  is  called  the  ventral  valve,  and  the  smaller,  the  dorsal.  In 
most  genera,  the  valves  are  locked  together  at  the  hinge.  The 
ventral  valve  is  usually  perforated  with  a  hole  called  the  foramen, 

1  Lat.  annulus  =  a,  little  ring. 

2  F.  A.  Bather,  The  Geological  Magazine,  Dec.  1911,  p.  549. 

3  Lat.  mollis  =  soit,  and  Lat.  oides  =  lik.e. 

4  Gr.  polus  =  ma,ny,  and  zocm=an  animal. 

5  Gr.  brachion=&n  arm,  and  pous,  podos=a,  foot. 


FOSSILS  I     OCCURRENCE,    CLASSIFICATION,    USES.        277 

for  the  passage  of  a  ligament  by  which  the  animal  attaches  itself 
to  solid  objects. 

Most  brachiopod  shells  contain  an  internal  calcareous  loop  or 
spiral  for  the  support  of  the  breathing  organs. 


d 


FIG.  166.— Showing  fossil 
polyzoan,  Monticuli- 
pora. 


FIG.  165. — Showing  fossil  poly- 
zoan, Fenestella. 


FIG.  167.  —  Fossil  brachiopod,  Spiri- 
fer,  showing  internal  spiral  loop. 


FIG.  168.  —  Fossil  brachiopod,  Tere- 
bratula  (lamp-shell),  showing 
foramen. 


Mollusca.  —  This  sub-kingdom  is  of  immense  importance.  It  is 
represented  by  thousands  of  living  and  extinct  species.  All  the 
land  shells,  and  practically  all  the  marine  shells,  so  numerous  in 
the  shallow  seas,  are  molluscs. 

Nearly  all  molluscs  possess  a  hard  calcareous  shell,  and  all  have 


278 


A    TEXT-BOOK    OF    GEOLOGY. 


an  elaborate  nervous  system  and  a  heart.     The  three  great  divisions 
of  the  mollusca  are  : — 

(1)  Lamellibranchiata. 

(2)  Gasteropoda. 

(3)  Cephalopoda. 


FIG.  IQQ.—Protocardium. 


FIG.  170. — Inoceramus. 


FIG.  171. — Cyrena. 


The  Lamellibranchiata  1  are  found  in  freshwater  lakes  and  the 
sea.  They  possess  a  bivalve  shell  which  consists  of  a  right  and  left 
valve.  Among  familiar  shells  of  this  class  we  have  the  mussel, 
cockle,  and  oyster. 


FIG.  172.—  Planorbis. 


FIG.  I13.—Paludina. 


FIG.  174.—  Valuta. 


The  Gasteropoda  2  are  molluscs  with  only  one  shell  or  valve, 
and  hence  are  spoken  of  as  univalve.  The  shell  may  be  basin- 
shaped,  as  in  Patella  ;  or  coiled  in  a  flat  or  a  turreted  spiral.  Some 

1  Lat.  lamella =a  little  plate,  and  branchiae— gills. 

2  Gr.  gaster  =  B,  belly,  and  pous,  podos  =  B,  foot. 


FOSSILS:    OCCURRENCE,  CLASSIFICATION,  USES.      279 

gasteropods  live  on  the  land,  some  in  fresh  water,  and  a  great  many 
in  the  sea.  All  possess  a  distinct  head  with  eyes  and  ears. 

The  Cephalopoda  l  are  the  most  highly  organised  of  the  mollusca. 
They  include  the  Nautilus,2  the  Octopus  or  Cuttle-fish)  the  Squid, 
and  two  important  orders  that  are  now  extinct,  the  Ammonites  3 
and  Belemnites*  The  Nautilus  possesses  a  beautiful  chambered 
shell. 

The  Ammonites  have  shells  resembling  those  of  the  Nautilus, 
but  more  highly  ornamented.  The  Belemnites  appear  to  have 


Nautilus.  FIG.  I16.—Goniatites.  FIG.  lll.—Ceratites. 


FIG.  178. — Ammonites. 


FIG.  H9.—Scaphites. 


resembled  the  modern  squids.  Both  the  Ammonites  and  Belem- 
nites became  extinct  about  the  close  of  the  Mesozoic  period. 

Arthropoda.5  —  These  are  animals  with  jointed  limbs  and  bodies 
divided  into  segments.  They  are  divided  into  (1)  the  Crustacea, 
(2)  Arachnoidea,  and  (3)  Insecta. 

The  Crustacea  include  the  crabs,  lobsters,  cray-fish,  shrimps, 
and  an  important  extinct  group  called  Trilobites  that  are  typically 


1  Gr.  kephale  =  B,  head,  and  pous,  podos  = 

2  Gr.  nautilus  =  a  sailor. 

3  So  named  after  Jupiter  Ammon. 

4  Gr.  belemnon=&  dart. 

5  Gr.  arthron=&  joint,  and  pous  =  a  foot. 


foot. 


280 


A    TEXT-BOOK    OF    GEOLOGY. 


characteristic  of  the  older  Palaeozoic  formations.     The  Trilobites  1 
owe    their   name    to   the   three-lobed   arrangement   of  the   body 


FIG.  180.— Orthoceras. 


FIG.  181.  — Guard  of 
Belemnites,  showing 
chambered  phragmo- 
cone  in. top  of  cavity. 


FIG.  I82.—Belemnitella. 


segments,  the  central  lobe  of  segments  being  flanked  by  two  other 
lobes,  one  on  each  side. 

The  Trilobites  are  the  most  distinctive  of  the  older  Palaeozoic 


FIG.  183.—  Illcenus. 


FIG.  184.—  Lichas. 


FIG.  185. — PrestwicJiia. 


fossils,  and  are  of  great  value  as  a  means  of  determining  the  age 
of  the  rocks  in  which  they  occur. 

The  Merostomata,2  which  are  represented  at  the  present  day  by 

1  Gr.  treis  =  three,  and  lobos=a,  lobe. 

2  Gr.  meros=&  thigh,  and  stoma=a,  mouth. 


FOSSILS:    OCCURRENCE,  CLASSIFICATION,  USES.      281 

the  King-crabs,  also  occur  in  the  older  formations,  a  well-known 
form  being  the  Pterygotus. 

The  Decapods  x  include  the  crabs  and  lobsters. 

The  Entomostraca  2  are  minute  crustaceans,  many  of  which  have 
the  entire  body  enclosed  in  a  shell  composed  of  two  valves  united 
along  the  back  by  a  hinge  which  permits  the  shell  to  be  opened 
and  shut  at  will.  The  best  known  of  this  class  are  the  Water-fleas, 
which  are  common  in  the  oldest  rocks,  and  are  still  represented 
by  many  living  species. 


FIG.  186.—  Pterygotus.     (Restored  by  H.  Woodward.) 

Vertebrata. — These  are  subdivided  into  five  great  classes  : — 

(1)  Pisces  =  Fishes. 

(2)  Amphibia  =  Frogs. 

(3)  Reptilia  =  Rep  tiles. 

(4)  Aves  =  Birds. 

(5)  Mammalia  =  Mammals. 

The  Pisces  or  fishes  are  the  oldest  known  vertebrates.  The  two 
orders  of  fishes  recognised  in  a  fossil  state  are  the  Ganoidei  3  and 
Tekostei* 

1  Gr.  deka  =  ten,  and  pous,  podos  =  &  foot. 

2  Gr.  entomon  =  &n  insect,  and  ostrakon  =  a,  shell. 
8  Gr.  ganos  =  brightness,  and  Lat.  aides =like. 

4  Gr.  teleos  =  complete,  and  osteon=a,  bone. 


282 


A    TEXT-BOOK    OF    GEOLOGY. 


Among  typical  ganoids  are  the  shark  and  sturgeon,  both  charac- 
terised by  the  possession  of  heterocercal l  tails.  The  former  are 
marine,  the  latter  freshwater  fishes. 

The  ganoids  first  appear  in  the  Silurian,  and  from  the  Devonian 
to  the  close  of  the  Mesozoic  they  predominate  among  fossil-fish. 


Heterocercal.  Homocercal. 

FIG.  187.— Fish-tails. 


The  Teleostei  are  in  many  respects  a  more  highly  organised  order 
than  the  ganoids,  of  which  they  are  the  lineal  descendants.  They 
first  appear  in  the  Cretaceous  and  include  most  existing  fishes. 


FIG.  188.  —  Showing  the  fossil  salamander-like  amphibian 
Branchiosaurus  salamandroides  (Fritsch),  twice  natural  size. 

The  Teleostei  are  characterised  by  the  presence  of  homocercal  2 
tails.  They  are  typically  represented  by  the  trout,  perch,  herring, 
cod,  mullet,  and  sole. 

The  Amphibia,3  sometimes  called  Batrachians,  are  animals  which 
begin  life  as  water-breathers,  like  fishes,  and  later  become  air- 


1  Gr.  heteros 

2  Gr.  homos 

3  Gr. 


other,  and  kerkos  =  a,  tail. 
the  same  or  whole,  and 
,  and  bios=  life. 


FOSSILS  :     OCCURRENCE,    CLASSIFICATION,    USES.        283 

breathers.  They  form  a  connecting-link  between  the  fishes  and 
reptiles. 

The  amphibians  are  represented  by  the  ancient  and  extinct  order 
of  Labyrinlhodonts,1  which  possessed  crocodile-like  bodies ;  and  by 
the  frogs,  toads,  and  newts. 

The  Reptilia  first  appeared  in  the  Carboniferous,  but  it  was  not  till 
the  Trias  that  they  became  numerous.  They  reached  their  fullest 
development  in  the  Jurassic  and  Cretaceous  epochs.  The  Mesozoic 
has  often  been  called  the  Age  of  Reptiles. 


FIG.  189. — Restoration  of  Ichthyornis  victor  (Marsh). 
From  the  Cretaceous  of  Kansas  (U.S.  Geo.  Surv.). 

Many  of  the  fossil-reptiles  assumed  grotesque  forms,  and  some 
of  them  grew  to  a  gigantic  size.  Among  the  best  known  are  the 
Palceosaurians  2  or  ancient  lizards,  of  which  the  Tuatara  (Sphenodon 
punctatum)  of  New  Zealand  is  the  sole  living  representative  ; 
Plesiosaurians*  Ichthyosaurians*  and  Deinosaurians.5 

The  Aves  or  birds  first  appeared  in  the  Mesozoic.     Many  have 

1  Gr.  labyrinthos  =  intricate,  and  odous,  odontos  =  &  tooth. 

2  Gr.  palaios=  ancient,  and  sauros  =  a,  lizard. 

3  Gr.  plesios  =  ne&T,  and  sauros  =  &  lizard. 

4  Gr.  icUhus=&  fish,  and  sauros=&  lizard. 

5  Gr.  deinos  =  terrible,  and  sauros  =  a,  lizard. 


284  A  TEXT-BOOK  OF  GEOLOGY. 

lizard-like  structure,  and  some  of  them  have  powerful  beaks  armed 
with  teeth.  Struthious  birds  of  the  ostrich,  emu,  and  moa  order 
have  been  found  in  the  Lower  Tertiary  of  Europe,  and  one  from 


FIG.  190. — Archceopteryx  macrura*     (After  Owen.) 

the  London  clay,  called  Dasornis,  is  considered  by  some  to  resemble 
the  lately  extinct  Dinornis  (moa)  of  New  Zealand. 

The  oldest  fossil-bird  is  the  Archceopteryx,  from  the  Jurassic  litho- 
graphic slaty  limestone  of  Solenhofen,  in  Bavaria. 

Mammalia. — This  sub-kingdom  includes  the  highest  class  of  the 
vertebrata,  and  is  characterised  by  the  young  being  nourished  for 


To  face  page  285.] 


FOSSILS  :     OCCURRENCE,    CLASSIFICATION,    USES.        285 

a  longer  or  shorter  time  by  milk  or  special  secretion  from  the 
mammary  glands. 

The  earliest  evidence  of  mammals  is  met  with  in  the  Upper  Trias, 
and  in  the  Lower  Jurassic  the  remains  of  small  mammals  become 
common.  All  the  earlier  forms  are  related  to  the  existing  Mar- 
supials. In  the  Pliocene  period  the  mammalian  fauna  assumes  a 
modern  appearance,  comprising  large  tiger-like  cats,  bears,  wolves, 
oxen,  numerous  antelopes,  giraffes,  deer,  horse-like  animals,  and 
elephants.  Most  of  the  mammals  of  the  Pleistocene  belong  to 
living  genera. 

The  last  group  to  appear  includes  the  apes  and  man. 

VEGETABLE  KINGDOM. 
Plants  are  divided  into  two  great  groups  : — 

I.  Cryptogamia1  or  flowerless  plants. 
II.  Phanerogamia  2  on  flowering  plants. 

The  Cryptogams  are  typically  represented  by  the  ferns,  horse-tails, 
mosses,  fungi,  diatoms,  and  algce  or  sea-weeds.  These  are  the  oldest 
and  lowest  forms  of  plant-life. 

The  Phanerogams  include  all  flowering  plants  which  bear  seeds, 
by  means  of  which  they  reproduce  themselves.  They  are  sub- 
divided into  two  groups  as  under  : — 

(1)  Gymnosperms,3  i.e.  plants  with  naked  seeds  =cycads  4  or  palms, 

and  conifer ce  or  pines. 

(2)  Angiosperms,5  i.e.  plants  with  seeds  enclosed  in  a  seed-case  or 

vessel  -oak,  walnut,  and  most  forest  trees  (except  pines)  ; 
roses  and  most  garden  plants. 

Of  these  two  groups,  the  Gymnosperms  represent  the  lowest  types 
of  flowering  plants. 

The  Angiosperms  are  divided  into  two  well-marked  and  easily 
distinguished  groups  as  follow  :— 

/  (a)  Monocotyledons,6  with  one  seed-lobe—grasses, 
.  )  cereals,  etc. 

S\(b)  Dicotyledons,''   with   two   seed-lobes  =  oaks, 
beans,  peas,  etc. 

1  Gr.  kryptos  =  hidden,  and  gamos  —  marriage. 

2  Gr.  phaneros  =  evident,  and  gamos  =  marriage. 

3  Gr.  gymnos  =  naked,  and  sperma  =  a  seed. 

4  Gr.  kuka  =  cocoa-palm. 

5  Gr.  angeios  =  vessel,  and  sperma  —a  seed. 

6  Gr.  monos  =  single,  and  kotuledon  =  seed-lobe. 

7  Gr.  di  —  double,  and  kotuledon  =  seed-lobe. 


286 


A   TEXT-BOOK    OF    GEOLOGY. 


The  Monocotyledons  usually  possess  hollow  stems,  and  increase 
in  size  by  internal  growth  and  elongation  at  the  summit,  and  hence 
are  often  called  Endogens.1 

The  Dicotyledons  possess  a  solid  stem,  and  usually  increase  in  size 
by  the  yearly  addition  of  a  new  layer  of  wood  on  the  outside,  and 
hence  are  called  Exogens.2 

The  leaves  of  the  Endogens  are  generally  distinguished  by  straight 
or  parallel  venation,  and  the  leaves  of  the  Exogens  by  reticulate  or 
net-like  venation. 

The  Palaeozoic  floras  are  mainly  Cryptogamic,  comprising  ferns, 
mosses,  algae,  and  diatoms.  The  Middle  Mesozoic  floras,  besides 


AN      IMA      L     S 

PLANTS 

1.  Age  of  Man 
(Recent  and  Pleisto- 
cene). 

2.  Age  of  Mammals 
(Oainozoic  or  Terti- 
ary). 

3.  Age  of  Reptiles 
(Mesozoic  or  Second- 
ary). 

4.  Great  Coal  Age 
(Carboniferous 
Period). 

5.  Age  of  Fishes 
(Devonian). 

(3.  Age  of  Q-raptolites  and 
Trilobites      (Silurian 
and  Ordpvician). 

7.  Eozoic  Era. 

1 

1 

1 

1 

T 

• 

^ 

' 

Dana. 

FIG.  193. 

Cryptogams,  include  numerous  coniferse  (pines)  and  cycads 
(palms).  The  Cretaceous  and  Tertiary  floras  are  characterised  by 
a  predominance  of  Phanerogams. 

The  above  diagram  approximately  illustrates  the  progress  of 
animal  and  plant-life  throughout  the  geological  record. 

Uses  of  Fossils. 

Perhaps  the  first  and  most  obvious  lesson  to  be  gleaned  from 
the  study  of  fossils  is  the  elementary  truth  that  life,  even  in  the 
earliest  times,  in  its  various  functions  and  characteristics,  differed 
in  no  way  from  the  life  of  to-day. 

1  Gr.  endon  =  within,  and  genes  =  born  or  produced. 

2  Gr.  exo  =  outside,  and  genes  =  born. 


FOSSILS:    OCCURRENCE,  CLASSIFICATION,  USES.     287 

Further,  we  observe  (a)  that  the  lowly  types  of  life  that  appear 
in  the  oldest  rocks  have  persisted  through  all  geological  times  up  to 
the  present  day  ;  (6)  that  new  genera  of  progressively  higher  types 
suddenly  appear  as  we  ascend  the  geological  scale  ;  and  (c)  that 
many  genera  have  a  limited  range  in  time. 

From  our  knowledge  of  the  distribution  and  habits  of  related 
existing  faunas  and  floras,  we  have  no  difficulty  in  distinguishing 
terrestrial  and  marine  organisms,  or  the  inhabitants  of  warm  and 
arctic  seas,  or  the  littoral  from  the  pelagic. 

Hence  the  fossils  contained  in  a  rock-formation  form  a  permanent 
record  of  the  climatic  and  physical  conditions  prevailing  at  the  time 
the  sediments  which  enclosed  them  were  being  deposited.  They  tell 
of  the  former  existence  of  continents,  rivers,  lakes,  estuaries,  and 
seas  ;  of  tropical  heat  and  arctic  cold. 

Fossils  as  Time-Registers. — As  stratified  rocks  are  composed  of 
more  or  less  parallel  layers  of  sediment  that  were  laid  down  one  after 
another,  it  follows  that  the  lower  beds  must  be  older  than  those  that 
overlie  them.  This  simple  truth  embodies  what  is  called  the  Law 
of  Superposition,  and  by  means  of  it  the  geologist  is  able  to  determine 
the  chronological  order  or  succession  of  stratified  rocks.  The  only 
exception  to  this  law  is  when  strata  have  been  inverted  by  acute 
folding. 

In  the  year^lYQO,  William  Smith,  as  the  result  of  an  examination 
of  the  Jurassic  rocks  of  West  England,  established  the  fact  that 
there  was  a  regular  order  in  the  succession  of  the  beds,  and  that 
each  bed  might  be  identified  by  its  fossils.  This  apparently 
simple  discovery  gave  a  new  direction  to  geological  investigation. 
It  laid  the  foundation  of  modern  Stratigraphical  Geology,  and 
established  a  principle  which  at  once  raised  geology  to  the  status 
of  a  science. 

Subsequent  investigation  has  shown  that  not  only  are  the  larger 
groups  of  beds  distinguished  by  particular  genera  and  species,  but 
that  particular  horizons  or  layers  may  possess  forms  that  are  limited 
to  them,  and  are  therefore  distinctively  their  own. 

The  Lias  is  now  known  to  be  divisible  into  zones,  each  character- 
ised by  one  or  more  species  of  Ammonite.  In  the  same  way  the 
Ordovician  may  be  divided  into  horizons  or  zones,  each  distinguished 
by  one  or  more  species  of  Graptolite  limited  to  it.  The  same  zonal 
distribution  of  fossils  may  be  seen  in  the  Chalk,  and  probably  the 
same  principle  prevails  throughout  all  the  geological  succession. 

When  once  the  order  of  succession  of  the  strata  in  any  region  has 
been  made  out,  the  fossils  found  in  the  different  beds  become  a 
valuable  means  of  identifying  the  same,  or  contemporaneous,  beds 
in  other  regions. 

Lithological  character  alone  is  never  a  safe  guide  for  the  identifica- 


288         A  TEXT-BOOK  OF  GEOLOGY. 

tion  or  correlation  of  distant  groups  of  stratified  rock.  A  group  of 
beds  may,  like  the  Desert  Sandstone  of  Queensland,  present  the 
same  lithological  characters  over  tens  of  thousands  of  square  miles, 
and  contain  the  same  fossils  throughout.  Frequently,  however,  as 
already  remarked  in  another  chapter,  a  sandstone  may  pass  in  the 
same  plane  into  a  shale,  and  a  shale  into  a  sandstone.  The  sand- 
stone and  shale  are  contemporaneous,  but  lithologically  they  are 
very  different  rocks.  Moreover,  the  fossils  in  these  rocks  will 
possess  the  same  general  fades,  minor  faunal  differences  that  may 
exist  being  due  to  the  different  conditions  of  deposition. 

When,  therefore,  the  chronological  succession  of  the  stratified 
rocks  of  the  globe  has  been  established,  and  the  distinctive  fossils  of 
each  group  identified,  the  fossils  become  time-registers,  by  which  the 
age  of  distant  rock-formations  may  be  determined  without  regard 
to  their  lithological  character.  In  other  words,  when  the  fossils  of 
a  rock-formation  in  a  new  region  have  been  examined,  the  geologist 
will  be  able  by  their  means  to  fix  the  age  of  the  rocks  relatively  to 
the  general  succession. 

Homotaxis. 

Investigation  has  shown  that  the  general  succession  of  animal 
and  plant  life,  throughout  geological  time,  has  been  the  same  over 
the  whole  of  the  globe.  For  this  similarity  of  succession  Huxley 
adopted  the  biological  term  homotaxis.^  For  example,  the  genera 
of  corals,  graptolites,  trilobites,  fishes,  reptiles,  brachiopods,  and 
plants  that  characterise  the  rocks  of  England,  appear  in  the  same 
general  order  in  New  Zealand  ;  but  it  does  not  necessarily  follow 
that  because  there  is  homotaxial  parallelism  that  the  groups  of 
beds  containing  the  same  fauna  in  these  distant  lands  are  chrono- 
logically contemporaneous. 

If  the  same  organic  types  appeared  simultaneously  over  the  whole 
globe,  it  is  obvious  that  all  rocks  containing  the  same  fossils  would 
be  coeval.  But  this  postulate  is  inconceivable.  It  is  more  probable 
that  particular  genera  made  their  first  appearance  in  the  Northern, 
or  in  the  Southern  Hemisphere,  and  slowly  spread  by  various  pro- 
cesses of  dispersion  from  one  hemisphere  to  the  other. 

As  would  naturally  be  expected,  the  marine  faunas  would  show 
a  closer  parallelism  than  the  floras,  a  circumstance  due  to  the 
greater  facilities  for  rapid  migration  possessed  by  marine  inhabi- 
tants in  a  continuous  sea,  compared  with  the  slower  dispersion  of 
terrestrial  organisms,  perchance  checked  by  physical  obstructions, 
such  as  wide  stretches  of  sea  and  mountain-chains. 

During  the  process  of  dispersion,  the  genera  would  to  some  extent 

1  Gr.  homos  =  the  same,  and  taxis  =  arrangement. 


FOSSILS  !     OCCURRENCE,    CLASSIFICATION,    USES.        289 

be  modified  by  accidents  of  climate  and  changes  in  the  distribution 
of  land  and  water,  arising  from  earth-movements  ;  and  the  slower 
the  rate  of  migration,  the  greater  would  be  the  differentiation. 

But  contemporaneity  is  in  some  respects  a  relative  term.  Recent 
events,  that  are  separated  by  a  year,  seem  far  apart,  while  events 
that  took  place  before  the  Christian  era  seem  close  together,  even 
when  separated  by  many  decades.  The  geological  day  is  not 
measured  by  years,  and  events  possibly  separated  by  thousands  of 
decades  of  our  limited  chronology  seem  to  converge  when  viewed 
in  the  distant  perspective  of  geological  time. 

The  marine  faunas,  on  account  of  their  greater  opportunity  for 
dispersion,  have  usually  been  taken  as  the  basis  of  comparison  and 
correlation  throughout  the  geological  record.  If  we  regard  the 
genus  as  the  organic  unit  and  not  the  species,  which  is  merely  the 
variant  arising  from  adaptation  to  local  environment,  we  cannot 
fail  to  be  impressed  with  the  extraordinary  similarity  of  the  marine 
types  existing  to-day  in  the  corresponding  latitudes  of  the  two 
hemispheres.  And  when  we. find  that  the  same  correspondence  of 
marine  types,  as  between  such  widely  separated  regions  as  Western 
Europe  and  New  Zealand,  can  be  traced  down  through  the  Pliocene, 
Miocene,  Eocene,  Cretaceous,  Jurassic,  and  Triassic  formations 
without  a  break,  or  the  interpolation  of  a  fauna  in  one  region  that 
is  not  represented  in  the  other,  we  are  forced  to  conclude  that  in 
this  portion  of  the  geological  record  there  is  little  room  for  chrono- 
logical divergence.  The  parallelism  of  the  more  primitive  Palaeo- 
zoic faunas  was  doubtless  as  close,  if  not  closer,  than  that  of  later 
ages. 

The  divergence  of  the  successive  land  faunas  of  the  two  hemi- 
spheres might  possibly  be  considerable  in  special  areas  in  view  of 
the  greater  opportunity  for  the  survival  of  ancient  types  in  regions 
isolated  by  deep  seas,  great  mountain  chains,  or  other  geographical 
barriers. 

The  great  Australian  continent,  on  account  of  its  permanency  and 
isolation,  is  pre-eminently  a  land  of  survivals.  Here  we  have  a 
remarkable  persistence  of  the  marsupials — a  primitive  type  of 
mammal — and  an  equally  ancient  type  of  flora. 

SUMMARY. 

(1)  The  remains  of  plants  and  animals  that  have  been  preserved 
in  rocks  are  called  fossils. 

(2)  Most  fossils  are  sea-shells  and  other  marine  organisms.     In 
many  rocks,  particularly  in  limestones  and  those  composed  of  fine 
sediments,  the  original  shell  or  calcareous  covering  of  the  animal  is 
preserved  ;    but  in  rocks  of  a  porous  character  the  shells  have 

19 


290  A  TEXT-BOOK  OF  GEOLOGY. 

frequently  been  dissolved  away,  leaving  only  an  external  or  internal 
cast,  or  perhaps  both. 

(3)  The  rocks  most  frequently  found  fossiliferous  are  limestones, 
clays,  marls,  shales,  and  sandstones. 

(4)  The  fossils  are  contemporaneous  with  the  sediments  or  rocks 
in  which  they  are  enclosed.     But  a  Cretaceous  rock  may  contain 
blocks  of  stone  that  enclose  Silurian  fossils.     Such  fossils  are  called 
derived  fossils . 

(5)  Igneous  rocks  do  not  contain  fossils,  but  fossiliferous  blocks  of 
stratified  rock  are  not  uncommon  among  the  fragmentary  detritus 
ejected  by  volcanoes.     Such  blocks  were  doubtless  torn  from  the 
sides  of  the  volcanic  vent. 

(6)  The  most  important  fossils  in  the  animal  kingdom,  beginning 
with  the  earliest  and  simplest  forms,  are  foraminifera,  graptolites, 
corals,  sea-lilies,  brachiopods,  molluscs,  fishes,  reptiles,  birds,  and 
mammals  ;  and  in  the  vegetable  kingdom,  ferns,  mosses,  horse-tails, 
cycads,  pines,  and  forest  trees  related  to  existing  types. 

(7)  The  chronological  succession  of  stratified  rocks  is  determined 
by  superposition.     When  the  order  of  succession  of  stratified  rocks 
has  been  determined,  the  contained  fossils  become  of  great  value 
for  the  determination  of  the  age  of  rock-formations  in  distant 
regions. 

(8)  The  faunas  and  floras  of  geological  time  appear  throughout 
the  globe  in  the  same  orderly  succession.     Although  genera  may 
appear  in  the  same  order  in  the  Northern  and  Southern  Hemi- 
spheres, it  does  not  necessarily  follow  that  they  are  contempor- 
aneous.    Time  would  be  required  for  dispersal  from  the  cradle 
where  the  new  genera  appeared.     But  when  a  great  succession  of 
strata  in  widely  separated  regions  contains  faunas  that  appear  in 
the  same  order  in  each  region,  we  seem  justified  in  assuming  that 
although   bed   for  bed  the  strata  may  not  lie   in  precisely  the 
same  time-plane,  for  all  practical  purposes  they  are  geologically 
contemporaneous. 


CHAPTER   XIX. 
CONFORMITY   AND   UNCONFORMITY. 

Conformity. — When  a  series  of  strata  has  been  laid  down  in  such 
a  way  that  the  stratification-planes  are  parallel  with  one  another, 
the  strata  are  said  to  be  conformable. 


FIG.  194. — Showing  conformable  series  of  strata. 

The  meaning  to  be  gathered  from  conformable  stratification 
is  that  the  deposition  of  the  sediments  composing  the  beds  was 
continuous  and  uninterrupted,  which  is  only  another  way  of  saying 
that  no  change  of  any  moment  took  place  in  the  physical  geography 
of  the  area  during  the  period  covered  by  the  deposition  of  the 
beds  in  question. 

The  beds  may  be  laid  down  on  a  slowly  sinking  or  rising  sea- 
floor,  and  coarse  sediments  may  be  followed  by  fine,  or  fine  by 
coarse,  due  to  the  overlap  resulting  from  an  advancing  or  receding 
shore-line,  but  the  distinctive  life  of  each  zone  will  remain  the 
same  so  long  as  the  same  physical  conditions  prevail. 

Hence  it  is  found  that  in  a  series  of  conformable  strata  there  is 
no  violent  biological  break  or  change  in  the  character  of  the  con- 
tained fauna,  always  provided  that  in  a  vast  pile  of  sediments 
representing  a  great  range  of  time,  some  of  the  older  forms  may 
disappear  before  the  invasion  of  newer  and  more  vigorous  kinds 
of  life  in  the  uppermost  strata. 

Unconformity. — When  a  series  of  conformable  strata  rests  on 

291 


292 


A   TEXT-BOOK    OF   GEOLOGY. 


the  upturned,  folded,  or  denuded  surface  of  an  older  series  of  beds, 
there  is  said  to  be  an  unconformity  between  the  two  formations. 
The  younger  series  lies  unconformably  on  the  older. 

In  fig.  195  the  younger  series,  a,  6,  c,  rests  unconformably  on  the 
upturned  edges  of  the  older  series,  d,  e,  f. 

The  meaning  to  be  gathered  from  this  relationship  is  that  the 
older  formation  was  deposited,  consolidated,  elevated,  denuded, 
and  again  submerged  before  the  younger  formation  was  laid  down. 
That  is,  the  older  area  of  deposition  was  elevated  so  as  to  form 
dry  land,  remained  dry  land  for  some  time,  and  then  became  sub- 
merged before  deposition  once  more  began. 

An  unconformity  is  therefore  an  evidence  of  a  break  in  the  con- 
tinuity of  the  geographical  conditions  which  existed  when  the  older 
formation  was  deposited.  This  break  may  represent  a  period  of 
time  of  greater  or  less  duration,  depending  on  the  rate  of  uplift, 


d  e  f 

FIG.  196. — Showing  an  unconformity. 

the  length  of  time  the  raised  sea-floor  remained  dry  land,  and  the 
rate  of  the  subsidence.  That  is,  some  unconformities  may  be 
slight,  others  very  great. 

If  the  uplift  is  slow,  the  sediments  as  they  emerge  from  the 
sea  may  be  little  disturbed  in  their  stratification  ;  and  denudation 
may  wear  away  only  the  upper  layers  before  subsidence  begins. 
In  this  case  the  unconformity  will  be  slight,  as  the  new  sediments 
will  be  laid  down  with  their  bedding  planes  almost  parallel  with 
those  of  the  older  partially  denuded  formation. 

But  if  the  uplift  is  of  long  duration,  permitting  the  land  to  be 
worn  down  by  denudation  to  a  surface  of  low  relief  before  subsidence 
takes  place,  the  newer  sediments  will  be  laid  down  on  an  approx- 
imately level  surface  of  the  older  formations.  In  this  case  the 
older  strata  will  be  separated  from  the  younger  by  a  decided 
unconformity.  When  the  older  formation  has  been  tilted  or  folded 
before  the  deposition  of  the  younger,  the  unconformity  may  be  as 
conspicuous  as  that  shown  in  fig.  195  between  points  d  and  a  ;  or 
in  fig.  197  between  b  and  a. 


CONFORMITY   AND    UNCONFORMITY.  293 

Unconformity  is  therefore  a  record  of  a  change  in  the  geographical 
conditions  in  the  area  of  deposition. 

A  physical  break,  as  might  be  expected,  is  usually  marked  by 
a  diversity  in  the  faunas  of  the  unconformable  rock-formations. 
It  is  obvious  that  the  uplift  of  the  sea-floor,  after  the  deposition 
of  the  older  sediments,  must  cause  the  migration  or  destruction 
of  the  existing  fauna. 

When  submergence  once  more  takes  place  and  the  deposition 
of  sediments  again  commences,  the  new  sea-floor  will  be  peopled 
by  colonists  from  the  neighbouring  seas.  If  the  unconformity 
is  slight,  the  incoming  fauna  will  be  the  lineal  descendants  of  the 
fauna  displaced  by  the  uplift ;  but  if  the  unconformity  is  decided, 
the  new  fauna  may  possess  little  or  no  relationship  to  the  old. 

Evidences  of  Unconformity. — The  most  obvious  proof  is  usually 
the  discordance  of  the  stratification-planes  of  the  two  formations. 
Moreover,  as  the  older  rocks  were  exposed  to  denudation  before 
the  deposition  of  the  younger  began,  fragments  and  pebbles  of  the 
older  rocks  are  frequently  found  in  the  younger. 

Beds  of  conglomerate  or  grit  in  many  cases  'form  the  bottom 
or  basal  members  of  a  rock-formation.  Hence  they  frequently 
occur  at  the  break  between  two  unconformable  formations. 

Thick  beds  of  coarse  conglomerate  interbedded  with  shales  or 
sandstones,  although  they  do  not  indicate  a  physical  break  in  the 
continuity  of  deposition,  clearly  mark  a  considerable  change  in 
the  local  geographical  conditions  arising  either  from  elevation 
or  subsidence. 

When  a  younger  series  wraps  around  the  edges  of  an  older  series, 
there  is  clear  evidence  of  unconformity,  even  though  no  actual 
contact  may  be  exposed  between  the  two  formations.  Uncon- 
formity is  also  shown  by  the  younger  series  overlapping  the  various 
members  of  the  older. 

Fault-fractures,  mineral  lodes  or  igneous  dykes  that  are  present 
in  the  lower  formation,  but  end  abruptly  at  a  given  point  of  con- 
tact, in  other  words  do  not  penetrate  the  overlying  series,  afford 
convincing  proof  of  interrupted  deposition  of  sediments,  and 
therefore  of  unconformity. 

The  fossil  fauna  of  the  different  rock-formations  is  now  so  well 
ascertained  that  the  unconformable  relationship  of  two  series  of 
strata  can  be  postulated  even  when  no  physical  break  is  apparent. 
For  example,  when  we  find  rocks  with  a  Triassic  fauna  resting  on 
Silurian  strata,  or  rocks  with  a  Tertiary  fauna  in  contact  with  a 
Cretaceous  system,  we  know  that  the  Trias  is  unconformable  to 
the  Silurian,  and  the  Tertiary  to  the  Cretaceous,  even  if  we  are 
unable  to  trace  the  physical  break  in  the  field. 

Interformational  Unconformity. — Physical  unconformity  is  some- 


294  A  TEXT-BOOK  OF  GEOLOGY. 

times  seen  in  certain  places  between  members  of  the  same  forma- 
tion. Such  discordance  may  have  arisen  from  local  uplift,  or  from 
the  wash-out  caused  by  tidal  waves,  temporary  diversion  of  sea- 
currents,  hurricanes,  or  cloud-bursts. 

Deceptive  Physical  Conformity. — The  actual  line  of  contact 
between  two  rock-formations  is  rarely  well  exposed,  being  only 
seen  to  advantage  in  sea-cliffs,  rocky  gorges,  and  quarries.  More 
often  the  junction-line  is  obscured  with  soil,  loam,  glacial  drift, 
residual  clays,  or  the  peaty  accumulations  of  forest  or  other  vegeta- 
tion. Care  must  therefore  be  exercised  in  the  determination  of 
the  physical  relationships  of  two  adjoining  formations,  and  in  no 
case  should  final  pronouncement  be  made  on  the  evidence  of  one 
exposure. 

Every  formation  represented,  in  the  geological  record  is  dis- 
tinguished by  its  own  peculiar  assemblage  of  fossil-remains,  by 
means  of  which  it  can  always  be  recognised,  however  complicated 
and  obscure  its  stratigraphical  relationships  may  be.  Hence  in 
the  determination  of  relationships  the  palseontological  evidence  is 
of  supreme  importance. 

Mere  parallelism  of  the  stratification-planes  of  two  formations 
does  not  necessarily  imply  conformity.  It  may  easily  happen  in 
the  case  of  two  systems  not  separated  by  a  great  interval  of  time, 
that  steady  uplift  of  the  older  to  a  height  not  far  above  sea-level 
in  a  region  of  quiet  denudation,  followed  by  steady  subsidence, 
may  result  in  the  deposition  of  the  younger  in  layers  that  rest 
on  the  older  with  apparent  parallelism.  Such  deceptive  conformity 
is  found  in  South-East  England  and  at  Waipara,  New  Zealand, 
in  Lower  Egypt,  as  between  the  Cretaceous  and  Eocene  ;  in  Ireland 
as  between  the  horizontal  Trias  and  Upper  Cretaceous  near  Belfast  ; 
and  in  South  Africa,  as  between  the  Ecca  Beds  of  Upper  Carboni- 
ferous age  and  the  Lower  Cretaceous,  as  exposed  at  Worcester,  in 
Cape  Colony. 

On  the  east  side  of  the  Libyan  basin,  the  outcrops  of  the  Upper 
Cretaceous  and  Lower  Eocene  limestones  may  be  traced  for  scores 
of  miles  running  parallel  with  one  another  and  with  the  underlying 
quartzose  Nubian  Sandstone.  At  many  places  the  inclination 
of  the  Cretaceous  and  Eocene  is  so  nearly  the  same  that  no  physical 
break  can  be  detected  between  the  two  systems  ;  while  at  other 
places  the  unconformity  is  quite  distinct.  Nevertheless  the 
palaeontological  break  is  everywhere  great. 

It  is  obvious  that  when  subsidence  took  place  the  old  Cretaceous 
basin  became  an  Eocene  basin.  When  the  tilting  of  strata  takes 
place  after  the  deposition  of  the  younger  beds,  there  may  be  little 
or  no  apparent  stratigraphical  break,  except  in  places  where  the 
Cretaceous  strata  have  been  subject  to  considerable  denudation. 


CONFORMITY   AND    UNCONFORMITY. 


295 


In  fig.  196  Tertiary  beds  are  seen  resting  on  the  Cretaceous. 
As  viewed  near  C  and  along  section  A — B,  the  two  systems  appear 


Plan 


B 


FIG.  196. — Plan  and  sections  showing  deceptive  conformity. 

to  be  conformable,  and  the  true  relationship  is  only  disclosed  when 
the  whole  section  from  C  to  D  is  examined. 


FIG.  197. — Showing  deceptive  conformity  of  two  unconformable 
rock-formations. 

Thus  we  see  that  the  physical  break  between  two  systems  may 
not  be  everywhere  equally  marked,  and  in  some  localities  its 
detection  may  be  impossible. 

Moreover,  there  may  be  apparent  conformity  in  places  arising 
from  accidents  of  folding  or  form  of  denudation.  For  example, 


296  A  TEXT-BOOK  OF  GEOLOGY. 

in  fig.  197  beds  a  are  highly  unconformable  to  beds  b  between 
points  b  and  d,  but  between  d  and  c  there  is  apparent  conformity. 

Many  notable  examples  of  deceptive  conformity  are  found  in 
regions  where  orogenic  or  mountain-building  movements  have 
thrust  the  rock-formations  into  great  folds.  All  the  stratified 
rocks  involved  in  such  folds  are  tilted  so  as  to  run  parallel  with 
the  main  axes  of  elevation,  and  in  this  way  a  parallelism  of  strati- 
fication is  obtained  even  among  rocks  of  the  most  diverse  ages. 
In  this  way  Triassic  or  even  younger  formations  may  appear 
conformable  to  older  Palaeozoic  rocks. 

In  the  alpine  chain  of  New  Zealand,  the  Trias  rocks  are  over- 
folded  so  as  to  be  parallel  with,  and  to  appear  conformable  to,  the 
underlying  schists  and  gneisses  of  Cambrian  age  ;  while  in  the 
Bernese  Oberland,  on  the  west  side  of  the  Jungfrau,  the  strips  of 


N.E 

FIG.  198. — Showing  deceptive  conformity  due  to  involvement  of  Tertiaries 

among  older  Palaeozoic  mica -schists,  Moonlight  Creek,  New  Zealand. 

(a)  Mica-schist.  (6)  Tertiaries. 

Eocene  nummulitic  limestone  appear  to  be  conformable  to  the 
Malm  or  White  Jura  in  which  they  are  infolded.  Similarly  the 
Lower  Tertiaries  infolded  among  the  mica-schists  at  Moonlight 
Creek  in  New  Zealand  possess  for  many  miles  the  same  strike 
and  dip  as  the  older  rocks,  everywhere  exhibiting  a  striking  example 
of  deceptive  conformity,  as  shown  in  fig.  198. 

Value  of  Unconformities.— We  have  already  found  that  a  great 
succession  of  strata  with  its  contained  fossils  is  a  record  or  history 
of  the  sea  in  the  particular  area  in  which  the  deposition  took  place. 
It  proves  that  deposition  was  continuous,  hence  the  chronological 
importance  of  such  a  succession. 

But  an  unconformity  is  no  less  valuable.  It  tells  us  of  a  break 
in  the  continuity  of  deposition  arising  from  the  uplift  of  the  sea- 
floor  whereby  the  previously  existing  area  of  denudation  was 
augmented.  It  fixes  the  dates  of  earth-movements,  and  enables 
us  to  outline  approximately  the  form  of  the  land-areas  and  sea- 
margins  in  past  geological  times. 


CONFORMITY   AND    UNCONFORMITY.  297 

The  obvious  effect  of  the  uplift  of  the  sea-floor  with  its  newly 
formed  sediments  will  be  to  increase  the  previously  existing  area 
of  dry  land,  thereby  increasing  the  area  of  denudation.  Denudation 
will  still  be  as  active  as  ever,  and  its  products  will  be  deposited 
on  the  new  sea-margin  which,  before  the  uplift,  may  have  been 
the  floor  of  a  deep  or  shallow  sea. 

From  this  we  gather  that  unconformities  are  not  necessarily 
world- wide,  for  though  uplift  may  cause  deposition  to  cease  in  a 
particular  area,  its  ultimate  effect  is  merely  to  shift  the  scene  of 
deposition  to  some  other  portion  of  the  sea-floor.  Therefore  by 
tabulating  the  various  series  of  strata  and  the  unconformities 
in  some  island  or  continent,  we  are  able  to  tell  when  that  area  was 
submerged  and  when  it  was  dry  land.  The  breaks,  or  lost  chapters, 
as  unconformities  have  been  aptly  called,  can  only  be  filled  in  by 
the  study  of  some  area  where  deposition  was  continuous. 

In  some  continents  the  geological  record  is  almost  complete  ; 
in  others  it  is  broken  and  imperfect.  A  full  record  tells  us  that  the 
area  was  marginal  to  some  land  of  great  permanence  that  may 
have  been  subject  to  oscillations,  but  was  never  completely  sub- 
merged, being  an  area  of  denudation,  though  perhaps  of  constantly 
varying  form,  from  the  earliest  geological  times. 

A  region  containing  an  imperfect  record  is  an  area  of  still  greater 
permanence,  for  its  persistence  as  dry  land  is  the  main  cause  of 
the  imperfection  of  the  record,  for  obviously  while  it  remained 
dry  land  it  could  not  be  an  area  of  deposition. 

But  all  portions  of  a  continent  are  not  equally  stable,  as  shown 
by  the  circumstance  that  the  record  may  be  comparatively  com- 
plete in  one  portion  and  scanty  in  another. 

Changes  of  Life  during  Geological  Time.— It  is  sometimes  found 
that  the  same  or  closely  related  types  persist  through  a  considerable 
thickness  of  strata,  which  may  be  partly  due  to  the  rapid  accumula- 
tion of  the  sediments,  and  partly  to  the  continuance  of  the  same 
physical  conditions  of  deposition. 

In  other  cases  changes  in  the  fauna  occur  in  every  few  feet  of 
rock,  which  would  tend  to  show  that  the  rate  of  deposition  was 
relatively  slow  compared  with  the  organic  changes  in  the  fauna. 
This  rapid  change  of  faunal  types  is  met  with  in  many  fine-grained 
shales  and  in  certain  cherts,  all  of  which  would  appear  to  have 
accumulated  slowly  in  deep  water  far  from  land. 

In  a  pile  of  conformable  strata  consisting  of  basal  conglomer- 
ates followed  by  sandstones,  clays,  and  limestone,  the  basal  rocks 
will  be  distinguished  by  the  prevalence  of  littoral  shells.  If  a 
band  of  conglomerate  follows  the  clays,  there  will  in  all  prob- 
ability be  a  reappearance  of  the  prevalent  types  of  the  lower  con- 
glomerate ;  and  if  this  conglomerate  is  followed  by  clays,  these 


298  A  TEXT-BOOK  OF  GEOLOGY. 

will  probably  contain  many  of  the  dominant  types  of  the  lower 
clays.  In  other  words,  with  a  recurrence  of  the  same  conditions  of 
deposition,  there  is  frequently  a  reappearance  of  the  same  faunal 
types  by  migration  from  the  neighbouring  seas.  Just  how  much 
these  types  are  modified  will  depend  on  the  lapse  of  time  repre- 
sented by  the  intervening  strata. 

Some  forms  of  life,  like  the  Nautilus  and  Shark,  have  a  great 
vertical  range  in  geological  time  ;  while  others,  like  many  species  of 
Ammonites  and  Belemnites,  have  a  wide  geographical  distribution 
in  some  particular  horizon  or  time-plane,  but  a  limited  vertical 
range. 

The  chronological  classification  of  the  stratified  rock-formations 
is  mainly  based  on  the  interpretation  of  the  physical  breaks  and 
the  progressive  organic  changes  observed  throughout  the  geological 
record.  The  rocks  are  the  monuments,  and  the  fossils  the  hiero- 
glyphics, by  means  of  which  the  geologist  is  enabled  to  divide  the 
history  of  the  Earth  into  eras,  periods,  and  epochs,  and  to  unravel 
the  successive  physical  and  organic  changes  that  have  taken  place 
since  the  beginning  of  geological  time. 

Permanence  of  Continents. — It  has  now  been  established  as 
geological  axioms  : — 

(a)  That  stratified  rocks  are  composed  of  detrital  material 
derived  from  the  denudation  of  land  areas  ; 

(6)  That  stratified  rocks  are  marginal  to  the  land  from  which  the 
material  composing  them  was  derived. 

The  obvious  inference  to  be  drawn  from  these  simple  truths 
is,  that  the  continents,  though  constantly  varying  in  size,  shape, 
and  height,  through  subsidence  and  uplift,  have  existed  as  land 
areas  from  a  remote  geological  period. 

It  would  almost  appear  as  if  the  present  continents  and  deep 
seas  were  developed  by  the  first  crumpling  of  the  Earth's  crust, 
arising  from  cooling  and  contraction,  at  some  period  long  antecedent 
to  the  formation  of  the  oldest  known  stratified  rocks. 

The  continents  have  been  subject  to  denudation  throughout  all 
time,  and  the  younger  stratified  formations  have  been  derived 
from  the  waste  of  the  older.  The  same  material  has  appeared 
re-sorted  in  different  rock-formations  in  different  geological  ages. 
The  continents  have  been  wasted  while  being  reconstructed,  and 
have  thereby  been  preserved  from  destruction  by  the  continual 
accumulation  of  new  material  supplied  from  their  own  ruin.  In 
this  way  they  have  maintained  their  individuality. 

The  preservation  of  the  continents  has  been  solely  dependent 
on  oscillations  of  the  land.  For  it  is  obvious  that  if  the  continents 
had  remained  stationary,  let  us  say,  since  the  close  of  the  Palaeozoic, 


CONFORMITY   AND    UNCONFORMITY.  299 

neither  rising  nor  sinking,  they  would  in  the  course  of  time  have 
been  reduced  to  a  plain  of  marine  denudation.  They  would 
eventually  have  disappeared  beneath  the  surface  of  the  sea,  and 
ceased  to  exist  as  land-areas.  Deposition  would  then  have  come 
to  an  end,  and  from  then  onward  there  would  be  a  complete  blank 
in  the  geological  succession,  and  a  cessation  in  the  progressive 
development  of  all  animal  and  vegetable  life. 

Uplift  and  denudation  are  doubtless  responsible  for  the  incom- 
pleteness of  the  geological  record,  but  it  is  certain  that  without 
continual  oscillation  and  deposition  there  could  be  no  succession 
of  stratified  rocks  and  therefore  no  record  of  organic  life. 

SUMMARY. 

(1)  When  sediments  are  laid  down  on  the  floor  of  the  sea  or  a 
lake  in  layers  that  lie  parallel  with  one  another,  they  are  said  to 
be  conformable.     The  bottom  layers  will  be  older  than  the  upper, 
but  there  will  be  no  break  in  the  continuity  of  the  succession. 

(2)  When  a  series  of  sediments  is  uplifted  so  as  to  become  dry 
land   and   is   subjected   to    denudation   in   such   a  way   that   its 
surface  becomes  worn  into  hollows  and  ridges,  eventually  submerged 
and  then  covered  with  a  succession  of  sediments,  the  new  sediments 
are  said  to  be  unconformdble  to  the  older  underlying  series. 

When  the  older  series  of  beds  is  not  only  eroded  but  also  tilted 
by  Earth-movements  before  the  younger  series  is  laid  down  on  it, 
the  unconformity  is  considerable,  and  probably  represents  a  long 
interval  of  time  between  the  deposition  of  the  two  series. 

(3)  An  unconformity  marks  a  physical  break  in  the  continuity 
of  the  local  conditions  of  deposition.     The  uplift  which  preceded 
it  causes  a  migration  of  the  marine  fauna  to  adjacent  seas.     If 
the  uplift  lasted  a  considerable  time,  there  may  be  recognisable 
change  in  the  character  of  the  fauna  when  submergence  once  more 
permits  deposition  to  begin  in  that  area. 

(4)  All  stratified  rocks  are  marginal  to  continental  areas,  and  all 
are  composed  of  material  derived  from  the  waste  of  older  rocks. 
From  this  it  is  inferred  that  continents,  though  constantly  varying 
in   size   and  shape    due    to   oscillations,  have   existed    from  the 
remotest  geological  times. 


PART   II. 


CHAPTEE   XX. 
HISTORY   OF   THE   EARTH. 

Division  of  Geological  Record. 

WHEN  studying  an  ancient  language,  the  student  as  a  first  step 
must  acquire  a  knowledge  of  the  form  of  the  written  characters, 
of  the  significance  of  each  character  standing  by  itself,  and  of  the 
meaning  to  be  attached  to  a  number  of  the  characters  when  placed 
together.  And  so  it  is  in  geology.  The  study  of  rocks  and  the 
geological  processes  involved  in  the  formation  of  rocks ;  of  fossils, 
and  the  preservation  of  fossils,  is  the  preliminary  but  necessary 
preparation  that  must  be  undertaken  before  we  can  successfully 
read  the  past  history  of  the  Earth  as  presented  in  the  geological 
record.  In  geology  the  rocks  are  the  monuments,  the  fossils  the 
records  ;  and  when  we  have  acquired  a  working  knowledge  of  the 
A  B  C  of  the  science,  we  are  able  to  interpret  the  writing  which 
unfolds  a  fascinating  story  of  sunshine  and  shower,  of  brooks  and 
rivers,  of  lakes  and  seas,  of  jungle  and  forest,  of  deserts  and  swamps, 
of  volcanoes  and  earthquakes.  Moreover,  we  further  discover  the 
wonderful  procession  of  life  that  has  peopled  and  clothed  the  Earth 
throughout  the  geological  ages. 

The  geological  history  of  the  Earth  from  the  earliest  times  is  a 
record  of  uplift  and  subsidence,  of  retreating  and  advancing  seas, 
of  denudation  and  deposition. 

During  subsidence  the  sea  advanced  on  the  land,  and  the 
conditions  of  deposition  that  prevailed  were  marine.  During 
uplift  the  sea  retreated,  and  large  inland  seas  and  basins 
were  enclosed,  and  in  these  the  conditions  of  deposition  were 
lacustrine  or  terrestrial.  Frequent  alternations  of  uplift  and 
subsidence,  commonly  spoken  of  as  oscillations  of  the  land,  often 
led  to  the  deposition  of  alternating  marine  and  terrestrial  beds. 

Uplift  increased  the  size  of  the  continents  and  consequently 
augmented  the  area  of  land  exposed  to  denudation.  Conversely, 

300 


DIVISION    OF    GEOLOGICAL   RECORD.  301 

subsidence  diminished  the  area  of  the  dry  land  exposed  to 
denudation. 

The  Gaps  in  the  Record. — The  history  of  the  Earth  must  be  read 
from  the  story  of  the  rocks  and  their  fossil  contents  in  the  same  way 
as  ancient  Egyptian  history  is  interpreted  from  the  different 
types  of  sculpture  and  pottery  buried  in  the  successive  layers  of 
debris  that  cover  the  sites  of  the  ancient  cities  and  temples.  On 
some  sites  we  find  evidence  of  unbroken  occupation  through  a 
long  succession  of  dynasties,  in  others  there  are  wide  gaps  that 
mark  periods  of  desertion  and  ruin.  And  so  it  is  with  the  geological 
record.  In  some  regions  the  succession  is  relatively  complete, 
in  others  it  is  fragmentary  and  full  of  gaps.  In  most  continents 
the  geological  record  is  incomplete,  but  fortunately  the  gaps  in 
the  different  regions  do  not  always  coincide,  or  occur  in  the  same 
place  in  the  succession  ;  hence  we  are  able  by  a  bit  of  patching 
to  build  up  a  record  that,  while  admittedly  imperfect,  nevertheless 
affords  a  valuable  synopsis  of  the  physical  geography  and  life  of 
the  Earth  from  the  remotest  times. 

Before  we  proceed  further,  let  us  clearly  understand  what  is 
meant  by  gaps  in  the  stratigraphical  succession.  In  one  part 
of  a  continent,  the  Palaeozoic  formations  may  be  well  represented 
and  followed  directly  by  the  Tertiary  formations  resting  on  a  highly 
denuded  surface  of  the  older  rocks.  Here  we  have  a  gap  or  un- 
conformity representing  the  whole  of  the  Mesozoic  era  ;  and  from 
this  we  gather  that  one  of  two  things  has  happened.  Either  the 
sea-floor  in  this  region  was  uplifted  after  the  close  of  the  Palaeozoic 
and  remained  dry  land  throughout  the  whole  Mesozoic  era,  thus 
preventing  the  deposition  of  sediments,  or  else  deposition  was 
continuous  for  a  portion  of  the  Mesozoic,  but  the  sediments  thus 
formed  were  swept  away  by  denudation  during  an  interval  of 
uplift  before  the  deposition  of  the  Tertiaries  began.  So  far  as 
the  geological  record  is  concerned,  the  result  is  a  complete  blank 
from  the  close  of  the  Palaeozoic  to  the  beginning  of  the  Tertiary 
era. 

In  another  portion  of  the  same  continent  we  may  find  the  Palaeo- 
zoic rocks  followed  in  orderly  succession  by  all  the  Mesozoic 
formations ;  hence,  when  we  make  up  the  stratigraphic  suc- 
cession for  the  whole  continent,  we  are  able  to  show  a  complete 
record. 

The  gaps  in  the  stratigraphical  succession  in  any  given  region 
are  due  either  to  sweeping  denudation  prior  to  the  deposition  of 
the  younger  unconformable  strata,  or  to  uplift,  which  prevented 
the  deposition  of  sediments. 

Uplift  does  not  always  take  place  at  a  uniform  rate  over  a  whole 
continent.  One  border  may  rise  more  rapidly  than  another, 


302  A  TEXT-BOOK  OF  GEOLOGY. 

thereby  affording  an  example  of  what  is  called  differential  uplift. 
Or  one  side  of  a  continent  may  rise  and  the  other  sink,  the  move- 
ment resembling  the  tilting  of  a  plank  laid  across  a  beam.  In  this 
case  sediments  will  be  deposited  on  the  sinking  sea-floor  ;  while, 
on  the  uplifted  side,  not  only  will  there  be  no  deposition  of  sediments, 
but  the  strata  newly  raised  from  the  sea  will  be  worn  away,  thereby 
accentuating  and  widening  the  gap  that  will  exist  before  subsidence 
once  more  permits  deposition  to  take  place  in  that  area. 

Deposition  of  sediments  has  been  continuous  around  the  shores 
of  the  continents  ever  since  they  came  into  existence  ;  but  the 
sediments  have  not  accumulated  as  a  continuous  pile  in  any  one 
place  on  account  of  the  frequent  oscillations  of  the  land. 

Uplift  does  not  cause  a  complete  cessation  of  deposition  every- 
where, for  it  is  obvious  that  while  dry  land  and  seas  exist,  the  pro- 
ducts of  denudation  must  be  carried  to  the  sea.  Therefore, 
although  uplift  may  cause  a  cessation  of  deposition  in  one  place, 
its  general  effect  is  merely  to  shift  the  scene  of  deposition  to  the 
adjacent  seas.  Hence  it  is  that  gaps  in  the  succession  in  one 
place  are  represented  by  sediments  laid  down  in  some  other  area. 
But  the  sediments  that  should  fill  the  gaps  have  not  always 
been  preserved,  or  if  preserved,  they  are  not  accessible.  In  some 
regions  they  have  been  removed  by  denudation,  in  others  they 
have  become  obscured  in  earth-folds  or  submerged  beneath  the 
sea.  Hence  it  happens  that  with  all  the  patching  that  research 
has  made  possible,  there  still  remain  many  gaps  in  the  stratigraphical 
succession  that  cannot  be  filled. 

Even  if  the  stratigraphical  succession  were  complete,  it  is  certain 
that  the  record  of  life  contained  in  the  rocks  would  still  be  imperfect, 
for  we  know  that  only  a  small  proportion  of  the  organisms  that 
lived  in  past  times  have  been  preserved  as  fossils.  Our  knowledge 
of  the  marine  faunas  is  very  imperfect,  and  of  the  land  faunas, 
meagre  and  fragmentary,  the  opportunity  for  preservation  of 
terrestrial  animals  being  small  compared  with  that  of  organisms 
living  in  the  sea. 

Unconformities  represent  gaps  in  the  succession  of  stratified  rocks 
during  which  there  is  no  record  of  the  contemporaneous  fauna  and 
flora.  An  unconformity  is  merely  a  lapse  of  time  of  which  there 
is  no  local  record.  It  does  not  measure  the  interval,  the  duration 
of  which  can  only  be  demonstrated  by  the  fossil  evidence. 

When  the  characteristic  fossils  of  the  geological  record  have 
once  been  determined,  the  fossil  evidence  may  prove  the  existence 
of  gaps  where  they  are  not  physically  apparent.  That  is,  deceptive 
conformity  can  frequently  be  proved  by  the  fossil  evidence. 

The  Geological  Record. — Superposition  is  the  only  basis  of 
stratigraphical  succession. 


DIVISION   OF   GEOLOGICAL   RECORD.  303 

The  order  in  which  the  different  layers  of  debris  occur  on  the 
site  of  a  buried  city  is  of  greater  importance  than  the  remains  of 
pottery  and  works  of  art  found  in  each  layer,  for  obviously  it  is 
only  after  the  proper  order  of  succession  of  the  different  layers 
has  been  ascertained  and  verified  that  the  contents  become  of 
chronological  value.  When  the  order  of  stratigraphical  succession 
of  a  pile  of  strata  has  been  definitely  ascertained,  and  the  character- 
istic fossils  of  the  different  beds  determined,  the  fossils  at  once 
possess  a  chronological  value  and  become  useful  for  the  fixing 
of  the  age  of  strata  in  distant  regions. 

It  is  now  known  that  certain  fossils  occur  only  in  certain  groups 
of  beds,  and  advantage  has  been  taken  of  this  truth  to  divide  the 
geological  record  into  eras,  periods,  and  epochs,  in  the  same  way  as 
historic  time  is  divided  into  empires,  dynasties,  and  reigns,  or  as  a 
book  is  divided  into  chapters,  paragraphs,  and  sentences.  It  is 
well  to  remember  that  the  subdivision  of  geological  time  is  only 
an  empirical  arrangement  intended  to  facilitate  the  study  and 
investigation  of  the  past  history  of  the  Earth  as  revealed  by  the 
stratified  rocks  and  their  fossils. 

Stratified  rocks  are  arranged  in  groups  which  are  subdivided 
into  systems  which  in  turn  consist  of  series.  In  many  cases  a 
series  is  divided  into  stages,  i.e.  upper,  middle,  and  lower  divisions  ; 
and  sometimes  a  stage  is  found  to  consist  of  recognisable  zones, 
each  characterised  by  distinctive  fossils  limited  to  it. 

The  equivalent  divisions  of  time  corresponding  to  groups, 
systems,  series,  etc.,  are  as  follow  :  — 


Analogy.  Strata.  Example. 

Book,  .  Era       =  Group,  e.g.  Mesozoic  Group. 

Chapter,      .  Period  =  System,  e.g.  Cretaceous  System. 

Paragraph,  .  Epoch  =  Series,  e.g.  Chalk. 

Sentence,     .  Age      =  Stage,  e.g.  Upper  Chalk. 

Line,   .         .  Stage  =  Zone,  e.g.  Zone  of  Belemnitella  mucronata. 

When  we  speak  of  the  Cretaceous  Period  we  refer  to  a  particular 
interval  of  geological  time  ;  but  when  we  speak  of  the  Cretaceous 
System  we  have  in  mind  the  assemblage  of  strata  formed  in  the 
Cretaceous  Period. 

Geological  time  is  divided  into  four  grand  eras  or  books,  which 
are  separated  by  unconformities  or  by  great  palseontological 
changes  in  the  fauna  and  flora.  Each  book  is  subdivided  into 
chapters  or  periods,  and  each  chapter  into  paragraphs  or  epochs. 
In  each  paragraph  there  may  be  one,  two,  or  more  sentences  or 
ages,  and  each  sentence  may  consist  of  one  or  more  lines,  i.e.  time- 
planes  or  stages. 


304 


A    TEXT-BOOK    OF    GEOLOGY. 


Epoch. 


Cainozoic  or 
Tertiary. 


Mesozoic  or 
Secondary. 


Palaeozoic  or 
Primary. 


Eozoic  or 
Archaean. 


Period. 
Recent. 
Pleistocene. 
Pliocene. 
Miocene. 
Oligocene. 
^Eocene. 

PalcBontological  Break* 
f  Cretaceous. 

<  Jurassic. 
I^Triassic. 

Palceontological  Break. 
'Permian. 

Carboniferous. 

Devonian. 

Silurian. 

Ordovician. 
^Cambrian. 

Unconformity. 
("Algonkian  or  Torridonian. 

<  Unconformity. 
(^Laurentian  or  Lewisian. 


Such  terms  as  Permian,  Devonian,  etc.,  are  time-names  and 
cover  vast  aeons.  When  a  rock  is  said  to  be  of  Miocene  age,  a 
reference  to  the  table  will  show  that  it  is  comparatively  young  ; 
whereas  a  rock  of  Silurian  age  is  one  of  great  antiquity. 

Some  of  the  names  of  the  periods  are  lithological,  as  Cretaceous 
and  Carboniferous  ;  some  have  a  numerical  origin  like  Trias  ; 
but  the  majority  are  derived  from  the  names  of  the  localities  or 
regions  where  the  rocks  of  that  particular  age  are  typically  developed. 
The  last,  which  are  the  best  adapted  for  general  use,  comprise 
Cambrian,  Silurian,  Devonian,  Permian,  and  Jurassic,  which  are 
names  generally  adopted  by  all  geologists.  But  whatever  their 
origin,  it  must  always  be  remembered  that  these  names  have  no 
lithological  significance.  The  Silurian  period,  for  example,  is 
merely  an  interval  of  time,  and  the  rocks  ascribed  to  it  in  one 
place  may  consist  of  conglomerates,  sandstones,  and  shales ;  in 
another  of  limestones,  shales,  etc. 

Further,  the  periods  do  not  represent  equal  intervals  of  time 
any  more  than  the  reigns  of  the  kings  in  history. 

Geological  time  cannot  be  measured  in  years.  All  attempts 
to  gauge  the  age  of  the  Earth  since  it  became  habitable  on  the  basis 
of  the  rate  of  deposition  of  sediments  in  deltas  and  estuaries  have 


DIVISION    OF    GEOLOGICAL    RECORD.  305 

ended  in  failure.  All  that  can  be  safely  hazarded  is  that  the 
Tertiary  era  may  cover  several  million  years,  and  the  whole  geo- 
logical record  perhaps  scores  of  millions. 

Summary. 

(1)  The  primary  object  of  the  study  of  rocks,  fossils,  and  geologi- 
cal processes  is  to  enable  the  student  to  unravel  the  past  history 
of  the  Earth. 

(2)  The  unconformities  or  gaps  in  the  geological  record    are 
intervals  not  represented  by  sediments.     The  gaps  may  be  due  to 
uplift,  or  to  denudation,  or  to  both.     When  no  sediments  are  laid 
down,  there  is  no  record  of  the  fauna  or  flora  of  the  interval  covered 
by  the  unconformity  ;    consequently  stratigraphical  unconformity 
is  usually  marked  by  a  palseontological  break. 

Unconformities  do  not  measure  time.  The  intervals  they 
represent  can  only  be  estimated  in  a  relative  way  from  the  extent 
of  the  break  in  the  succession  of  life. 

(3)  Superposition   is    the    only    true    basis    of    stratigraphical 
succession.     When  the  order  in  which  the  rock-formations  occur 
has  once  been  determined,  the  fossils  contained  in  them  become 
of  great  value  in  fixing  the  age  of  rocks  in  distant  regions.     But 
since  the  succession  of  organic  types  throughout  the  geological 
record  has  been  definitely  ascertained  and  is  the  same  in  all  parts 
of  the  globe,  fossils  are  of  great  value  in  fixing  the  age  of  strata 
wherever  they  occur,  or  however  involved  they  may  be  in  crustal 
folds. 

(4)  The  different  groups  of  strata  are  characterised  by  certain 
distinctive  fossils,  and  advantage  has  been  taken  of  this  to  sub- 
divide geological  time  into  eras,  periods,  and  epochs. 

(5)  The  periods  are  not  of  equal  length  any  more  than  the  reigns 
of  the  kings  of  history.     Moreover,  the  time  that  has  elapsed  since 
the  Earth  became  habitable  cannot  be  estimated  in  years,  but  is 
vast  and  may  possibly  amount  to  many  score  million  years. 


20 


CHAPTER   XXL 
EOZOIC1  ERA. 

THIS  group  includes  all  the  rocks  of  pre-Cambrian  age  that  reach 
the  surface  or  have  been  laid  bare  by  denudation. 

The  Eozoic  rocks  are  not  only  the  oldest  and  thickest,  but  also 
the  most  widespread  of  all  the  rock-formations  taking  part  in 
the  structure  of  the  Earth's  crust.  Even  where  not  exposed  at 
the  surface,  it  may  safely  be  assumed  that  they  form  the  basement 
on  which  all  the  younger  formations  rest. 

In  North  America  these  ancient  rocks  occupy  an  area  of  nearly 
2,000,000  square  miles,  and  have  an  estimated  thickness  of  over 
50,000  feet.  Elsewhere  they  occur  in  numerous  isolated  patches, 
some  of  considerable  extent  in  the  British  Isles,  Scandinavia, 
Bohemia,  Alps,  Himalayas,  China,  Andes,  Australia,  and  South 
Africa. 

The  distinguishing  features  of  these  primitive  rocks  are  : — 

(a)  Their  universal  extent. 

(b)  Their  vast  thickness. 

(c)  Their  highly  metamorphic  character. 

(d)  Their  poverty  in  fossils. 

(e)  Their  richness  in  valuable  ores  and  minerals. 

The  time  covered  by  the  formation  of  the  pre-Cambrian  systems 
must  have  been  of  extraordinary  length,  probably  as  long  as  the 
Palseozic,  Mesozoic,  and  Cainozoic  eras  put  together. 

In  the  Lake  Superior  region  of  North  America  where  the  Eozoic 
has  its  greatest  and  perhaps  most  typical  development,  the  succes- 
sion recognised  by  American  geologists  is  as  follows,  the  name 
Archaean  being  restricted  to  the  lower  highly  crystalline  complex 
of  altered  rocks  : — 

1  Gr.  eos  =  the  dawn,  and  zoe  —  liie. 
306 


EOZOIC   ERA.  307 

Pre-Cambrian. 

(1)  Keweenawan  (Copper-bearing  series). 
Unconformity. 

Upper. 
Algonkian     J,      (2)  Huronian  j      ^Unconformity. 

Unconformity. 
Lower. 

f  Unconformity. 

A    ,  (3)  Keewatin. 

Eruptive  unconformity. 
[      (4)  Laurentian. 

Archaean.1 — The  greatest  and  perhaps  best  known  develop- 
ment of  rocks  of  this  age,  occurs  in  the  Laurentian  region;  but 
they  are  well  represented  in  the  British  Isles,  and  in  all  the  great 
continents. 

Characteristically  they  consist  of  granites,  gneisses,  and  various 
schists,  with  which  are  sometimes  associated  various  clastic  and 
pyroclastic  rocks,  usually  highly  altered.  No  trace  of  organic 
remains  has  ever  been  found  in  them,  and  this  circumstance  led 
the  geologists  of  last  century  to  call  them  Azoic,2  a  term  at  one 
time  loosely  applied  to  all  rocks  older  than  the  Palseozoic. 

LAURENTIAN  REGION. 

The  Archaean  rocks  of  this  region  are  easily  divisible  into  two 
great  formations,  namely,  the  Keewatin  schist  formation  and  the 
Laurentian  granitic  and  gneissic  complex,  which  is  in  some  places 
intrusive  in  the  rocks  of  the  Keewatin,  but  does  not  reach  into  the 
Cambrian. 

Keewatin. — The  lower  Archaean  of  North  America  consists 
mainly  of  crystalline  schists  resulting  from  the  metamorphism  of 
igneous  rocks  that  would  seem  to  have  been  principally  surface 
flows,  tuffs,  and  pyroclastic  sediments.  Associated  with  the  altered 
lavas  and  pyroclastics,  there  are  subordinate  beds  of  conglomerate 
and  shale,  which  are  the  oldest  sedimentary  rocks  of  which  there 
is  any  record. 

Intercalated  with  the  aqueous  rocks,  there  are  lenticular  beds  of 
jasper  and  iron  ore. 

The  more  abundant  metamorphic  rocks  are  hornblende-schist, 
greenstone-schist,  and  mica-schist,  which  are  everywhere  sharply 

1  Gr.  archaios  =  very  old. 

2  Gr.  a  =  without,  and  zoe  =  life. 


308 


A   TEXT-BOOK    OF   GEOLOGY. 


folded,  plicated,  and  sheared.  They  have  been  at  many  different 
times  intruded  by  plutonic  igneous  rocks  on  a  scale  not  equalled 
in  any  other  period  of  the  geological  record. 

The  thickness  of  the  Keewatin  system  amounts  to  many  thousand 
feet,  but  the  rocks  are  so  complicated  with  folding,  shearing,  and 
intrusives  that  it  is  difficult  to  make  a  trustworthy  estimate. 

The  massive  surface  flows  and  tuffs  of  which  the  schists  of  this 
period  are  mainly  composed  indicate  the  existence  of  older  rocks 
below  them ;  and  the  presence  of  sedimentary  rocks  implies 
denudation  and  contemporaneous  deposition.  Of  the  extent  of 
the  pre- Archaean  land  over  which  the  Keewatin  volcanoes  spread 
such  vast  piles  of  lavas  and  tuffs,  or  of  the  seas  in  which  the  sedi- 
mentaries  were  deposited,  nothing  whatever  is  known. 


w 


FIG.  199. — Ideal  cross-section  of  Black  Hills.     (After  Henry  Newton.) 

The  vortical  scale  is  about  six  times  the  horizontal ;   the  dotted  lines  indicate 
the  portion  of  the  uplift  removed  by  erosion. 

1,  Archaean  slates  and  schists.  5,  Red  beds,  with  included  limestones. 

2,  Granite.  6,  Jurassic. 

3,  Cambrian  resting  unconformably    7,  Cretaceous. 

on  1  and  2.  8,  White  River  Tertiary  resting  uncon- 

4,  Carboniferous.  formably  on  7. 

No  authentic  traces  of  life  have  so  far  been  found  in  the 
Archaean,  and  this  is  perhaps  not  surprising  when  we  consider  the 
subordinate  part  played  by  the  sedimentary  rocks  and  the  intense 
metamorphism  they  have  suffered.  Moreover,  it  is  not  improbable 
that  the  sedimentaries  we  now  see  are  but  the  remnants  of  piles 
of  clastic  rocks  that  were  deposited,  consolidated,  elevated,  and 
destroyed  long  before  the  advent  of  the  Palaeozoic  era.  There  is 
nothing  to  prove  that  life  did  not  exist  in  these  remote  times.  On 
the  contrary,  the  denudation  of  the  dry  land  by  aqueous  agencies, 
and  the  deposition  of  sediments,  indicate  the  prevalence  of  the 
physical  conditions  that  we  usually  associate  with  life. 

The  earlier  pages  of  historic  time  are  notoriously  fragmentary, 
blurred,  or  missing  ;  hence  it  should  cause  us  no  surprise  to  find 
the  first  pages  of  the  geological  record  even  more  incomplete,  dim, 
and  difficult  to  interpret. 


EOZOIC    ERA.  309 

Laurentian. — The  rocks  of  this  great  system  consist  of  granites 
and  gneisses  that  have  been  intruded  into  the  Keewatin  as  dykes 
and  great  bosses,  or  which  occur  as  the  country-rock,  covering  large 
areas.  At  one  time  the  Laurentian  was  considered  to  be  a  dis- 
tinctively basal  granite  complex,  but  wherever  it  comes  in  contact 
with  the  Keewatin,  it  is  found  to  be  intrusive  ;  hence  it  must  be 
younger  than  the  Keewatin. 

The  gneisses  include  ordinary  granitic  gneiss,  syenite-gneiss, 
and  diorite-gneiss,  all  of  which  are  deeply  involved  among  the 
granites.  Their  origin  is  still  obscure.  By  some  writers  they 
are  regarded  as  highly  metamorphosed  sedimentaries,  by  others, 
as  foliated  and  altered  eruptives. 


OE     OCBCDED     CBAG  N.R 


Soh. 


FIG.  200. — Hypothetical  section  across  the  Menominee  iron  region  in  the 
vicinity  of  Quinnesec  Valley.     (After  R.  D.  Irving,  1890.) 

A,  Basal  sericitic  quartz-slates.  E,  Slates  and  quartzites. 

B,  Quartzite.  G,  Granite. 

C,  Limestone.  Sch.,  Schists  of  the 
I),  Iron  horizon.  Laurentian. 

The  second  view  is  the  one  most  favoured  by  petrographers. 

Algonkian. — This  group  includes  most  of  the  North  American 
pre-Cambrian  sedimentaries,  many  of  which  are  highly  meta- 
morphosed, folded,  and  contorted.  It  is  associated  with  masses 
of  igneous  rock,  also  much  altered  and  folded. 

The  rocks  are  principally  conglomerates,  sandstones,  shales, 
quartzites,  limestones  (often  graphitic  or  dolomitic),  various 
schists,  gneisses,  and  granites.  Their  thickness  is  probably  not 
less  than  40,000  feet. 

In  the  Lake  Superior  region  where  the  succession  is  best  seen, 
the  Algonkian  may  be  divided  into  four  distinct  systems  that  are 
separated  by  well-marked  unconformities  : — 

(a)  Keweenawan.        "i 

(6)  Upper  Huronian   I  Alaonkian 

(c)  Middle  Huronian  f  Alg011 

(d)  Lower  Huronian  j 

The  largest  area  covered  by  the  Algonkian  is  that  of  the  Belt 
Series  of  Northern  Montana,  Idaho,  and  South  British  Columbia. 


310 


A    TEXT-BOOK    OF    GEOLOGY. 


Fossils  have  been  reported  from  many  places,  but  in  most  cases 
they  have  been  found  on  close  examination  to  be  of  inorganic 
origin.  Only  in  two  instances  are  true  fossils  known  in  the 
pre-Cambrian  areas  of  the  United  States,  namely,  in  the  shales 
of  the  Belt  Series  of  Montana,  and  in  the  Chuar  group  of  the 
Grand  Canyon.  These  represent  the  earliest  forms  of  life  yet 
found. 

The  fauna  of  the  Algonkian  Montana  shales  includes  four 
species  of  annelid  trails,  and  trails  that  appear  to  have  been 
made  by  a  minute  mollusc  or  crustacean.  The  same  shales 


FIG.  201. — Portion  of  Eozoon  magnified  100  diameters,  showing  the  supposed 
original  cell-wall  with  tubulation  and  the  supplemental  skeleton  with 
canals.  (After  W.  B.  Carpenter.) 

(a)  Original  tubulated  wall  or  "  Nummuline  layer,"  more  magnified  in  Fig.  A. 
(6  c)  Intermediate  skeleton  with  canals. 

also  contain  thousands  of  fragments  of  one  or  more  genera  of 
crustaceans. 

The  fossils  of  the  Grand  Canyon  Algonkians  are  a  small  discinoid 
shell  and  a  Stromatopora-]ike  form. 

The  discovery  by  Logan  in  1863  of  certain  forms  in  the  Algonkian 
limestones  of  Canada  and  the  Adirondacks  of  New  York,  which  he 
thought  were  of  organic  origin,  led  to  a  controversy  which  lasted 
nearly  forty  years.  Specimens  from  the  base  of  the  "  Grenville 
limestone"  weres  ubmitted  to  J.  W.  Dawson,  who,  in  1865,  recog- 
nised them  as  organic,  and  referred  them  to  protozoans  related 
to  the  foraminifera.  On  account  of  their  geological  position  he 
named  them  Eozoon  canadense. 


EOZOIC    ERA.  311 

W.  B.  Carpenter,  the  microscopist,  confirmed  the  conclusions  of 
Dawson  ;  but  William  King  and  T.  H.  Rowney  of  Queen's  Uni- 
versity, Ireland,  challenged  the  organic  origin  of  Eozoon,  and 
affirmed  their  belief  that  the  imitative  structures  were  purely 
mineral  and  of  crystalline  origin  resulting  from  chemical  change. 

Similar  structures  were  about  this  time  found  in  Bavaria  and 
elsewhere.  The  view  of  King  and  Rowney  was  subsequently 
supported  by  Mcebius  of  Kiel,  J.  W.  Gregory,  and  H.  J.  Johnston- 
Lavis. 

It  is  now  generally  believed  that  Eozoon  canadense  is  of  mineral 
origin. 

Pre-Cambrian  rocks  represented  by  numerous  isolated  patches 
of  granite,  gneiss,  and  crystalline  schists  are  exposed  at  the  surface 
in  all  the  continents.  They  form  the  framework  of  all  the  great 
mountain-chains,  and  also  appear  in  the  truncated  arid  plateaux 
of  Africa,  Asia,  and  Australia,  and  in  the  stumps  of  some  worn- 
down  or  sunken  chains  of  great  antiquity. 

In  many  regions  there  is  little  or  no  available  data  as  to  the  age 
of  the  rocks.  The  general  practice  almost  everywhere  is  to  refer 
all  gneissic  and  schistose  rock-formations  of  unknown  age  to  the 
Archaean.  At  the  present  time  the  information  as  to  the  general 
character  and  succession  of  the  pre-Cambrian  rocks  is  so  meagre 
that  no  satisfactory  basis  for  their  correlation  with  the  Eozoic 
of  North  America  has  yet  been  worked  out.  A  resemblance  in 
some  of  the  broader  features  is,  however,  becoming  apparent  in 
many  instances.  The  pre-Cambrian  of  Europe,  for  example,  is 
characterised  by  a  basal  complex  of  schist,  gneiss,  and  granite, 
unconformably  overlain  by  a  highly  altered  series  that  is  mainly 
sedimentary  and  devoid  of  recognisable  fossils.  The  former  may, 
in  a  general  way,  be  correlated  with  the  Archaean,  and  the  latter 
with  the  Algonkian. 

British  Isles. — Pre-Cambrian  rocks  are  well  exposed  in  the 
North- West  Highlands  of  Scotland  ;  and  in  Donegal,  Mayo,  and 
Galway  Counties,  in  North  and  West  Ireland.  Smaller  patches 
of  these  rocks  crop  out  in  Anglesey ;  at  the  Longmynd  and  the 
Wrekin  in  Shropshire  ;  at  Malvern  Hills  and  St  David's  in  Pem- 
brokeshire ;  and  at  Charnwood  Forest  in  Leicestershire. 

The  area  occupied  by  these  rocks  in  Scotland  is  the  largest  and 
most  important  in  the  British  Isles,  and  although  relatively  small 
when  compared  with  the  Eozoic  tracts  of  North  America  or  Scan- 
dinavia, there  is  perhaps  no  part  of  Europe  where  they  are  so 
well  displayed,  or  where  they  have  been  the  subject  of  more  critical 
examination. 

Mainly  as  the  result  of  the  researches  of  Peach  and  Home,  they 
have  been  divided  into  two  well-marked  systems,  namely,  the 


312 


A    TEXT-BOOK    OF    GEOLOGY. 


Torridonian,    dominantly    sedimentary ;     and    the    Lewisian, 
basal  complex  of  crystalline  schists,  gneisses,  and  granites. 


I.  Torridonian. — Comprising  a  thickness  of  8000  or  10,000 
feet  of  sandstones,  shales,  grits,  and  con- 
glomerates, with  subordinate  calcareous 
bands. 

Great  unconformity. 

II.  Lewisian. — Mainly  gneisses,  of  probably  igneous  origin, 
and  crystalline  schists,  which  may  be 
altered  sedimentaries. 


FIG.  202. — Diagrammatic  section  of  west  face  of  Glasven. 
Hor.  dist.  =  1£  mile.     (After  Peach  and  Home.) 

1,  Gneiss  covered  by  Torridon  sandstone  (2)  ;    3,  Cambrian  quartzite  ; 
4,  Pipe-rock  followed  by  fucoid  bed  (5),  and  the  limestone  (6). 

T,  T1,  T2,  powerful  thrust-planes ;  t,  t,  minor  thrusts. 

Lewisian. — This  great  system,  which  may  very  well  be  correlated 
with  the  Archaean  of  North  America,  forms  the  Isle  of  Lewis,  from 
which  it  takes  its  name,  and  extends  throughout  the  Outer  Hebrides, 
whence  it  passes  on  to  the  mainland. 

The  characteristic  rocks  are  gneisses  and  crystalline  schists  that 
are  frequently  closely  folded  and  contorted,  and  in  places  penetrated 
by  numerous  igneous  dykes. 

Generally  the  rocks  are  so  much  altered  that  it  is  impossible  to 
make  much  of  their  original  character  ;  but  where  they  are  less 
altered,  they  are  seen  to  pass  into  syenites,  diorites,  gabbros,  and 
other  plutonic  igneous  rocks. 

In  the  valley  of  Loch  Maree  there  is  a  remarkable  series  of  meta- 
morphic  rocks  consisting  chiefly  of  mica-schist,  quartz-schist, 
graphite-schist,  and  limestone,  for  which  Sir  Archibald  Geikie  has 


EOZOIC    ERA.  313 

proposed  the  name  Dalradian.  This  group  is  probably  sedimentary; 
and  it  appears  to  be  intruded  by  the  Lewisian  gneiss.  If  this  rela- 
tionship be  verified,  the  Dalradian l  must  take  its  place  as  the  oldest 
group  of  rocks  in  Britain,  if  not  in  Europe. 

Torridonian.— The  rocks  of  this  system,  like  the  American 
Algonkian,  are  typically  sedimentary.  They  rest  on  a  highly 
denuded  surface  of  the  Lewisian,  from  which  they  are  separated 
by  a  great  unconformity. 

The  Torridonian  system  extends  as  a  belt  along  the  west  coast  of 
Scotland  for  a  distance  of  100  miles,  and  take  its  name  from  Lake 
Torridon,  where  they  are  typically  displayed.  The  lowest  bed  is 
a  conglomerate  which  contains  fragments  of  Lewisian  gneiss,  and 
also  pebbles  of  unaltered  igneous  rocks  that  are  unknown  in  the 
Lewisian  of  Scotland,  but  resemble  some  of  the  Archaean  lavas  in 
Shropshire. 

The  upper  portion  of  the  Torridonian  is  composed  of  red  and 
chocolate-coloured  sandstones  that  appear  to  have  been  formed  in 
desert  or  continental  conditions. 

A  characteristic  feature  of  the  North- West  Highlands  of  Scotland 
is  the  remarkable  horizontal  shearing  of  the  overlying  Cambrian 
rocks,  which  have  been  fractured  and  deformed  by  a  series  of  power- 
ful thrust-planes,  the  most  easterly  and  greatest  of  which,  called 
the  Moine  Thrust,  has  carried  the  strata  overlying  it  westward  for 
a  distance  of  at  least  ten  miles  on  to  the  undisturbed  Cambrian 
rocks. 

Pre- Cambrian  of  other  Countries. — The  largest  continuous  tract 
of  pre-Cambrian  rocks  in  Europe  occupies  Scandinavia,  and  passes 
into  Finland.  The  rocks  are  principally  granites,  gneisses,  and 
crystalline  schists,  with  which  are  associated  bands  of  limestone. 

The  Algonkian  and  Archaean. divisions  of  the  pre-Cambrian  are 
recognised  in  France  ;  but  in  Central  Europe  the  rocks  have  not 
been  subdivided. 

Two  series  of  gneisses  have  been  recognised  in  Northern  India  ; 
and  in  China,  where  there  is  a  great  development  of  gneiss,  schists, 
quartzites,  and  limestone  of  pre-Cambrian  age,  the  succession,  as 
worked  out  by  Willis,  shows  a  singular  resemblance  to  that  of 
North  America. 

In  Australia  and  New  Zealand  there  are  extensive  tracts  occupied 
by  massifs  of  granite,  gneiss,  mica-schist,  and  limestone,  frequently 
much  folded  and  plicated,  faulted,  and  sheared,  and  in  many  places 
intruded  by  numerous  igneous  dykes.  The  age  and  relationships 
of  these  rocks  to  lower  Palaeozoic  formations  have  not  yet  been 
worked  out. 

The  evidence  as  to  the  great  antiquity  of  the  rocks  reputed  to 
1  So  named  after  the  old  Celtic  Kingdom  of  Dalradia  in  North  Ireland. 


314 


A    TEXT-BOOK    OF    GEOLOGY. 


be  pre-Cambrian  is  not  always  satisfactory,  and  is  only  capable  of 
proof  where  the  older  Palaeozoic  formations  are  present.  Detailed 
geological  surveys  have,  in  the  past  few  decades,  greatly  reduced 
the  areas  formerly  ascribed  to  the  Archaean,  and  in  all  probability 
future  observations  will  still  further  reduce  their  extent. 

ECONOMIC  MINERALS. 

The  crystalline  rocks  of  pre-Cambrian  age  are  everywhere 
remarkable  for  their  richness  in  metallic  ores  and  precious 
stones. 

The  famous  copper  deposits  of  Lake  Superior,  and  the  vast  bodies 


Wesf 


FIG.  203. — Longitudinal  section  of  Loretto  Mine,  Menominee,  Wisconsin. 

of  iron  ore  in  the   same   region,  occur  in  the   Keweenawan  and 
Huronian  divisions  of  the  Algonkian  respectively. 

The  iron  ores  are  of  secondary  origin,  and  mostly  occur  in  the 
trough-like  folds  of  the  rocks,  as  shown  in  figs.  200  and  203. 


HISTORY  OF  THE  PRE-CAMBRIAN. 

The  first  pages  of  the  geological  record  are  exceedingly  frag- 
mentary, and  refer  to  a  time  so  veiled  in  obscurity  that  it  is 
almost  impossible  to  construct  a  picture  of  the  contemporary 
physical  geography  that  can  claim  to  be  more  than  a  shadowy 
approximation. 

We  have  already  observed  that  the  Archaean  is  chiefly  composed 
of  altered  igneous  rocks,  mostly  of  intermediate  and  basic  types, 


EOZOIC    ERA.  315 

with  subordinate  intercalated  bands  of  sedimentaries,  among  which 
calcareous  members  are  conspicuously  absent. 

Of  the  land  surface  on  which  these  piles  of  Archaean  igneous  rocks 
were  spread,  we  have  no  record  of  any  kind.  It  may  have  been  a 
portion  of  the  primitive,  wrinkled,  and  gnarled  crust  that  formed 
when  the  globe  first  cooled  down,  or  a  surface  composed  of  sedi- 
mentary and  igneous  rocks  spread  over  the  primitive  crust.  The 
subject  is  one  that  affords  ample  scope  for  controversy,  and,  although 
full  of  interest,  is,  after  all,  more  academic  than  material.  What  we 
do  know  is  that  on  this  ancient  land  surface,  whatever  its  character 
and  origin,  there  was  poured  a  stupendous  accumulation  of  lavas 
mingled  with  piles  of  fragmentary  ejectamenta.  The  distribution 
of  these  Archaean  rocks  would  seem  to  warrant  the  belief  that  . 
the  regions  in  which  these  titanic  outbursts  took  place  already 
formed  the  nucleus  or  framework  of  the  existing  continents. 

So  far  as  we  know,  the  Archaean  rocks  were  not  the  product  of 
a  single  world-wide  paroxysmal  outburst,  but  the  accumulation 
of  many  eruptions,  possibly  separated  by  long  intervals  of  com- 
parative quiescence.  During  the  periods  of  cessation  from  volcanic 
activity,  the  still  smoking  piles  of  lavas  and  ashes  became  subject 
to  denudation,  the  detritus  being  deposited  in  the  adjacent  seas, 
where  it  afterwards  became  covered  with  the  ejecta  of  later  erup- 
tions. In  this  way  were  formed  the  bands  of  conglomerate  and 
shale  intercalated  among  the  igneous  rocks. 

The  absence  of  calcareous  bands  has  been  thought  by  some 
writers  to  indicate  that  the  seas  of  these  Archaean  times  were  devoid 
of  lime  and  other  dissolved  salts,  but  of  the  truth  of  this  we  are  in 
complete  ignorance. 

After  an  interval  of  unknown  length  the  lavas  and  tuffs  became 
invaded  by  enormous  plutonic  intrusions  of  acid  magmas,  which 
now  constitute  the  granites  and  gneisses  of  the  Laurentian. 

This  was  apparently  a  period  of  general  uplift  during  which  the 
continents  attained  a  great  area,  particularly  in  North  America  and 
Western  Europe.  The  beginning  of  this  uplift  ends  the  first  or 
Archaean  chapter  of  the  Earth's  history,  which  was  mainly  character- 
ised, as  we  have  seen,  by  unparalleled  volcanic  activity. 

The  duration  of  the  post-Laurentian  uplift  is  unknown.  That  it 
was  very  great  may  be  gathered  from  the  enormous  alteration, 
folding,  and  erosion  suffered  by  the  Archaean  rocks  before  the  Algon- 
kian  period  began. 

The  Algonkian  is  mainly  sedimentary,  and  comprises  four  great 
systems  separated  by  unconformities.  The  systems  are  a  record 
of  subsidence  and  deposition,  and  the  unconformities  of  long 
intervals  of  uplift  and  denudation. 

The  vast  thickness  of  the  Algonkian  is  a  witness  of  prolonged  and 


316  A  TEXT-BOOK  OF  GEOLOGY. 

probably  rapid  denudation  of  land-areas  long  since  worn  down  to 
a  low  relief,  and  covered  over  with  later  rock-formations. 

The  alternating  subsidence  and  uplift,  deposition,  and  denudation 
resulting  in  the  formation  of  conglomerates,  sandstones,  shales,  and 
limestones,  now  highly  altered  and  deformed,  indicate  the  preval- 
ence of  physical  conditions  in  the  Algonkian  not  unlike  those  of  the 
present  day.  With  such  conditions  it  is  not  surprising  to  find  many 
evidences  of  life  ;  although  this  truth  might  have  been  postulated 
from  the  existence  of  the  limestone  bands,  and  the  highly  organised 
character  of  the  succeeding  Cambrian  faunas  which  it  is  reasonable 
to  suppose  must  have  been  preceded  by  a  long  line  of  ancestors. 


CHAPTER   XXII. 
PALAEOZOIC   ERA. 

THE    Palaeozoic  formations   have   been  subdivided  on  palaeonto- 
logical  grounds  into  six  easily  recognised  systems  : — 

6.  Permian. 
5.  Carboniferous. 
4.  Devonian. 
3.  Silurian. 
2.  Ordovician. 
1.  Cambrian. 

The  Palaeozoic  is  the  lowest  of  the  three  great  fossiliferous 
divisions  of  sedimentary  rocks.  It  is  characterised  by  the  presence 
of  the  oldest  known  organic  remains,  if  we  except  the  few  in- 
distinct fossils  found  in  the  Eozoic  of  North  America. 

The  flora  is  mainly  cryptogamic,  comprising  gigantic  ferns,  club- 
mosses,  and  horse-tails,  which  in  the  upper  half  are  associated  with 
conifers  and  cycads. 

The  fauna  is  specially  distinguished  by  its  crinoids,  corals,  grapto- 
lites,  peculiar  brachiopods,  ancient  nautili,  trilobites,  and  ganoid 
fishes.  Reptiles  just  appear  at  the  close,  and  birds  are  entirely 
absent. 

The  rocks  are  represented  by  sandstones,  grits,  conglomerates, 
shales,  slates,  and  limestones,  frequently  tilted,  folded,  faulted, 
cleaved,  and  metamorphosed. 

In  many  places  they  are  intercalated  with  contemporaneous 
sheets  of  lava,  and  intruded  by  igneous  dykes. 

The  total  thickness  of  the  formations  included  in  the  Palaeozoic 
is  estimated  at  100,000  feet. 

In  the  lower  half  the  Palaeozoic  contains  deposits  of  gold,  silver, 
tin,  and  iron  ;  and  in  the  upper  half  valuable  seams  of  coal. 

When  dealing  with  the  Eozoic,  in  the  absence  of  fossils,  we  found 
it  impossible  to  correlate  the  sandstones  of  Loch  Torridon  with  those 
of  Longmynd,  or  the  Lewisian  gneiss  of  Scotland  with  that  of 
Ireland  or  Anglesey,  and  still  less  possible  to  correlate  the  ancient 

317 


318 


A   TEXT-BOOK   OF   GEOLOGY. 


rocks  of  the  British  Isles  with  those  of  Scandinavia  or  Canada  ; 
but  when  we  reach  the  fossiliferous  Palaeozoic  formations,  all  this 
is  changed.  The  contained  fossils  enable  us  to  say  within  narrow 
limits  of  error  that  a  certain  rock  in  Great  Britain  is  contempo- 
raneous with  such  a*one  in  Bohemia,  India,  Tasmania,  or  Canada. 
Moreover,  by  a  careful  study  of  the  character  of  the  fossils  and  of 
the  sediments  in  which  they  are  embedded,  we  are  able  to  determine 
the  physical  conditions  that  prevailed  simultaneously  on  the 
different  continents  with  an  approximate  degree  of  certainty. 
Relationship  of  Outcrop  to  Actual  Extent  of  Formations. — When 


Section 


FIG.  204. — Showing  relationship  of  outcrop  to  actual  extent  of  a 
rock-formation. 

(a)  Archaean.       (6  and  c)  Cambrian.       (d)  Ordovician.       (e)  Silurian. 

we  say  that  a  rock-formation  or  system  "  occupies,"  "  covers,"  or 
-"  occurs  in  "  a  certain  tract  of  country  ;  or  when  we  speak  of  it  as 
being  "  well-developed  "  or  "  represented  "  in  a  specified  locality, 
we  refer  only  to  the  portion  of  the  formation  exposed  at  the  surface. 
Obviously  the  portion  so  exposed  may  be  only  a  fraction  of  the  total 
area  in  which  the  formation  exists,  which  is  only  another  way  of 
saying  that  the  greater  portion  of  the  formation  may  lie  buried 
beneath  younger  rocks. 

For  example,  we  know  that  the  surface  exposures  of  the  Carboni- 
ferous Coal-Measures  of  England  cover  but  a  small  portion  of  the 
area  actually  occupied  by  that  formation.  Bore-holes  have  proved 
a  great  easterly  extension  into  Kent  under  the  overlying  Oolite, 


PALEOZOIC    EEA.  319 

Greensands,  and  Chalk  ;  and  coal  has  been  discovered  at  Dover, 
1100  feet  below  the  sea,  over  100  miles  from  the  nearest  outcrop 
of  the  Coal-Measures  in  England. 

In  this  plan  and  section  we  see  that  the  surface  exposure  of  the 
Cambrian  a  amounts  only  to  x,  while  as  a  matter  of  fact  that  system 
exists  far  to  the  westward  of  the  outcrop  below  d  and  e  ;  and  so  far 
as  we  can  see  it  appears  to  be  co-extensive  with  the  Ordovician  and 
Silurian. 

The  surface  exposure  of  a  rock-formation  is  usually  the  result  of 
some  accident  of  folding,  tilting,  or  faulting  followed  by  denudation. 
Hence  a  map  which  shows  only  the  present  exposure  of  a  rock- 
formation  conveys  no  information  as  to  the  extent  of  the  sea-floor 
on  which  the  original  sediments  were  laid  down. 

CAMBRIAN  SYSTEM. 

The  name  Cambrian  is  derived  from  Cambria,  the  ancient  name 
of  Wales.  It  was  first  proposed  by  Sedgwick  in  1833  for  a  certain 
group  of  fossiliferous  rocks  in  North  Wales,  which  subsequent 
research  showed  to  be  Silurian  and  Ordovician.  The  name  is  now 
confined  to  the  oldest  group  of  Palaeozoic  fossiliferous  strata,  and 
is  recognised  as  a  time-name  by  geologists  in  all  countries. 

Distribution. — Cambrian  rocks  are  found  in  all  the  continents. 
They  are  typically  developed  in  North  Wales,  where  they  attain  a 
thickness  of  12,000  feet.  They  are  well  represented  in  North 
Scotland,  also  in  Spain,  France,  Belgium,  Bohemia,  and  other  parts 
of  Central  Europe,  where  their  thickness  is  estimated  at  10,000  feet. 

In  Scandinavia  the  Cambrian  system  dwindles  down  to  a  few 
hundred  feet  of  shales  and  thin-bedded  limestones,  which  indicate 
deep-water  conditions  of  deposition.  Obviously  the  ancient 
continent,  near  which  the  thicker  arenaceous  Cambrian  rocks  of 
Wales  were  laid  down,  existed  somewhere  to  the  westward. 

The  Cambrian  rocks  in  North  America  occupy  a  wider  extent  of 
country  and  attain  a  greater  thickness  than  in  any  other  known 
part  of  the  globe.  They  also  present  a  type  of  sedimentation 
typically  distinct  from  that  of  Continental  Europe.  In  the 
Adirondack  Mountains  of  New  York,  in  East  Canada,  in  the 
Appalachian  Chain,  stretching  through  Pennsylvania,  Virginia, 
Tennessee,  Georgia,  and  Alabama,  in  Central  Nevada,  and  British 
Columbia,  they  are  represented  by  sandstones,  shales,  and  massive 
beds  of  limestone  that  are  frequently  dolomitic.  The  thickness  of 
the  system  in  North  America  varies  from  2000  to  10,000  feet.  Of 
a  thickness  of  7700  feet  in  Nevada,  more  than  4000  feet  are  beds 
of  massive  limestones  ;  and  in  all  the  States  the  calcareous  members 
are  conspicuous,  particularly  in  the  upper  divisions  of  the  system. 


320  A  TEXT-BOOK  OF  GEOLOGY. 

Cambrian  rocks  crop  out  from  below  later  accumulations  in  the 
Andes  in  North- West  Argentina,  in  the  Salt  Range  in  India,  in 
Korea,  in  South-East  Australia,  and  in  Tasmania.  Cambrian 
corals  have  been  found  in  South  Victoria  Land. 

Rocks. — The  rocks  of  the  Cambrian  system  mainly  consist  of 
sandstones,  grits,  greywackes,  shales,  slates,  and  limestones.  They 
are  frequently  associated  with  bands  of  quartzite,  quartz-schist, 
phyllite,  and  mica-schist. 

As  might  be  expected  from  their  great  antiquity,  the  Cambrian 
rocks  are  much  disturbed,  particularly  in  Great  Britain,  where 
they  are  tilted  at  high  angles,  sharply  folded  and  metamorphosed. 
In  North  America,  Scandinavia,  and  Russia  they  lie  comparatively 
undisturbed  over  considerable  areas  ;  but  in  Eastern  Russia  they 
rise  up  in  folds  as  they  approach  the  Ural  Mountains. 

In  England,  France,  and  Belgium  the  Cambrian  rocks  are  inter- 
calated with  sheets  of  diabase  and  diabase-tuff,  and  intruded  by 
dykes  of  quartz-porphyry  and  diorite. 

In  North  America  the  Cambrian  rocks  are  comparatively  free 
from  contemporaneous  volcanic  materials,  but  in  many  of  the 
States  they  are  intruded  by  igneous  dykes  of  later  date. 

Fauna. — The  fauna  of  the  Cambrian  System  is  remarkably  rich 
and  varied  for  rocks  of  such  great  antiquity.  The  lowest  forms  of 
life  represented  are  radiolaria  and  sponges.  Graptolites  appear  at 
the  close,  and  many  beautiful  jelly-fish  (Medusw)  have  been 
described  by  Walcott  from  the  Middle  Cambrian  of  Alabama. 

Corals  are  abundant  in  the  limestones  of  North  America,  but 
comparatively  rare  in  Europe,  where  the  conditions  of  arenaceous 
deposition  were  not  favourable  for  their  existence.  Hence  it  is  that 
in  Europe  limestones  are  scarce,  except  in  a  limited  area  in  North- 
West  Scotland,  where  massive  beds  of  Cambrian  limestone  are  well 
developed.  Crinoids  and  starfish  are  fairly  common. 

Annelids,  which  first  made  their  appearance  in  the  Algonkian, 
are  known  to  have  been  numerous,  as  shown  by  the  plentiful  occur- 
rence of  their  trails  and  burrows.  Specimens  of  Salterella  (Serpu- 
lites)  Maccullochi,  a  tubicolous  worm,  are  not  uncommon  in  the 
Lower  Cambrian  of  Scotland  and  North  America. 

The  most  distinctive  of  the  Cambrian  fossils  are  the  Crustacea, 
most  of  which  belong  to  the  extinct  trilobites.  These  exhibit  a 
relatively  high  state  of  development,  which  would  point  to  a  long 
line  of  ancestors  in  pre-Cambrian  times.  Shrimp-like  crustaceans 
appear  for  the  first  time  in  the  Upper  Cambrian. 

Brachiopods  of  a  peculiar  type  are  represented  by  several  hingeless 
forms,  among  which  Paterina  labradorica  is  common.  Among 
other  types  are  Lingulella  and  Orthis,  which  become  plentiful  in  the 
next  period  ;  and  of  the  Mollusca  we  have  numerous  Lamellibranchs, 


. 

. 

uH  .6 

. 

.- 

. 


PLATE   XIX. 

CAMBRIAN  FOSSILS. 

1.  Oldhamia  radiata.     Lower  Cambrian.     Bray  Head,  Wexford,  Ireland. 

2.  Oldhamia  antiqua.     Lower  Cambrian.     Bray  Head,  Wexford,  Ireland. 

3.  Histioderma  Hibernica  (Kinn.).     Lower  Cambrian.     Bray  Head,  Wexford, 

Ireland. 

4.  Histioderma  Hibernica  (Kinn.).     Lower  Cambrian.     Bray  Head,  showing 

fine  transverse  lines. 

5.  Hymenocaris  vermicauda  (Salt.).     Lingula  Flags,  North  Wales. 

6.  Paradoxides  Davidis  (Salt.).     Menevian  and  Lingula  Flags,  South  Wales. 

7.  Olenus  micrurus  (Salt.).     Lingula  Flags. 

8.  Lingulella  Davisii  (M'Coy).     Lingula  Flags  and  Tremadoc  Slates. 


To  face  page  320.] 


[PLATE  XIX. 


CAMBRIAN  FOSSILS. 


PALEOZOIC    ERA. 


321 


Gasteropods,  and  Cephalopods,  the  latter  including  Orthoceras  and 
Nautilus.  Among  the  Gasteropods  we  find  representatives  of  the 
genera  Murchisonia  and  Pleurotomaria. 

No  plant  remains  have  yet  been  found  in  the  Cambrian,  though 
the  so-called  fucoid  markings  in  the  Middle  Series  (Fucoid  Beds), 
in  North-  West  Scotland  and  elsewhere  have  been  described  as  the 
prints  of  sea-weeds. 

Associated  with  worm  trails  there  are  found  in  the  Cambrian 
rocks  at  Bray  Head,  Wexford,  Ireland,  peculiar  fossil  markings, 
called  Oldhamia,  the  organic  nature  of  which  has  been  doubted.  The 
two  species  recognised  are  0.  radiata  and  0.  antiqua  (Plate  XIX.). 

Subdivision  of  Cambrian.  —  The  Cambrian  System  has  been 
divided  into  three  main  groups  of  beds,  each  characterised  by  its 
own  assemblage  of  fossils.  This  threefold  subdivision  has  been 
identified  in  Western  Europe,  North  America,  and  Tasmania. 


FIG.  205.  —  Section  across  Harlech  Dome  from  Cader  Idris  to  Snowdon. 

(a)  Harlech  and  Llanberis  beds.  (d)  Tremadoc. 

(6)  Menevian.  (e)  Ordovician. 

(c)  Lingula  flags. 

{Upper  — 
Middle— 
Lower  —  Olenellus  Beds. 


—  Olenus  Beds. 
Middle—  Paradoxides  Beds. 


The  local  divisions  in  England,  as  seen  in  the  Harlech  anticlinal 
in  Western  Merionethshire  where  the  succession  is  complete,  are 
as  follow  :  — 

Olenus  Beds         {  j;  Jg£SJf" 

ParadoxidesReds  —  2.  Menevian  Series  —  Middle  Cambrian. 
Olenellus  Eels         L  }  Lower  Cambrian. 


}uPper 


Cambrian. 


Series 

The  three  divisions  recognised  in  North  America  are  as  under 

Potsdam  Sandstone. 
St  John  or  Acadia  Group. 
^Georgia  Group. 


Upper  Cambrian  or 

Olenus  Beds 
Middle  Cambrian  or 

Paradoxides  Beds 
Lower  Cambrian  or 

Olenellus  Beds 


21 


322  A  TEXT-BOOK  OF  GEOLOGY. 

OlenellUS  Beds. — These  beds  are  everywhere  considered  to  form 
the  base  of  the  Cambrian  System.  They  contain  the  oldest  fauna 
of  which  we  have  any  accurate  knowledge.  The  trilobite  Olenellus 
is  characteristic  of  this  series,  and  hence  Olenellus  Beds  and  Lower 
Cambrian  are  synonyms. 

At  Harlech,  in  Wales,  and  in  West  England,  the  Lower  Cambrian 
consists  mainly  of  grits  interbedded  with  bands  of  grey  and  purple 
slates ;  but  in  North- West  Scotland  the  Cambrian  begins  with 
arenaceous  deposits  and  becomes  more  and  more  calcareous  and 
dolomitic  towards  the  top,  in  this  showing  a  curious  resemblance 
to  the  Cambrian  of  North  America. 

Paradoxides  Beds.  —  These  constitute  the  Middle  Cambrian 
and  are  distinguished  by  the  presence  of  the  trilobite  Paradoxides 


FIG.  2QQ.— Olenellus. 

(Plate  XIX.).  In  North  Wales  they  consist  chiefly  of  dark  slates  ; 
and  in  Canada  of  slates  and  shales,  with  which  are  correlated  the 
limestones  of  Central  Nevada  and  British  Columbia.  A  fossil  crab 
of  a  primitive  type,  Sidney ia  inexpectans,  occurs  in  the  Middle 
Cambrian  rocks  of  Mount  Wapta,  in  British  Columbia. 

Olenus  Beds. — In  North  Wales,  this  series  comprises  two  groups  of 
beds,  the  Lingula  Flags  and  the  Tremadoc  Slates,  the  latter  forming 
the  closing  member  of  the  Cambrian  System  in  Britain  (Plate  XIX.). 

The  Durness  Limestone  which  closes  the  Cambrian  System  in 
North- West  Scotland  contains  a  fauna  unlike  any  other  in  the  British 
Isles,  comprising  a  number  of  species  that  are  unknown  in  other 
British  areas,  but  most  of  which  have  been  identified  in  the  Upper 
Cambrian  Calciferous  Series  of  the  United  States  and  Canada. 

The  Calciferous  Series  of  North  America  is  approximately  the 
equivalent  of  the  Tremadoc  Slates  of  England,  with  which  the 


PALAEOZOIC    ERA.  323 

upper  and   perhaps   greater   portion   of  the   Durness   Limestone 
should  also  be  correlated. 

ECONOMIC  PRODUCTS. 

Cambrian  rocks  contain  veins  of  gold,  silver,  and  copper,  but  they 
have  nowhere  proved  very  productive.  The  most  valuable  pro- 
ducts obtained  from  this  system  in  Great  Britain  are  the  roofing 
slates  of  the  Llanberis  Beds  in  North  Wales.  They  alternate  with 
conglomerates  and  underlie  the  Harlech  grits.  Their  colour  is 
bluish-purple  ;  they  cleave  with  ease  and  regularity,  and  are 
probably  the  finest  roofing-slates  in  the  world. 

THE  HISTORY  OF  DEPOSITION. 

The  known  exposures  of  the  Cambrian  in  the  different  continents 
form  only  a  small  proportion  of  the  area  actually  occupied  by  that 
system,  and  since  the  area  and  distribution  of  the  portions  lying 
buried  beneath  the  younger  formations  are  unknown  and  cannot 
be  ascertained,  we  are  unable  to  reconstruct  a  map  that  will  show 
even  approximately  the  borders  and  extent  of  the  seas  in  which 
the  Cambrian  sediments  were  laid  down.  Obviously,  the  same 
obscurity  surrounds  the  area  and  distribution  of  the  continents 
that  furnished  the  sediments. 

The  fossil  fauna  even  in  the  most  diverse  kinds  of  rock  shows  a 
remarkable  uniformity  throughout  both  hemispheres,  from  which 
we  may  reasonably  draw  the  inference  that  the  climatic  conditions 
were  fairly  uniform  throughout  the  globe  during  the  Cambrian 
period. 

The  Cambrian  sediments  were  laid  down  as  marginal  deposits 
around  the  shores  of  the  then  existing  continents  ;  and  we  may  infer 
from  the  arenaceous  character  of  the  material,  and  the  prevalence 
of  worm  trails  and  burrows,  that  a  large  proportion  of  the  sediments 
were  deposited  in  shallow  seas.  The  red  and  purple  colour  of  many 
of  the  Cambrian  sandstones,  the  frequent  ripple-marks,  sun-cracks, 
and  false-bedding  might  even  suggest  that  in  many  instances 
deposition  took  place  in  shallow  inland  lakes,  or  in  land-locked 
estuaries. 

The  palseontological  evidence  shows  that  the  Lower  Cambrian 
fauna  of  the  North- West  Highlands  is  almost  identical  with  that  of 
the  Georgian  terrain  of  North  America,  but  essentially  different 
from  the  Lower  Cambrian  fauna  of  the  rest  of  Europe. 

In  the  case  of  the  Northern  Hemisphere,  it  seems  not  improbable 
that  the  continent  which  provided  the  sediments  was  situated  in 
the  North  Atlantic  region,  with  long  prolongations  stretching  far 


324  A  TEXT-BOOK  OF  GEOLOGY. 

to  the  east  and  to  the  west,  but  separated  from  the  rest  of  Europe 
by  a  deep  sea. 

CAMBRIAN  GLACIATION. — In  Northern  Norway  there  is  a  coarse 
breccia-conglomerate  resting  on  a  polished  and  striated  pavement 
that  is  believed  to  be  glacial.  The  glacial  beds  belong  to  a  series 
of  sedimentary  beds  known  as  the  Gaisa  Beds,  regarded  by  Reusch 
as  equivalent  to  the  Sparagmite  formation  which  underlies  rocks 
containing  the  Olenellus  fauna. 

A  glacial  boulder-rock  formation  containing  numerous  striated 
stones  has  recently  been  described  in  the  upper  Yang-tse  Valley, 
in  China,  as  lying  beneath  a  series  of  rocks  containing  Cambrian 
trilobites. 

Cambrian  glacial  deposits  have  been  described  by  Howchin  and 
David  as  occurring  in  South  Australia  at  intervals  over  a  distance 
of  150  miles  ;  and  Schwarz  has  reported  Cambrian  glacial  beds  in 
South  Africa. 

It  would  appear  from  these  evidences  of  glaciation  that  the 
Eozoic  rocks  must,  in  Cambrian  times,  have  formed  high  mountain 
chains  from  which  glaciers  descended  into  the  sea,  where  they 
deposited  their  load  of  rocky  detritus. 

The  general  facies  of  the  Cambrian  life  shows  that  the  glaciation 
was  not  general,  but  confined  to  certain  mountain  chains  bordering 
the  sea-coasts  in  some  of  the  continents. 


CHAPTER   XXIII. 
ORDOVICIAN   SYSTEM. 

(Lower  Silurian  of  Murchison.) 

THE  rocks  now  recognised  as  Ordovician  are  still  included  in  the 
Lower  Silurian  by  the  Geological  Survey  of  Great  Britain,  which 
is  the  position  originally  assigned  to  them  by  Sir  Roderick  Murchi- 
son, who  was  the  first  to  describe  and  name  the  Silurian  rocks  of 
Wales.  Sedgwick  claimed  them  as  part  of  his  Cambrian  System  ; 
and  to  avoid  confusion  Lapworth,  at  a  later  date,  suggested  placing 
them  in  a  separate  system,  to  which  he  gave  the  name  Ordovician 
after  the  ancient  British  tribe  Ordovices  in  whose  territory  in  East 
Wales  and  Shropshire  the  rocks  are  typically  developed.  The 
name  has  now  been  adopted  by  geologists  in  many  parts  of  the 
globe. 

Relationships. — The  Ordovician  System,  like  the  Cambrian, 
has  been  recognised  in  all  the  great  continents.  It  rests  conformably 
on  the  Cambrian  and  is  conformably  overlain  by  the  Silurian, 
except  in  places  where  there  is  a  break  in  the  succession  due  to  one 
or  more  of  the  members  of  the  system  being  absent. 

It  will  assist  us  to  a  better  understanding  of  the  relationship  of 
the  Cambrian,  Ordovician,  and  Silurian,  if  we  remember  that  these 
are  closely  related  systems  of  conformable  strata,  laid  down  on  the 
same  sea-floor,  and  marginal  to  the  same  continents.  In  each 
region  there  was  a  continuance  of  the  same  physical  conditions 
of  deposition,  and  though  the  character  of  the  sediments  in  many 
instances  indicates  frequent  oscillations  of  the  land,  the  general 
movement  was  that  of  subsidence.  As  a  result  of  uplift  in  some 
areas,  the  continuance  of  deposition  was  interrupted  for  a  time, 
and  in  such  regions  we  have  stratigraphical  breaks  in  the  succession, 
many  of  them  small,  others  of  great  magnitude.  Of  the  latter  we 
have  a  good  example  in  India,  where,  in  the  Peninsula  and  Salt 
Range,  there  is  a  great  hiatus  between  the  Cambrian  and  Permian, 
arising  from  a  long  persistent  uplift  after  the  deposition  of  the 
Cambrian. 

The  fauna  throughout  this  gigantic  succession  of  strata  is  closely 

325 


326  A  TEXT-BOOK  OF  GEOLOGY. 

related  and  stamped  with  the  same  general  facies  ;  and  this  is 
what  we  should  expect  to  find  in  sediments  laid  down  in  the  same 
continuous  sea.  In  a  rich  and  varied  fauna,  the  dominant  and 
characteristic  organisms  are  trilobites  and  graptolites. 

The  threefold  subdivision  into  Cambrian,  Ordovician,  and 
Silurian  is  purely  empirical  and  based  on  the  range  of  certain 
well-marked  genera.  The  great  thickness  of  the  strata,  and  the 
sudden  appearance,  prevalence,  and  gradual  disappearance  of  many 
generations  of  distinctive  genera  would  lead  to  the  inference  that 
these  systems  covered  a  vast  period  of  time  of  which  we  can  make 
no  trustworthy  estimate. 

Distribution. — In  the  British  Isles,  the  Ordovician  rocks  cover  a 
much  larger  area  than  the  Cambrian.  They  occupy  a  considerable 
portion  of  Wales  and  the  Lake  District,  and  are  also  found  in 
Shropshire,  Isle  of  Man,  and  Cornwall.  In  Scotland  they  are 
found  in  the  North- West  Highlands,  and  in  the  Southern  Uplands, 
where  they  stretch  in  a  belt  from  the  Firth  of  Clyde  to  the  Forth. 
They  also  occur  in  South-East  Ireland,  notably  in  Ulster,  and  in 
Bounties  Gal  way  and  Mayo. 

In  Continental  Europe,  Ordovician  rocks  occupy  large  areas  in 
Spain,  North  France,  Scandinavia,  and  the  Baltic  provinces  of 
Germany  and  Russia. 

The  greatest  known  developments  of  rocks  of  this  age  are  found 
in  the  United  States  and  Canada,  the  largest  tracts  occurring  in 
the  Black  Hills  of  South  Dakota,  in  New  Mexico,  Arizona,  California, 
Utah,  Nevada,  Wyoming,  Montana,  Colorado,  and  further  north 
in  British  Columbia. 

Considerable  areas  of  rocks  that  have  been  referred  to  the 
Ordovician  occur  in  the  Northern  Himalayas,  in  Northern  China, 
central  South  Siberia,  and  Arctic  regions. 

The  Ordovician  System  is  well  represented  in  the  Commonwealth 
of  Australia,  notably  in  the  States  of  Victoria  and  New  South  Wales, 
and  also  in  Western  Tasmania  and  New  Zealand.  Graptolites  were 
discovered  in  the  South  Orkney  Islands  by  Pirie,  which  would 
support  the  view  that  the  older  Palaeozoic  formations  are  repre- 
sented in  the  American  quadrant  of  the  Antarctic  continent. 

Rocks. — In  Europe,  except  in  the  southern  regions,  the  Ordo- 
vician consists  mainly  of  detrital  material  which  composes  massive 
beds  of  grit,  conglomerates,  sandstones  or  quartzites,  greywackes, 
and  shales  with  which  are  associated  subordinate  lenses  of  lime- 
stone. The  scarcity  of  calcareous  rocks  in  the  Ordovician  of 
Europe  is  in  marked  contrast  with  that  of  North  America,  where 
limestone?  are  conspicuously  abundant.  It  would  appear  as  if  the 
conditions  of  deposition  that  prevailed  in  Europe  and  North 
America,  in  the  Cambrian,  were  continued  into  the  Ordovician. 


To  /ace  page  327.] 


[PLATE  XXI. 


PUCKERED  MICA-SCHIST  FROM  TACONIC  RANGE. 
(After  Dale,  U.S.  Geol.  Survey.) 


ORDOVICIAN   SYSTEM.,  ;      ;  \'/ / *    >,-,  ,327 

Generally,  throughout  the  United  States  and  Canada,  the  rocks 
of  that  age  are  limestones,  shales,  and  sandstones,  the  former 
always  conspicuous  and  usually  richly  fossiliferous. 

In  the  central  valley  of  Tennessee,  in  Cincinnati,  Alabama,  and 
many  of  the  neighbouring  States,  the  Ordovician  rocks  lie  horizontal 
or  appear  in  gentle  folds  ;  but  in  Vermont  (Plate  XX.),  Eastern 
Tennessee,  West  Virginia,  and  in  the  mountains  of  Arkansas  they 
are  tilted  at  high  angles.  In  Wales  and  England  they  are  tilted 
at  various  angles,  and  in  North- West  Scotland  sharply  folded, 
contorted,  and  involved  in  overthrusts. 

In  Wales  and  Shropshire  the  Ordovician  grits,  which  consist 
mainly  of  resorted  volcanic  debris,  are  intercalated  with  thick  sheets 
of  lava  and  tuffs  ;  and  in  the  Lake  District,  vast  piles  of  volcanic 
material  dominate  the  system.  In  these  areas  we  thus  have 


FIG.  207. — Showing  structure  of  Taconic  Mountains. 
(After  Walcott.) 

(a)  Hudson  shales.  (6)  Trenton,  Chazy,  and  other  limestones. 

(c)  Cambrian  (Potsdam)  micaceous  shales. 

(d)  Lower  Cambrian  (Georgian).  (a  and  6)  =  Ordovician. 

conclusive  proof  of  intense  and  prolonged  volcanic  activity,  con- 
temporaneous with  the  deposition  of  the  fossiliferous  Ordovician. 

In  America  there  is  little  evidence  of  contemporaneous  volcanic 
disturbance,  the  general  tranquillity  of  the  Cambrian  having 
continued  into  the  Silurian.  The  relationship  of  the  Ordovician 
to  the  Cambrian  in  the  Taconic  Mountains  is  shown  in  the  above 
figure.  Plate  XXI.  shows  a  mica -schist. 

In  a  general  review  of  this  period,  we  observe  that  the  Ordovician, 
like  the  Cambrian,  contains  two  distinct  facies  of  sediments,  an 
arenaceous  and  calcareous,  the  former  typically  European,  the 
latter  typically  American. 

Fauna. — There  was  a  marked  change  in  the  life  at  the  close  of 
the  Cambrian,  and  many  genera  and  species  abundant  in  that 
system  are  absent  in  the  Ordovician,  in  which,  however,  many 
new  forms  appear  for  the  first  time. 

In  a  rich  and  varied  fauna  graptolites  and  trilobites  are  the 
most  important  organisms,  and  of  these  the  graptolites  may 
perhaps  be  regarded  as  distinctively  Ordovician.  They  comprise 


328 


;  A :  TEXT-BOOK    OF    GEOLOGY. 


a  great  many  genera  and  include  two-,  four-,  and  many-branched 
kinds.     Among  the  best  known  are  : — 

Diplograptus  .l 
Tetragraptus.2 
Ccenograptus.3 

Corals  and  crinoids  are  numerous  in  the  limestones  and  calcareous 
strata  ;  while  Brachiopods  are  well  represented  by  the  genera 
Orthis,  Strophomena.  and  Leptcena,  which  occur  in  the  arenaceous 
facies  of  sediments,  together  with  many  Lamellibranchs  and 
Gasteropods.  Of  Cephalopods  we  still  have  the  persistent  Ortho- 
ceras*  which  first  appeared  in  the  Cambrian  (Plate  XXIII. ). 

The  trilobites  attain  their  maximum  development  in  this  period, 


FIG.  208. 
Illcenus. 


FIG.  209. 
Asaphus. 


FIG.  21Q.—Trinudeus. 


more  than  half  of  all  the  known  genera  being  peculiarly  Ordovician. 
A  few.  like  Agnostus  and  Calymene,  survived  from  the  Cambrian, 
while  the  others  appear  for  the  first  time.  In  the  Silurian  they  fall 
to  half  the  number,  and  in  the  succeeding  formations  dwindle 
rapidly,  until  they  finally  disappear  at  the  close  of  the  Palaeozoic. 

Among  the  prominent  Ordovician  trilobites  we  find  Agnostus, 
Ogygia,  Asaphus,  Trinucleus,  and  Illcenus  (Plate  XXIV.). 

The  remains  of  fish-like  organisms,  the  earliest  known  verte- 
brates, are  found  abundantly  in  the  Ordovician  of  Colorado. 

No  fossil  remains  of  land  plants  are  known  in  this  period,  but 
at  Olonetz,  in  Finland,  in  a  series  of  sandstones  and  dolomites 
ascribed  to  this  or  even  an  older  system,  there  is  a  seam  of  anthracite, 

1  Gr.  diplous  =  double,  and  grapko  =  I  write  (i.e.  pen). 

2  Gr.  tetra  =  four,  and  grapho. 

3  Gr.  koinos  =  kindred,  and  grapho. 

4  Gr.  orthos  =  straight,  and  keras  =  a  horn. 


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PLATE   XXIII. 

ORDOVICIAN  FOSSILS. 

1.  Maclurea  Peachii  (Salt.).     Llandeilo  of  Durness,  North-West  Highlands. 

2.  Operculum  of  Maclurea  Peachii. 

3.  Maclurea  Logani  and  operculum.     Llandeilo  of  Ayrshire. 

4.  Raphistoma  cequalis  (Salt.).     Bala  beds  of  America. 

5.  Oncoceras,  sp.  (Phragmoceras).     Llandeilo  of  Durness. 

6.  Orthoceras  mendax  (Salt.).     Durness. 

7.  Ophileta  compacta  (Salt.).     Llandeilo  of  Durness. 

8.  Murchisonia  sub-rotundata.     Bala  bfeds. 

9.  Cyclonema  rupestris  (Eichw.).     Bala  beds. 

10.  Halysites  catenulatus  (Lim.).     Chain  coral,  common  in  Ordovician  and 

Silurian. 

11.  Echinosphcerites   balticus    (Eichw.).      Abundant   in    Llandeilo    of    South 

Wales  and  Scandinavia. 

12.  Echinosphcerites  granatus  (WahL).     Llandeilo  and  Bala  of  South  Wales 

and  Scandinavia. 

13.  Sphceronites  punctatus  (Forbes).     Bala  beds. 

14.  Agelaccrinites  Buchianus  (Forbes).     Caradoc,  South  Wales. 

15.  Sphceronites  (Caryocystites)  munitus  (Forbes).     Bala  beds. 

16.  Ovarium  pyramid  of  Echinosphcerites. 

17.  Palceaster  asperrimus  (Salt.).     Bala  beds. 

18.  Palceaster  obtusus  (Forbes).     Bala  of  Wales  and  Ireland. 

19.  Murchisonia  obscura  (Portl.).     Caradoc.     Ireland. 

20.  Helminthochiton  Griffithii  (Salt.). 

21.  Patella  ?  Saturni  (Discina  ?).     Llandeilo. 


To  face  page  328.] 


[PlATE 


10 


ORDOVICIAN  FOSSILS. 


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•••••••• 


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11  10  t>ijc4jvo 


PLATE   XXIV. 
ORDOVICIAN  FOSSIL  CRUSTACEA. 

1.  Asaphus  tyrannus  (Murch.).     Llandeilo  rocks, 
la.  Do.  do. 

2.  Ogygia  Buchii  (Brong.).     Llandeilo  Flags. 

3.  Calymene  duplicata  (Murch.).     Llandeilo  and  Caradoc  rocks. 

4.  Calymene   brevicapitata    (Portlock).      Ranges    from    Llandeilo    Flags    to 

Wenlock  Shale. 

5.  Trinucleus  concentricus  (Eaton).     Caradoc  and  Llandovery  rocks. 

6.  Trinucleus  Lloydii  (Murch.).     Llandeilo  and  Caradoc. 

7.  Trinucleus  fimbriatus  (Murch.).     Llandeilo  and  Caradoc. 

8.  Agnostus  M'Coyi  (Salt.).     Llandeilo  beds. 

9.  Agnostus  trinodus  (Salt.).     Caradoc. 

10.  Asaphus  Powisii  and  labrum  (Murch.).     Llandeilo  and  Caradoc. 

11.  Illcenus  Davisii  (Salt.).     Caradoc  and  Bala. 

12.  Lichas  taxatus  (M'Coy).     Caradoc  and  Llandovery.     Ireland. 

13.  Calymene  allied  to  brevicapitata.     Caradoc. 

14.  Beyrichia  complicata  (Salt.).     Llandeilo  and  Caradoc. 

15.  Beyrichia  tuberculata  or  Wilckensiana.     Caradoc  or  Bala. 

16.  Pygidium  of  Lichas  Barrandii  ? 

17.  Trinucleus  concentricus  (Eaton),  in  three  stages  of  development.     (After 

Barrande). 

18.  Stygina  latifrons  (Portl.).     Caradoc.     Ireland.     (After  Murchison.) 


To  face  page  328. 


XX/V/, 


17  16  15 

ORDOVICIAN  FOSSIL  CRUSTACEA. 


18 


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PLATE   XXV. 

ORDOVICIAN  AND  SILURIAN  FOSSILS. 
OBDOVICIAN. 

Trilobites. 

1.  Phacops  conophthalamus.     Caradoc  Sandstone. 

2.  Cybele  verrucosa.     Caradoc  and  Lower  Llandovery. 

3.  Cheirurus  clavifrons.     Caradoc. 

4.  Remopleurides  dorso-fipinifer.     Caradoc  rocks. 

5.  Harpes  flanagani.     Caradoc  rocks. 

6.  Ampyx  nudus.     Llandeilo  and  Caradoc. 

7.  Acidaspis  bispinosus.     Caradoc  rocks. 

8.  Cyphoniscus  socialis.     Caradoc  rocks. 

9.  Lichas  anglicus  (Beyr.).     Dudley,  etc. 

10.  Calymene  tuberculata  (Salt.).     Kendal.     Barrington. 

11.  Trinucleus  concentricus.     Llandeilo  and  Lower  Llandovery. 

12.  Mglina  mirabilis.     Llandeilo  and  Caradoc. 

Brachiopods. 

13.  Orthis  alata.     Arenig  rocks. 

14.  Orthis  striatula.     Arenig.     Llandeilo  and  Caradoc. 

15.  Lingula  attenuata.     Arenig.     Llandeilo  and  Caradoc. 

16.  Lingula  granulata.     Llandeilo  and  Caradoc. 

17.  Lingula  Ramsay i.     Llandeilo. 

18.  Siphonotreta  micula.     Llandeilo  and  Caradoc. 

19.  Theca  reversa  (Pteropod). 

SILURIAN. 

20.  Avicula  Danbyi.     Wenlock  and  Ludlow. 

21.  Pterincea  asperula.     Wenlock. 

22.  Pterincea  tenuistriata.     Wenlock  and  Ludlow. 

23.  Pterincea  planulata.     Upper  Llandovery  to  Wenlock. 


To  face,  page  328.] 


[PLATE  XXV. 


'21 


22 


23 


ORDOVICIAN  AND  SILURIAN  FOSSILS. 


ORDOVICIAN    SYSTEM. 


329 


the  presence  of  which  would  appear  to  be  evidence  of  the  existence 
of  peat-bogs  even  in  these  remote  times. 

Subdivision. — The  typical  subdivision  of  the  Ordovician  as  seen 
in  Shropshire  is  as  follows,  the  oldest  being  at  the  bottom  : — 

f  3.  Caradoc  or  Bala  Beds — Grits  and  shales  with  their 

bands  of  limestone  (4000  feet). 
Llandeilo    Beds — Black    flags,  shales,  and    lime- 
stones (3000  feet). 

I  1.  Arenig    Beds — Black    flags,    grits,    or    quartzite 
(3000  feet). 

With  these  beds  there  are  associated  great  intercalated  masses 
of  lavas,  tuffs,  and  agglomerates. 

In  the  Lake  District,  where  the  total  thickness  is  20,000  feet,  more 


Ordovician 


FIG.  211. — Ordovician  graptolites. 

(a)  Tetragraptus.  (c)  Didymograptus  Murchisoni. 

(b)  Diplograptus.  (d)  Cwnograptus. 

than  half  of  which  is  volcanic  material,  the  succession  is  as  given 
below  : — 

f  Ashgffl  Beds  "I 

3.  Bala  Group          -I  Coniston  Limestone  V3000  feet. 

[_Dufton  Shales  J 

2.  Llandeilo  Group — Borrowdale  Volcanic  Series  (10,000  feet). 
1.  Arenig  Group     — Skiddaw  Slates,  upper  part  (7000  feet). 

The  Ordovician  of  England  contains  a  great  diversity  of  deposits, 
some  containing  shelly  fossils,  others  graptolites.  This  has  led  to 
the  construction  of  two  parallel  classifications,  one  for  the  shelly 
facies  distinguished  by  characteristic  trilobites,  the  other  for  the 
graptolitic  facies,  distinguished  by  dominant  graptolites. 

Trilobites.  Graptolites. 

I  Trinucleus  seticornis.  (Plate  XXV.)    |  "    ? 

Ashgillian^  Cybele  verrucosa.  „  i       anceps. 

\  ni   •  •         •  \  Diploqraptus 

Cheirurus  yuvems.  ,,  i      /        / 

^  J      truncatus. 


330  A  TEXT-BOOK  OF  GEOLOGY. 

Trilobites.  Graptolites. 

f  Trinucleus  concentricus  .  ^  Dicranograptus  Clingani. 

Caradocian    ~l  Phacops  apiculatus.  >  Pleurograptus. 

[_  Asaphus  Powisi.  J   Amphigraptus. 

f  Trinucleus  fav  us.  "1  Ccenograptus. 

Llandeilian   <   Asaphus  tyrannus.  >Didymograptus 

{^Ogygia  Buchi.  J       Murchisoni. 

Gibbsi.  1  Phograptus. 


Skiddavian      Placoparia.  [ 

or  Arenigianl  Mglina,  binodosa.  \  Didymograptus  -  -  open- 

/-»       •    o  i        -  branching  torms.  as  D. 

0gygm  Selwym.  • 


The  first  two  comprise  the  Upper  Ordovician  =Caradoc  or  Bala 
Group. 

Usually  the  trilobites  and  graptolites  do  not  occur  together  in 
the  same  beds,  except  in  the  Skiddavian. 

Graptolite  zones  were  first  established  in  1879  by  Lap  worth, 
whose  researches  showed  that  the  vertical  range  of  the  species  of 
graptolites  is  comparatively  limited.  He  recognised  twenty  zones, 
one  in  the  Upper  Cambrian,  eight  in  the  Lower  Silurian  (Ordovician), 
and  eleven  in  the  Upper  Silurian.  In  recent  years  the  list  of  zones 
has  been  greatly  extended. 

The  best  recognised  subdivisions  in  North  America,  where  the 
calcareous  facies  of  sediments  predominates,  are  as  follow  : — 

n-     •        ,  •  fllichmond  Beds. 

Cincmnatian  or  •      T>  j 

TT          r\  j     •  •      -s  Lorraine  Beds. 
Upper  Ordovician  ]  TT, .      ^   -, 
I  Utica  Beds. 


nt-r  i       i  •  fTrenton  Limestone. 

Mohawkian  or  ^     ,   -r,.        T . 

A/r-jji    rv  j     •  •     -\  Black  Hi ver  Limestone. 
Middle  Ordovician  1  T         .„    T  . 

Limestone. 


Canadian  or  /Chazy  Limestone. 

Lower  Ordovician  \Beckmantown  Limestone. 

These  three  divisions  exhibit  an  approximate  parallelism  with 
the  Bala,  Llandeilo,  and  Arenig  of  Great  Britain. 

Ordovician  rocks  are  well  developed  in  South-East  Australia, 
in  the  States  of  Victoria  and  New  South  Wales.  In  Victoria, 
where  they  are  best  known,  they  consist  of  alternating  bands  of 
slates,  shales,  greywackes,  and  quartzites  that  are  usually  tilted  at 
high  angles.  Calcareous  rocks  are  absent  or  but  feebly  represented. 
Fossils  are  abundant,  and  of  these  many  European  species  of 
graptolites  have  been  recognised.  The  divisions  suggested  by  Hall 
for  the  Ordovician  of  Victoria  are  as  follow  ; — 


OBDOVTCTAN    SYSTEM.  331 

Upper  Ordovician — 4.  Darriwell  Series. 

("3.  Castlemain  Series. 
Lower  Ordovician <  2.  Bendigo  Series. 

(_1.  Lancefield  Series. 

The  total  thickness  of  these  rocks  amounts  to  many  thousand  feet. 

Arenig  Group  (Lower  Ordovician). — This  group  of  rocks  derives 
its  name  from  the  Arenig  Mountains  in  North  Wales.  It  consists 
of  dark  slates,  shales,  flags,  and  sandstones  intercalated  in  the 
Shropshire  area  with  a  considerable  quantity  of  volcanic  debris. 
Many  of  the  highest  mountains  in  Wales,  such  as  Cader  Idris, 
Arenig,  Arans,  and  Berwyns,  are  composed  of  these  intercalated 
masses  of  lava  and  tuffs. 

The  most  abundant  fossils  in  this  group  are  graptolites,  although 
trilobites  are  also  common.  The  characteristic  graptolites  are 
Tetragraptus  serra,  Didymograptus  extensus,  and  D.  bifidus. 

In  the  North  of  England,  where  only  the  upper  part  of  the 
Skiddaw  slates  appears  to  be  Ordovician,  there  is  little  or  no 
evidence  of  volcanic  activity  during  Arenig  times. 

Llandeilo  Group  (Middle  Ordovician). — This  group  consists  of 
dark-coloured  flagstones,  sandstones,  and  shales  ;  all  sometimes 
more  or  less  calcareous.  It  also  contains  a  bed  of  limestone  with 
a  rich  assemblage  of  fossils,  including  many  trilobites  and  shells. 
The  graptolites  are  abundant  and  best  preserved  in  the  shales. 

In  Shropshire  the  Llandeilo  beds  contain  many  evidences  of 
contemporaneous  volcanic  activity,  and  in  the  Lake  District 
the  Skiddaw  slates  are  followed  by  an  enormous  accumulation 
of  basic,  andesitic,  and  rhyolitic  lavas,  tuffs,  and  agglomerates, 
to  which  the  name  Borrowdale  Series  has  been  applied.  The 
estimated  thickness  of  this  volcanic  pile  is  10,000  feet. 

The  alternating  hard  and  soft  bands  of  volcanic  rock  have  given 
rise  under  the  influence  of  denudation  to  the  great  diversity  of 
surface  features  which  has  made  the  Lake  District  one  of  the  most 
attractive  and  picturesque  regions  in  Britain.  Conspicuous 
among  the  mountains  composed  of  the  volcanic  rocks  of  the 
Borrowdale  series  are  Scawfell  and  Helvellyn. 

Among  the  characteristic  graptolites  of  the  Llandeilo  Group  is 
Didymograptus  Murchisoni,  which  is  abundant  in  the  lower  beds, 
and  having  only  a  limited  range,  possesses  a  zonal  value.  Trilo- 
bites are  numerous  and  include  Asaphus  tyrannus  (Plate  XXIV. 
fig.  1)  in  the  lower,  and  Ogygia  Buchii  (Plate  XXIV.  fig.  2)  in  the 
upper  portion. 

Bala^Group  (Upper  Ordovician).— This  group,  which  closes  the 
Ordovician  System,  is  named  after  the  town  of  Bala,  where  two 
bands  of  fossiliferous  limestone  are  well  exposed.  The  Caradoc 


332  A  TEXT-BOOK  OF  GEOLOGY. 

Sandstone  in  Shropshire  is  also  of  the  same  age,  hence  the  dual 
name  Bala  or  Caradoc  frequently  applied  to  this  group  of  beds. 

The  Bala  Limestone  of  Wales  is  believed  to  be  the  horizontal 
equivalent  of  the  Coniston  Limestone  of  the  Lake  District. 

The  most  abundant  genera  of  Bala  graptolites  are  Diplograptus 
and  Climacograptus. 

The  Bala  period  was  characterised  by  great  volcanic  activity,  and 
thick  masses  of  lava  and  ashes  were  intercalated  with  the  marine 
sediments.  In  many  cases  the  volcanic  ash-beds  are  fossiliferous. 

Lying  below  the  Bala  Ash  there  is  a  vast  pile  of  rhyolitic  lavas 
and  tuffs  which  culminates  in  the  peaks  of  Snowdon,  Glyders,  and 
Y-Tryfaen. 

Conditions  "of  Ordovician  Deposition. — The  physical  geography 
of  this  period  was  a  continuance  of  that  of  the  Cambrian  ;  and 

Cader  Idris 

i 


NW  .X^Sfcx      A   .  SE 


FIG.  212. — Section  across  Cader  Idris.     (After  Murchison.) 

(a)  Lingula  flags  and  Tremadoc  slates  with  bands  of  porphyry. 
(6)  Massive  porphyrites  and  greenstones,  alternating  with  (c). 
(c)  Arenig  and  Llandeilo  beds, 
(a  and  6)  =  Upper  Cambrian.  (c)  =  Ordovician. 

deposition  appears  to  have  taken  place  in  the  Northern  Hemisphere 
around  the  southern  shores  of  a  great  North  Atlantic  continent. 
Intense  local  volcanic  activity  prevailed  in  the  Lake  District  and 
in  Shropshire  in  England  ;  but  elsewhere  there  appears  to  have 
been  little  or  no  disturbance.  The  character  of  the  sediments 
and  contained  fauna  afford  some  evidence  of  minor  oscillations  of 
the  land,  but  the  general  movement  was  that  of  subsidence. 

Economic  Products. — The  economic  importance  of  the  Ordovician 
is  considerable.  In  the  United  States  the  Trenton  formation 
constitutes  one  of  the  most  productive  oil  and  gas  horizons.  In 
Central  Tennessee  the  Ordovician  limestones  contain  large  deposits 
of  rock- phosphate  ;  and  in  Wisconsin  and  the  adjoining  States  of 
Iowa  and  Illinois,  valuable  ores  of  lead  and  zinc  occur  as  replace- 
ment deposits  in  cavities  in  the  limestones  of  this  period. 

The  mineral-bearing  value  of  the  Ordovician  rocks  in  Europe 
is  unimportant.  In  Australia  they  contain  the  celebrated  gold- 
bearing  saddle-reefs  of  Bendigo,  which  have  already  added  about 
£75,000,000  to  the  wealth  yielded  by  the  State  of  Victoria. 


CHAPTER   XXIV. 
SILURIAN    SYSTEM. 

( Upper  Silurian,  Murchison  and  British  Geological  Survey.} 

SILURIAN  rocks  are  typically  developed  in  Shropshire,  and  in  Central 
and  South  Wales,  the  country  of  the  ancient  British  tribe  Silures. 
They  were  first  described  by  Sir  Roderick  Murchison,  whose 
"  Siluria  "  embraced  what  is  now  known  as  the  Ordovician  and 
Silurian  systems. 

Distribution. — Besides  occurring  in  Shropshire  and  Wales, 
Silurian  rocks  occupy  nearly  the  whole  of  the  southern  portion 
of  the  Lake  District ;  while  further  north  they  are  extensively 
developed  in  the  Southern  Uplands  of  Scotland,  where  they  stretch 
as  a  wide  belt  from  the  Mull  of  Galloway  on  the  south-west  coast 
to  St  Abb's  Head,  near  the  Firth  of  Forth. 

The  continuation  of  this  belt  is  found  across  the  Irish  Sea  in 
West  Ireland  where  it  occupies  the  greater  portion  of  County 
Down,  whence  it  extends  in  a  south-west  direction  through  the 
adjoining  counties  until  it  eventually  disappears  beneath  the 
Carboniferous  Limestone.  There  is  a  patch  of  Silurians  in  County 
Dublin,  and  many  isolated  outcrops  occur  in  the  provinces  of 
Connaught  and  Munster. 

The  Silurian  system  occupies  a  large  area  in  North-East  France, 
also  in  Scandinavia,  Finland,  and  Russia,  where  it  forms  a  wide 
belt  that  runs  parallel  with  the  Ordovician  as  far  east  as  the  Ural 
Mountains.  It  covers  large  tracts  in  Western  China,  Northern 
India,  Burma,  New  South  Wales,  Victoria,  Tasmania,  New  Zealand, 
Brazil,  Peru,  and  Bolivia. 

In  North  America  Silurian  rocks  are  typically  developed  in  the 
Appalachian  Mountains  of  New  York,  and  in  the  States  of  Penn- 
sylvania, Ohio,  Michigan,  Indiana,  and  Illinois,  all  bordering  the 
Lake  Country.  West  of  the  Mississippi,  they  extend  into  Missouri 
and  Arkansas.  North  of  the  Lakes,  the  Silurians  extend  into 
Ontario  and  adjoining  States  of  Canada  ;  and  a  considerable 
development  is  found  west  of  Hudson's  Bay  and  in  Greenland. 

Rocks. — The  rocks  of  this  system  are  almost  everywhere  sand- 

333 


334  A  TEXT-BOOK  OF  GEOLOGY. 

stones,  shales,  and  limestones.     The  latter  are  sometimes  dolo- 
mitic. 

In  the  European  and  Asiatic  regions  the  Silurian  rocks  are 
more  or  less  arenaceous  and  calcareous,  and  show  a  close  ap- 
proach to  the  calcareous  facies  which  characterises  the  lower 
Palaeozoic  rocks  of  North  America.  As  a  consequence  of  this 
new  phase  of  Silurian  deposition  in  Europe,  the  two  facies  of  life 
—  the  shelly  and  graptolitic  —  so  characteristic  of  the  British 
Ordovician,  is  not  well  marked,  and  in  the  higher  beds  is  hardly 
recognisable. 

Silurian  rocks  in  all  parts  of  the  globe  are  remarkably  free  from 
contemporaneous  volcanic  material,  from  which  it  would  appear 
that  a  period  of  general  tranquillity  followed  the  close  of  the 
Ordovician. 

Fauna. — The  general  facies  of  the  fauna  is  similar  to  that  of 
the  closely  related  Ordovician  System  ;  and  trilobites  and  grapto- 
lites  still  remain  the  dominant  organisms. 

The  distinctive  Ordovician  genera  of  graptolites  are  now  mostly 
replaced  by  the  uniserial  forms  belonging  to  the  Monograptidce. 

Corals  are  still  abundant,  but  the  coral-like  bryozoans  show  a 
marked  decline.  Crinoids  now  reached  the  summit  of  their 
development,  being  so  numerous  as  to  form  almost  the  whole  of 
some  massive  beds  of  limestone. 

Sea-urchins  and  starfish  are  still  well  represented,  especially 
in  the  higher  beds  of  the  system. 

Brachiopods  are  particularly  abundant,  and  include  some  new 
genera,  among  which  we  find  Pentamerus,  Stricklandia,  and  Dayia. 
Spirifers,  a  distinctive  type  of  straight-hinged  Brachiopod,  first 
appear  in  the  Silurian,  but  they  attain  their  greatest  development 
in  the  Devonian  and  Carboniferous. 

The  molluscs  are  still  represented  by  Lamellibranchs,  Gastero- 
pods,  and  Cephalopods ;  but  it  should  be  noted  that  the  large 
straight  Orthoceras,  which  is  the  sole  representative  of  the 
Cephalopods  in  the  Cambrian,  is  now  less  abundant  though  still 
common  ;  while  the  curved  and  coiled  forms  which  first  appeared 
in  the  Ordovician  are  plentiful,  and  represented  by  a  great  many 
genera  and  species. 

The  trilobites  are  still  represented  by  Illcenus,  Calymene, 
Phacops,  and  Homalonotus ;  but  the  new  genera  that  appear  are 
insufficient  to  balance  the  losses  due  to  the  disappearance  of  many 
Ordovician  types  ;  and  generally  throughout  this  system  there  is 
a  sensible  decline  of  the  trilobites. 

The  decline  of  these  ancient  crustaceans  is  more  than  com- 
pensated by  the  advent  in  the  late  Silurian  of  a  group  of  remarkable 
crustaceans  mainly  distinguished  for  their  abnormal  size.  The 


SILURIAN    SYSTEM.  335 

most  characteristic  of  these  are  the  gigantic  Pterygotus  and  great 
Eurypterus.  The  former  attained  a  length  of  six  feet,  while 
examples  of  the  latter  from  one  foot  to  a  foot  and  a  half  are 
common. 

The  fish  remains  found  so  abundantly  in  the  Bone  Bed  in  the 
Ludlow  Series  are  the  earliest  British  vertebrates. 

Fossil  insects  are  plentiful  and  include  scorpions  ;  but  of  the 
land  flora  and  fauna  which  clothed  and  peopled  the  Silurian 
continents  singularly  little  is  known.  Many  of  the  Ordovician 
and  Silurian  shales  are  black  with  diffused  anthracite,  which 
probably  represents  the  altered  form  of  land  and  aquatic 
plants. 

Relationships. — The  Silurian  is  normally  conformable  to  the 
Ordovician  with  which  it  is  usually  co-existent ;  although  in  some 
regions  in  Europe  and  North  America  it  is  absent  where  the  latter 
is  present.,  and  in  Northern  Canada  is  present  where  the  Ordovician 
is  missing. 

In  regions  where  uplift  took  place  after  the  close  of  the 
Ordovician,  the  Silurian  is  missing;  and,  conversely,  where  the 
submergence  of  some  of  the  continental  tracts  that  had  remained 
dry  land  since  pre-Cambrian  times  took  place,  Silurian  sediments 
were  deposited  in  areas  in  which  no  Ordovician  or  Cambrian 
rock  existed. 

In  many  places  the  Silurian  rocks  overlap  the  Ordovician,  and 
rest  unconformably  on  older  rocks.  This  overlap  is  landward, 
and  arises  from  the  subsidence  of  low  flat  shelving  coastal  lands 
that  permitted  a  rapid  advance  of  the  sea. 

In  the  British  Isles  the  Silurian  and  the  related  Ordovician  beds 
are  usually  tilted  and  folded,  and  in  the  Southern  Uplands  of 
Scotland  are  compressed  into  numerous  isoclinal  folds. 

In  Northern  Europe  and  North  America  the  Silurian  rocks  are 
comparatively  undisturbed. 

Subdivision. — The  Silurian  system  on  the  borders  of  Wales, 
where  Murchison  first  worked  out  the  succession,  begins  with 
conglomerates  and  sandstones — that  is  with  beach  deposits — and 
these  are  followed  by  the  deeper  water  shales  and  limestones 
which  alternate  with  one  another,  the  shales  being  for  the  most 
part  graptolitic  and  the  limestones  shelly. 

Towards  the  top  of  the  system  the  rocks  again  become  sandy, 
and  as  we  ascend,  the  sandstones  become  redder  and  brighter,  and 
finally  pass  into  the  overlying  Old  Red  Sandstone  of  Devonian 
age. 

In  the  prevailing  life  of  these  three  groups  of  beds  we  have  in 
Britain  the  basis  of  a  threefold  division  of  the  Silurian  System, 
which,  omitting  the  details,  is  as  follows  : — 


336 


A   TEXT-BOOK   OF   GEOLOGY. 


Clunian  or    J 
Downtonian  I  3. 

r2 

Salopian       < 

Passage  Beds 

Ludlow  Series 
(1800  feet) 

Wenlock  Series 
(2000  feet) 

Valentian 


1.  Llandovery  Series 
(1000  to  3000  feet) 


J'Ledbury  Shales. 
\Downton  Sandstone. 

C  Upper  Ludlow. 
<  Aymestry  Limestone. 

|^  Lower  Ludlow  Shale. 

f  Wenlock  Limestone. 
J  Wenlock  Shale. 
I^Woolhope  Limestone. 

Tarannon  Shale. 
Upper  Llandovery. 
Lower  Llandovery. 


Nowhere  are  the  Silurian  rocks  so  well  developed  as  at  Woolhope, 
near  Hereford;  where  they  mantle  round  the  central  dome  composed 
of  the  Upper  Llandovery  Sandstones,  as  shown  in  fig.  213. 

Llandovery  Series. — The  rocks  of  this  series  consist  mainly  of 
sandstones  and  conglomerates,  and  like  all  shore-deposits  vary 
greatly  in  character  and  thickness.  The  numerous  shells  they 
sometimes  contain  render  them  calcareous. 

In  the  Lake  District  and  Moffat  in  Dumfriesshire,  the  Llandovery 
is  represented  by  graptolitic  shales,  but  at  Girvan,  in  Ayrshire,  we 
have  the  normal  conglomerates,  sandstones,  and  limestones. 

In  some  places  there  is  a  break  at  the  base  of  the  series,  in  others, 
in  the  middle.  Frequently  the  higher  beds  overlap  the  lower, 
and  rest  directly  on  the  Ordovician.  The  breaks  arise  from  minor 
uplifts,  and  the  overlap  from  subsidence  of  a  sea-littoral  of  low  relief. 

Among  the  characteristic  Brachiopods  of  the  Llandovery  beds 
and  the  genera  Pentamerus,  Stricklandia,  and  Meristella.  Orthis, 
Atrypa,  and  Strophomena  are  also  present.  Pentamerus  undatus 
is  perhaps  the  most  prevalent  species  in  the  lower  division,  and 
P.  oblongus  in  the  upper.  Trilobites  are  also  found  in  these  beds. 

The  Tarannon  beds  which  form  the  upper  member  of  the  Llan- 
dovery series  consist  of  soft  green  and  purple  slates.  They  contain 
few  fossils. 

Wenlock  Series. — The  two  bands  of  limestone  in  this  series 
are  merely  local  intercalations  in  the  Wenlock  Shales.  They 
contain  an  abundance  of  well-preserved  fossils. 

The  Wenlock  Shales  contain  several  species  of  graptolites, 
notably  the  uniserial  Monograptus 1  priodon,  which  is  a  useful 
zonal  form,  and  Cyrthograptus . 

Among  the  most  numerous  fossils  in  the  calcareous  bands  are 
the  corals  Holy sites,  Heliolites,  and  Favosites  (Plate  XXVI.).  Other 
fossils  are  the  trilobites  Calymene,  Phacops,  and  Illcenus  ;  the 

1  Gr.  monos  =  single,  and  grapho=I  write. 


io  <*fl9mq< 

§0iwOil&     . 


.9-C 


JUT* 

oobiBi^y^i 


•  -*li.- 


(D  .8 


I 


riiJ 


PLATE   XXVI. 

SILURIAN  FOSSILS. 

1.  Acervularia  ananas.     Wenlock  Limestone. 

2.  Arachnophyllum  typus.     Wenlock  Limestone. 

2a.  Arachnophyllum   typus.     Showing   calicular   development  or  budding 
from  a  single  corallite. 

3.  Cyathophyllum   truncatum.      With    single    corallite,    showing    calicular 

development.     Wenlock  Limestone. 

4.  Cyathophyllum  articulatum.     Wenlock  Limestone. 

5.  Omphyma  turbinata.     Exhibiting   the   characteristic   rootlets   springing 

from  the  epitheca  or  wall. 
5a.  Omphyma  turbinata  and  section  of  calice,  showing  the  four  fossulse. 

6.  Omphyma  turbinata.     Cut  through  to  show  the  tabulae  and  arched  vesi- 

cular wall  tissue  of  the  coral. 

7.  Cystiphyllum  vesiculosum.     Section  showing  cellular  structure. 

8.  Ptychophyllum  patellatum.     Wenlock  Limestone  and  Shale. 

9.  Sphcerexochus  mirus.     Woolhope  and  Wenlock  rocks. 

10.  Cheirurus  bimucronatus.     Ranges  from  the  Caradoc  rocks  up  to  the 

Aymestry  Limestone. 

11.  Encrinurus  punctatus.     Caradoc  to  Upper  Ludlow. 

12.  Encrinurus  variolaris.     Wenlock  rocks. 

13.  Phacops  DowningicB.     Upper  Llandovery  to  Upper  Ludlow,  abundant. 

14.  Acidaspis  Brightii.     Caradoc  to  Wenlock. 

15.  Acidaspis  Barrandii.     Wenlock  Limestone. 

16.  Cyphaspis  megalops.     Caradoc  to  Ludlow. 

17.  Proetus  latifrons.     Upper  Llandovery  to  Wenlock  Limestone. 

18.  Favosites  Gothlandicus. .   Wenlock  Limestone. 

18a.  Enlarged  section  of  corallite,  walls  showing  perforations. 


To  face  page  336/ 


[PLATE  XXVI. 


SILURIAN  FOSSILS 


SILURIAN    SYSTEM. 


337 


•I,1 

I  Ml 

illlll 

rt     hs*.  • 

"S^.-^O 


22 


338 


A   TEXT-BOOK    OF    GEOLOGY. 


Brachiopods  Atrypa  and  Or  this  ;  and  the  Cephalopod  Orthoceras 
primcevum. 

Ludlow  Series. — The  Lower  Ludlow  shaly  mudstones  of  this 
series  are  more  sandy  than  the  underlying  Wenlock  Shales,  and 
in  places  contain  a  graptolitic  fauna  ;  in  others  a  shelly.  Near 
Ludlow  they  contain  a  number  of  graptolites,  including  the  charac- 
teristic Monograptus  colonus  of  zonal  value,  and  Cyrthograptus. 

The  Aymestry  Limestone  also  contains  many  fossils,  including 
the  Brachiopods  Pentamerus  and  Dayia. 

The  Upper  Ludlow  beds  are  soft  grey  shales  that  alternate  with 
thin  bands  of  limestone.  Towards  the  top;  where  they  are  sandy, 
they  contain  the  well-known  Bone  Bed,  which  is  a  thin  bed  full 


FIG.  214. — Showing  uniserial  graptolites. 
(a)  Monograptus  priodon  (Brown).         (6)  Monograptus  colonus  (Barr). 

of  the  bones  and  spines  of  fishes  together  with  fragments  of  the 
great  Eurypterids. 

The  principal  genera  of  Eurypterida  are  Eurypterus,1  Pterygotus,2 
and  Slimonia.  Pterygotus  anglicus  is  the  largest  known  crustacean, 
and  is  the  seraphim  of  quarry  men. 

North  American  Divisions. — The  three  main  divisions  of  the 
Silurian  recognised  in  North  America  are  as  follow  : — 

f3.  Salina  (or  Cayugan)  Series. 
Silurians  2.  Niagaran  Series. 
\\.  Oswegan  Series. 

The  rocks  are  mainly  conglomerates  and  grits  at  the  base  followed 
by  sandstones,  shales,  limestones,  and  dolomites.  The  fauna 
follows  the  same  general  succession  as  in  Western  Europe,  but 

1  Gr.  eurys  =  broad,  and  pteron  =  a,  fin. 

2  Gr.  pteryx  =  a,  wing,  and  otos  =  a,n  ear. 


SILURIAN    SYSTEM.  339 

an  exact  parallelism  cannot  be   established  between  the  three 
main  British  and  North  American  divisions. 

Conditions  of  Deposition. — At  the  base  of  the  Salina  Series  in 
New  York  there  are  lenticular  beds  of  rock-salt  varying  from 
40  to  80  feet  thick.  These  cover  an  area  of  nearly  10,000  square 
miles,  and  would  tend  to  show  that  after  the  Niagaran  period 
the  general  uplift,  which  seems  to  have  affected  the  whole  of  the 
northern  continents  after  the  mid-Silurian,  enclosed  great  shallow 
lagoons  or  land-locked  seas.  The  precipitation  of  the  salt  would 
indicate  the  prevalence  of  arid  climatic  conditions  at  this  time. 


FIG.  ZlZ.—Pterygotus.     (Restored  by  Dr  H.  Woodward.) 

This  uplift,  as  we  have  seen,  was  universal  throughout  North 
America  and  Northern  Europe,  and  by  changing  the  marine  condi- 
tions of  deposition  that  prevailed  at  the  close  of  the  mid- Silurian 
(  =  Salopian  of  Europe  and  Niagaran  of  North  America)  to  brackish 
water  and  lacustrine,  it  introduced  new  conditions  which  led  to 
the  advent  of  the  remarkable  Eurypterids.  These  belonged  to  a 
type  barely  represented  before  ;  and  they  attained  a  size  that 
would  justify  the  surmise  that  they  lived  in  a  warm  climate  and 
possessed  a  plentiful  supply  of  food. 

In  North  America  they  are  abundant  in  the  Waterlime  Hydraulic 
Limestone,  the  closing  beds  of  the  Salina  Series. 

Eurypterids  appeared  as  suddenly  and  prominently  in  the  top 


340  A  TEXT-BOOK  OF  GEOLOGY. 

of  the  Silurian  in  Wales,  England,  Scotland,  Sweden,  and  Russia 
as  in  North  America,  but  in  these  regions  there  is  no  association 
of  salt  deposits.  Nowhere  are  they  associated  with  marine  shells, 
and  they  range  upward  into  the  Old  Red  Sandstone,  in  which  their 
associates  are  land  plants  and  fishes. 

Australasia. — Silurian  rocks  cover  large  tracts  of  country^in  New 
South  Wales,  Victoria,  and  Tasmania.  They^co'nsist  chiefly  of 
sandstones,  shales,  quartzites,  limestones,  and  cherts,  which  are 
frequently  sharply  tilted  and  folded.  In  many  places  the  shales 
and  limestones  are  richly  f ossiliferous.  The  fossils  include  trilobites, 
brachiopods,  molluscs,  corals,  and  bryozoans. 

The  celebrated  Jenolan  caves,  to  the  west  of  the  Blue,  Mountains 
in  New  South  Wales,  occur  in  Silurian  limestone. 

Silurian  rocks  are  present  in  South- West  Tasmania,  at  Zeehan, 
and  Queen  River.  They  consist  of  sandstones,  slates,  and  lime- 
stones, and  contain  a  marine  fauna. 

The  Silurian  System  in  the  South  Island  of  New  Zealand  is  repre- 
sented by  slates,  quartzites,  cherts,  and  limestones.  Among  the 
fossils  in  these  rocks  are  numerous  trilobites,  brachiopods,  corals, 
and  bryozoans.  Many  of  the  trilobites  and  brachiopods  are  almost 
identical  with  species  characteristic  of  the  Silurian  of  England  and 
North  America,  but  singularly  enough  the  New  Zealand  Silurian 
fauna  is  quite  unlike  the  Australian.  This  would  tend  to  show  the 
existence  in  the  Silurian  period  of  a  continuous  sea-littoral  between 
New  Zealand,  America,  and  North- West  Europe,  and  of  a  deep 
sea  or  land  barrier  between  New  Zealand  and  Australia. 

It  is  noticeable  that  all  the  known  Silurian  rocks  in  the  Southern 
Hemisphere  belong  to  the  marine  facies.  The  terrestrial  or  semi- 
terrestrial  facies,  with  its  characteristic  fauna,  which  occupies  such 
a  conspicuous  place  in  the  close  of  the  Silurian  in  Europe  and  North 
America,  appears  to  be  missing. 

Economic  Products. — The  limestones  of  this  system  are  valuable 
as  a  source  of  lime  and  as  building-stone.  The  rock-salt  and 
associated  gypsum  deposits  in  the  State  of  New  York  are  of  great 
economic  value.  Elsewhere  the  Silurian  rocks  are*  not  notable  for 
their  mineral  contents. 

SUMMARY. 

(1)  The  lower  Palaeozoic  is  divided  into  three  great  systems, 
namely  : — 

3.  Silurian. 
2.  Ordovician. 
1.  Cambrian. 

1.  These  systems  consist  of  a  continuous  succession  of  rocks  that 


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PLATE   XXVII. 
SILURIAN  FOSSILS. 

1.  Marsupiocrinus  ccelatus  (Phill.).     Wenlock  Limestone,  Dudley,  etc. 

2.  Proboscis  of  Marsupiocrinus  ccelatus.     Inserted  in  the  shell  of  Acroculia 

haliotis.     Dudley  and  Wenlock  Edge. 

3.  Crotalocrinus  rugosus  (Miller).     Wenlock  Limestone,  Dudley,  showing  the 

arms  above  the  small  pelvis. 
3a.  Stem  with  rootlets. 

4.  Crotalocrinus  rugosus.     Stomach  plates  removed  to  show  the  base  of  the 

many-fingered  arms.     Dudley,  etc. 

5.  The  flat  stomachal  surface,  showing  branching  arms  from  their  bases. 

6.  Protaster  Miltoni  (Salt.).     Lower  Ludlow  rocks.     Leintwardine. 

7.  Protaster.     Showing  base  or  ventral  side. 

8.  Palceocoma  Marstoni  (Salt.).     Lower  Ludlow  rocks.     Leintwardine. 

9.  Palceocoma  Colvini  (Salt.).     Lower  Ludlow. 

10.  Palasterina  primceva  (Foibes).     Ludlow  rocks. 

11.  Palceaster  hirundo  (Forbes).     Upper  Ludlow.     Kendal. 

12.  Palceaster  Ruthveni  (Forbes).     Upper  Ludlow.     Kendal. 

13.  Protaster  Sedgwicki  (Forbes).     Upper  Ludlow.     Kendal. 

14.  Pseudocrinites  bifasciatus  (Pearce).     Wenlock  Limestone,   Dudley,   etc. 

After  Murchison  ;  Siluria,  4th  ed. 


To  face  page  340.] 


[PLATE    XXVII. 


SILURIAN  FOSSILS. 


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PLATE   XXVIII. 
SILURIAN  FOSSILS. 

1.  Cyathophyllum  truncatum  (Lim.).     Upper  Silurian.     Wenlock  Limestone. 

2.  Discina  Forbesii  (Orbicula)  (Dav.).    Wenlock  beds.    Dormington,  Malvern, 

etc. 

3.  Pentamerus  Knightii  (Sow.).     Aymestry  Limestone.     Sedgley,  Aymestry, 

Malvern. 

4.  Pentamerus     galeatus     (Dalm.).     Upper     Silurian.     Ludlow,     Aymestry, 

Woolhope,  Ledbury. 

5.  Lituites  cornu-arietes  (Sow.).     Presteign. 

6.  Phragmoceras  ventricosum  (Sow.).     Upper  Silurian.     Aymestry,  Dudley, 

etc. 

7.  Phragmoceras  pyriforme  (Sow.).     Upper  Silurian.      Ledbury,  Aymestry, 

Leintwardine  Hill  (Ludlow). 

8.  Orthis,  sp.,  loc.  ? 


To  face  page  340.] 


[PLATE  XXVIII. 


SILURIAN  FOSSILS. 


SILURIAN    SYSTEM.  341 

are  normally  conformable  to  one  another.  In  many  wide  tracts 
in  Europe  and  North  America  all  three  systems  are  present,  follow- 
ing one  another  in  orderly  succession,  but  in  other  places  there  may 
be  considerable  breaks  in  the  succession,  and  a  system  or  portions 
of  a  system  may  be  missing.  These  stratigraphical  breaks  are  due 
to  regional  oscillations  of  the  land. 

2.  The  three  systems  are  present  in  all  the  continents,  and  each 
is  divisible  into  three  groups,  as  follow  : — 

System.  British  Isles.  North  America. 

{3.  Ludlow  Series.  T3.  Salina. 

2.  Wenlock  Series.  <  2.  Niagaran. 

1.  Llandovery  Series.  [_!.  Oswegan. 

{3.  Caradoc  or  Bala.        f  3.  Cincinnatian. 
2.  Llandeilo.  <  2.  Mohawkian. 

1.  Arenig.  |^1.  Canadian. 

{3.  Olenus  Beds.  f3.  Potsdam. 

2.  Paradoxides  Beds.    <  2.  Acadian. 
1.  Olenellus  Beds.  [I.  Georgian. 

3.  The  fauna  preserved  as  fossils  in  the  great  pile  of  sediments 
comprising  the  Cambrian,  Ordovician,  and  Silurian,  shows  a  closely 
related  facies  throughout,  as  might  be  expected  in  the  life  inhabit- 
ing a  continuous  sea. 

4.  The  life  of  each  system  (or  period)  is  dominated  by  trilobites 
and  graptolites,  which  appear  suddenly  in  the  Cambrian,  attain 
their  greatest  development  in  the  Ordovician,  and  begin  to  wane 
in  the  Silurian. 

Besides  trilobites  and  graptolites,  there  is  a  rich  mixed  fauna  of 
corals,  bryozoans,  echinoderms,  brachiopods,  and  molluscs. 

5.  In   Northern  Europe   and   North   America,   after  the   mid- 
Silurian,  there  began  an  upward  movement  which  culminated  in 
the   Old  Red  Sandstone  (Devonian)  period.     At  the  close  of  the 
Silurian  this  uplift  enclosed  great  lakes  and  inland  seas,  particularly 
in  eastern  North  America,  where  the  drying  up  of  the  enclosed  sea- 
basin  led  to  the  formation  of  valuable  deposits  of  rock-salt. 

The  brackish- water  conditions  arising  from  the  uplift  led  to  the 
advent  of  a  remarkable  group  of  crustaceans,  which  included  among 
other  forms  the  gigantic  Pterygotus. 


CHAPTER   XXV. 
DEVONIAN   SYSTEM. 

THE  name  Devonian  was  first  applied  by  Murchison  and  Sedgwick 
to  a  great  succession  of  greywackes,  slates,  and  limestones  occurring 
in  the  counties  of  Devon  and  Cornwall. 

Marine  and  Lacustrine  Types. — The  Devonian  System  is  char- 
acterised by  the  presence  of  two  distinct  facies  of  deposits,  namely, 
a  marine  and  a  lacustrine. 

The  marine  type  or  facies  forms  continuous  sheets  of  great  extent, 
and  is  found  in  all  parts  of  the  globe  ;  while  the  terrestrial,  or  con- 
tinental as  it  is  sometimes  called,  occurs  in  disconnected  areas,  and 
is  mostly  confined  to  the  British  Isles,  Western  and  Northern 
Europe. 

The  marine  type  of  deposits  was  laid  down  on  the  floor  of  seas  that 
were  a  continuance  of  the  Silurian  seas,  and  the  continental  type  in 
basins  situated  in  regions  where  denudation  was  extremely  active. 

The  marine  Devonians  comprise  the  usual  succession  of  sand- 
stones, grits,  slates,  and  limestones,  and  contain  a  mixed  fauna  of 
trilobites,  molluscs,  brachiopods,  and  corals,  that  do  not  differ  in 
general  character  from  the  fauna  of  the  Silurian.  The  continental 
type,  on  the  other  hand,  consists  mainly  of  brightly  coloured  red  and 
brown  sandstones  and  marls  that  contain  no  brachiopods  or  corals, 
but  a  fauna  characterised  by  the  presence  of  the  giant  Eurypterids, 
land  plants,  and  armoured  fishes. 

The  marine  facies  of  rocks  is  usually  called  the  Devonian  type, 
and  the  continental  or  lacustrine,  the  Old  Red  Sandstone  type. 

Very  few  fossils  are  common  to  the  two  types,  which  are  never- 
theless believed  to  be  contemporaneous  on  stratigraphical  evidence. 

In  Devon  and  Cornwall  the  Devonian  succession  lies  between  the 
Silurian  and  Carboniferous,  and  passes  conformably  into  the  latter. 
In  France,  Belgium,  North  Germany,  North  Russia,  and  Southern 
Europe  rocks  of  Devonian  age  also  lie  between  the  Silurian  and 
Carboniferous. 

In  Scotland  the  Old  Red  Sandstone  passes  upward  into  the  Car- 
boniferous, and  in  Wales  it  passes  downward  into  the  Silurian  and 
upward  into  the  Carboniferous. 

342 


DEVONIAN    SYSTEM.  343 

The  stratigraphical  evidence  would  thus  seem  to  show  con- 
clusively that  the  marine  Devonian  rocks  are  the  equivalent  of  the 
continental  Old  Bed  Sandstone  Series. 

Conditions  of  Deposition. — The  differential  uplift,  which  began 
in  the  mid-Silurian,  continued  into  the  next  period  ;  and  in  Scot- 
land, Ireland,  and  South  Wales,  owing  to  the  peculiar  configuration 
of  the  land,  was  able  to  enclose  large  inland  basins  in  which  the 
deposition  of  sediments  took  place  contemporaneously  with  the 
deposition  of  sediments  in  the  neighbouring  seas. 

Most  of  the  basins  were  completely  detached  from  the  sea,  but 
others  were  situated  near  the  sea-coasts  in  situations  where  minor 
oscillations  of  the  land  sometimes  permitted  the  sea  to  invade  the 
basins. 

As  the  uplift  was  differential  and  faster  in  Scotland  than  in  the 
south,  the  inland  basins  came  into  existence  in  Scotland  some  time 
before  those  in  Wales. 

The  Caledonian  Movement. — There  is  abundant  evidence  of  con- 
siderable differential  movement  in  some  parts  of  Western  Europe 
during  the  early  Palaeozoic  period.  In  the  Southern  Uplands  of 
Scotland  the  Ludlow  Series  and  Passage  Beds  are  absent,  while  in 
the  North- West  Highlands  the  Silurian  is  entirely  missing. 

We  may  therefore  infer  that  the  final  folding  and  ridging  up  of 
the  Highlands  took  place  after  the  close  of  the  Ordovician  and 
before  the  advent  of  the  Carboniferous  period  ;  and  it  first  affected 
the  North- West  Highlands,  and  afterwards  the  Southern  Uplands, 
where,  as  we  have  observed,  only  a  portion  of  the  Silurian  is  absent. 

The  effects  of  this  folding  and  differential  uplift  can  be  traced  in 
the  Lake  District,  Isle  of  Man,  and  North  Wales. 

This  movement  or  series  of  movements,  usually  known  as  the 
Caledonian,  ridged  the  rocks  into  a  number  of  approximately 
parallel  folds  which  run  from  north-east  to  south-west.  It  con- 
stitutes one  of  the  dominant  structural  features  of  the  British  Isles, 
and  its  effects  are  at  once  seen  when  we  examine  a  geological 
map  of  the  United  Kingdom,  for  nearly  all  the  boundaries  of  the 
older  geological  formations  in  Scotland,  North-East  Ireland,  the 
Lake  District,  and  North  Wales  have  an  approximate  north-east 
and  south-west  bearing.  Moreover,  the  Caledonian  folds  can  be 
traced  into  Norway. 

These  crustal  folds  produced  mountain-chains,  of  which  the 
present  mountains  of  the  Scottish  Highlands  and  Norway  are  but 
the  worn- down  and  dissected  stumps.  At  the  same  time  a  great 
tract  of  land  appeared  in  North- West  Europe  which  played  an 
important  part  in  the  subsequent  history  of  the  Palaeozoic. 

Distribution. — In  the  British  Isles  the  marine  Devonian  is  most 
fully  developed  in  Cornwall,  Devon,  and  West  Somerset. 


344  A  TEXT-BOOK  OF  GEOLOGY. 

The  Old  Red  Sandstone  occupies  a  triangular  area  in  South 
Wales,  north-west  of  the  Severn.  It  also  occurs  in  the  Cheviot 
Hills  ;  and  further  north  forms  a  broad  belt  which  runs  across 
the  island  from  the  Firth  of  Clyde  to  the  Forfar  coast.  A  con- 
siderable tract  occurs  around  the  Moray  Firth,  and  practically  the 
whole  of  the  county  of  Caithness  and  the  Orkney  Islands  is  occupied 
by  the  Old  Red  Sandstone. 

In  North  Ireland  the  Old  Red  Sandstone  is  well  developed  in  the 
counties  of  Tyrone  and  Fermanagh,  and  in  South- West  Ireland  it 
forms  the  greater  portion  of  the  mountains  of  the  province  of 
Munster,  and  occupies  nearly  all  the  south-west  corner  of  the  island. 

In  Central  Europe  only  the  marine  Devonian  facies  is  repre- 
sented. A  large  tract  of  these  rocks  extends  from  the  north  of 
France,  through  the  broken  and  wooded  Ardennes  to  the  south 
of  Belgium,  and  thence  into  Rhenish  Prussia,  Westphalia,  and 
Nassau.  They  even  pass  as  far  east  as  the  Harz  Mountains  and 
Thuringia. 

In  Southern  Europe  the  Devonian  covers  a  considerable  area 
in  Spain  and  Portugal. 

In  Russia  it  occupies  an  area  many  thousand  square  miles  in 
extent,  and  stretches  from  Kurland  through  Livonia  to  the  White 
Sea.  There  is  also  a  wide  development  in  the  Urals,  Siberia, 
Altai  Mountains,  South- West  China,  Asia  Minor,  and  Turkish 
Bosphorus. 

The  Devonian  rocks  also  cover  large  tracts  in  North  and  South 
Africa.  In  South  Africa  the  rocks  of  this  age,  known  as  the  Cape 
System,  play  an  important  part  in  the  structure  of  Cape  Colony  and 
Natal. 

No  rocks  of  Devonian  age  have  so  far  been  recognised  in  India, 
but  in  Australia  the  marine  type  occupies  extensive  tracts  in 
Queensland,  New  South  Wales,  Victoria,  Western  Australia,  and 
Tasmania. 

In  North  America  the  Devonians  are  well  represented  in  the 
Appalachian  Mountains  of  New  York,  in  the  States  bordering  the 
Great  Lakes,  in  Ontario  and  Nova  Scotia,  in  Arizona,  Colorado, 
Utah,  Nevada,  Wyoming,  Montana,  North  and  South  California, 
and  many  parts  of  Alaska. 

Rocks. — The  rocks  of  the  marine  or  Devonian  facies  are  mostly 
sandstones,  conglomerates,  grits,  shales  or  slates,  and  limestones ; 
and  of  the  Old  Red  Sandstone  type,  red  and  brown  sandstones, 
and  marls. 

The  total  thickness  of  the  English  Devonian  is  about  8000  feet, 
and  of  that  of  Central  Europe  20,000  feet. 

The  Devonian  was  generally  a  period  of  comparative  tranquillity 
except  in  Great  Britain  and  Central  Europe. 


To  face  page  345. J 


[PLATE    XXIX. 


DEVONIAN  FOSSILS. 


PLATE   XXIX. 

DEVONIAN  FOSSILS. 

1.  Spirifer  disjuncta  (Sow.).      Middle  and  Upper  Devonian.      British  and 

Foreign. 

2.  Stringocephalus  Burtini  (Def.).     Middle  Devonian. 

3.  Cucullcea  trapezium  (Sow.).     Middle  and  Upper  Devonian. 

4.  Calceola  sandalina  (Lim.).     Middle  Devonian. 

5.  Cyrtoceras  tridecimale  (Phill.).     Middle  Devonian. 

6.  Murchisonia  spinosa  (Phill.).     Middle  Devonian. 

7.  Section  of  Clymenia  Icevigata,  showing  position  of  siphuncle  at  base  of 

chamber. 

8.  Cucullcea  Hardingii  (Sow.).     Middle  and  Upper  Devonian. 

9.  Strophalosia  productoides  (Murch.).     Middle  and  Upper  Devonian. 

10.  Head  of  Phacops  granulatus  (Miinst.).     Upper  Devonian. 

11.  Cystipliyllum    vesiculosum    (Goldf.).     Coral,    and   characteristic   of    the 

Middle  Devonian. 


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*    DEVONIAN    SYSTEM.  345 

In  Scotland  the  Old  Red  Sandstone  is  intercalated  with  vast 
masses  of  andesitic  lavas,  tuffs,  and  agglomerates,  from  which  we 
gather  that  the  continental  movement  was  in  Britain  accompanied 
by  intense  volcanic  activity.  The  volcanic  outbursts  took  place 
during  the  first  half  of  the  Old  Red  Sandstone  period.  The  greatest 
eruptions  occurred  in  the  Cheviot  Hills  and  in  the  Midland  Valley, 
which  stretches  across  the  country  from  the  north-east  to  south- 
west between  the  Highlands  and  the  Southern  Uplands.  In  this 
region  the  hard  masses  of  lava  and  agglomerate  stand  up  as  con- 
spicuous ridges,  as  in  the  Ochil  and  Sidlaw  Hills.  The  aggregate 
thickness  of  the  igneous  rocks  in  Scotland  is  believed  to  be  about 
6000  feet. 

In  Germany  and  Devon  the  marine  Devonians  contain  a  large 
proportion  of  igneous  material,  mostly  diabase  and  diabase  tuffs. 
These  rocks  occur  in  many  separate  horizons,  showing  that  the 
eruptions  were  separated  by  intervals  of  rest. 

Fauna. — The  general  character  of  the  Devonian  marine  fauna 
is  similar  to  that  of  the  Silurian,  and  many  of  the  characteristic 
Silurian  genera  still  survive. 

Grapt elites  are  entirely  absent,  the  last  of  them  being  seen  in  the 
Ludlow  Beds. 

Corals  and  crinoids  are  still  abundant,  but  the  former  show  a 
marked  decrease  as  compared  with  the  Silurian. 

Brachiopods  are  numerous,  and  represented  by  the  genera 
Spirifer  (Plate  XXIX.),  Rhynchonella,  Airy-pa  (Plate  XXIX.), 
Chonetes,  Stringocephalus  (Plate  XXIX.),  and  Uncites,  the  last  two 
being  limited  to  the  Devonian.  Productus  appears  for  the  first  time. 

Molluscs  are  still  abundant,  although  the  gasteropods  now  occupy 
a  subordinate  place  ;  while  the  cephalopods  show  a  notable  advance, 
being  represented  by  many  old  forms  and  a  new  type,  the  lobate- 
sutured  Goniatites. 

The  trilobites  show  a  decline  in  England  both  in  number  of 
genera  and  species,  and  those  that  survive  exhibit  a  tendency  to 
develop  into  the  spiny,  highly  ornamented  forms  which  are 
regarded  as  degenerate  types  of  Crustacea. 

In  North  America  the  trilobites  present  a  notable  increase  over 
the  number  of  species  appearing  in  the  same  region  in  the  Silurian 
epoch,  but,  as  in  England,  the  ornamented  forms  are  conspicuous. 

The  Old  Red  Sandstone  rocks  contain  very  few  fossils,  but  in  a 
few  places  in  Scotland  the  giant  Eurypterus  and  Pterygotus  are  found 
in  great  abundance  associated  with  the  remains  of  land-plants  and 
fishes. 

Many  of  the  fishes  are  protected  with  large  bony  plates  that 
form  a  more  or  less  rigid  coat  of  armour.  Among  the  genera  so 
protected  are  Pterichthys,  Cephalaspis,  and  Coccosteus. 


346  A  TEXT-BOOK  OF  GEOLOGY. 

The  plants  are  principally  Lycopods  and  ferns,  which  are  repre- 
sented by  the  genera,  Knorria  and  Palceopteris. 

The  Old  Bed  Sandstone  also  contains  a  freshwater  mussel, 
Anodonta  Jukesii  (Plate  XXXI.  fig.  4),£which  closely  resembles 
living  species. 

Subdivisions. — The  marine  Devonian  rocks  of  North  Devon  are 
divided  into  eight  groups  of  beds  as  follow  : — 

{8.  Pilton  Beds. 
7.  Baggy  Beds. 
6.  Pickwell  Down  Sandstone. 

Tur-jji    Tk         •          f  5-  Morte  Slates. 
Middle  Devonian    |4    nfracombe  Beds. 

{3.  Hangman  Grits. 
2.  Lynton  Slates. 
1.  Foreland  Sandstone. 

The  strata  are  so  much  disturbed  by  folding  and  faulting  that 
there  is  still  some  doubt  as  to  the  correct  order  of  succession  of  the 
beds. 

Fossils  are  numerous  in  the  limestones,  scarce  in  the  slates,  and 
usually  absent  in  the  sandstones.  The  limestones  and  slates  are 
marine  and  the  sandstones  probably  estuarine. 

Perhaps  some  of  the  sandstones  were  formed  in  brackish-water 
basins  near  the  sea  in  conditions  not  dissimilar  to  those  in  which 
some  portions  of  the  Old  Bed  Sandstone  were  laid  down. 

In  the  Pickwell  Down  Sandstone  beds,  which  are  red  and  purple 
in  colour,  there  are  found  the  remains  of  fishes  and  land  plants.  A 
few  of  the  fishes  are  characteristic  of  the  Old  Bed  Sandstone,  the 
commonest  genus  being  Pteraspis,  which  first  appeared  in  the 
Upper  Ludlow  towards  the  close  of  the  Silurian. 

The  weight  of  the  evidence  would  seem  to  support  the  view  that 
the  conditions  of  deposition  of  the  Pickwell  Down  Sandstone  were 
related  to  those  of  the  Mediterranean  type. 

In  Scotland  the  Old  Bed  Sandstone  is  divided  into  two  groups, 
an  Upper  and  a  Lower  Series,  which  are  separated  by  a  well-marked 
unconformity  ;  but  it  should  be  noted  that  recent  research  tends 
to  show  that  a  portion  of  the  Lower  Series  may  belong  to  the 
Silurian,  and  a  portion  of  the  Upper  Series  to  the  Carboniferous.  It 
would  seem  from  this  that  the  Old  Bed  Sandstone  conditions 
appeared  in  Scotland  earlier  than  in  South  Wales,  and  ended  later  ; 
and  this  is  what  we  should  look  for,  since  the  uplift,  which  we 
know  began  after  the  mid-Silurian,  was  differential,  being  faster 
in  North  Britain  than  in  England.  As  a  natural  consequence 
of  this  the  terrestrial  or  continental  conditions  of  deposition 


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PLATE   XXX. 

DEVONIAN  FOSSILS. 

1.  Stromatopora  polymorpha  (Goldf.).     Middle  Devonian.     South  Devon. 

2.  Hexacrinus    interscapularis   (Phill.).     Middle   Devonian.     South   Devon 

(basal  plates). 

3.  Apex  of  Hex.  acrinus  interscapularis. 

4.  Heliolites  porosus  (Goldf.).     Middle  Devonian.     South  Devon. 

5.  Spirifer  disjuncta  (Sow.).     Upper  Devonian.     North  and  South  Devon — 

passim. 

6.  Stropfialosia  productoides  (Murch.).     Middle  and  Upper  Devonian. 

7.  Stringocephalus  Burtini  (Defr.).     Middle  Devonian.     North  and  South 

Devon. 

8.  Atrypa  desquamate  (Sow.).     Lower  and  Middle  Devonian.     North  and 

South  Devon. 

9.  Megalodon  cucullatum  (Sow.).     Middle  Devonian.     South  Devon. 

10.  Clymenia    linearis    or    undulata    (Miinst.).     Middle    Devonian.     South 

Devon. 

11.  Murchisonia  bigranulosa  (D'Arch.).     Middle  Devonian.     South  Devon. 


To  face  page  346.J 


[PLATE    XXX. 


DEVONIAN  FOSSILS. 


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PLATE   XXXI. 

DEVONIAN  AND  CARBONIFEROUS  FOSSILS. 

1.  Pterygotus  anglicus  (Ag.).     Lower  Old  Red  Sandstone.     Scotland.     Ven- 

tral aspect.     After  Dr  Woodward,  F.R.S. 

(a)  The  oval  Carapace.     Showing  sessile  eyes  at  the  anterior  angles. 
(6)  The  Metastoma.     (Post  oval  plate.) 
(c.c.)  Antennules.     (Chelate  appendages.) 

(d)  Antennce,  or  first  pair  of  simple  palpi. 

(e)  Mandibles.     Second  pair  of  simple  palpi. 
(/)  First  Maxillce.     Third  pair  of  simple  palpi. 

(g)  Swimming  feet.     The  serrated  edges  of  the  basal  joints  serve  as 

maxillce. 
(h)  Thoracic  plate.     Covering  the  first  two  thoracic  segments. 

{1-6.  Thoracic  segments  or  Somites. 
7-12.  Abdominal  segments  or  Somites. 
13.  Telson  or  Tail  plate. 

2.  Bronteus  flabellifer.     Middle  Devonian. 

3.  Homalonotus,  sp. 

4.  Anodonta  Jukesii.     Upper  Devonian.     Ireland. 

5.  Clymenia  Icevigata.     Front  view  and  Siphuncle. 

6.  Calceola  sandalina.     Middle  Devonian. 

7.  Euomphalus  pentangulatus.     Carboniferous   Limestone,     (a)  Dorsal   sur- 

face.    (6)  Ventral  surface,     (c)  Pentagonal  mouth,     (d)  Section  show- 
ing chambers. 


To  face  page  346.] 


DEVONIAN  AND  CARBONIFEROUS  FOSSILS. 


DEVONIAN    SYSTEM.  347 

would  obviously  come  into  existence  in  the  north  sooner  than  in 
the  south. 

From  the  evidence  before  us  we  are  led  to  infer  that,  as  the  uplift 
progressed,  the  sea  retreated  southward  from  the  Caledonian  region 
until  it  reached  the  ancient  coasts  of  Devon,  on  the  borders  of  which 
was  formed  a  land-locked  basin  to  which  the  sea  had  occasional 
access,  and  in  which  the  Pickwell  Down  Sandstone  was  laid  down. 

The  uplift  had  now  reached  its  climax  and  was  soon  followed  by 
subsidence  which  lasted  until  the  basal  limestones  of  the  Carboni- 
ferous System  were  laid  down.  As  the  downward  movement 
progressed,  the  sea  advanced  northward,  and  deposition  of  sedi- 
ments began  long  before  deposition  could  commence  in  the  north. 
Obviously,  then,  the  beds  of  the  Lower  Carboniferous  laid  down  in 
the  Devonian  seas  would  be  missing  in  the  north. 

Devonian  rocks,  are  more  fully  developed  in  Rhenish  Prussia 
than  elsewhere  in  Europe.  They  are  arranged  in  a  series  of  reversed 
folds,  and  their  estimated  thickness  is  20,000  feet. 

The  Lower  Devonian  of  this  region  consists  mainly  of  sandy  and 
clayey  beds  in  which  fossils  are  not  abundant,  the  most  common 
being  brachiopods,  among  which  the  characteristic  species  are 
Spirifer  auriculatus,  S.  curvatus,  S.  paradoxus,  and  Chonetes  dilatata. 

The  Middle  Devonian  is  mainly  calcareous,  and  contains  in  the 
well-known  Calceola  Beds  the  rich  fauna  for  which  the  Devonian 
of  Eifel  has  become  so  famous.  Among  the  typical  forms  are  the 
Lamellibranchs  String 'ocephalus  Burtini  (Plate  XXX.  fig.  7)  and 
Megalodon  cucullatum  (Plate  XXX.  fig.  9)  ;  the  Gasteropods 
Murchisonia  bilineata  and  Pleurotomaria  delphinuloides  ;  and  the 
Cephalopods  Orthoceras  triangular e  and  Goniatites  gracilis. 

The  Upper  Devonian  is  chiefly  represented  by  calcareous  slates 
and  limestones  rich  in  fossils.  Among  the  brachiopods  are 
Rhynchonella  cuboides,  Spirifer  Verneuili,  and  Productus  subacu- 
leatus.  The  ammonoid  Cephalopod  Clymenia  is  entirely  limited 
to  the  upper  part  of  the  Upper  Devonian. 

The  threefold  division  of  the  Devonian  System  recognised  in 
North  America  is  as  follows  : — 

Upper  Devonian          fChautauquan. 
(4000  to  8000  feet)  \Senecan. 

Middle  Devonian         f  Erian. 
(1000  to  4500  feet)  \Ulsterian. 

Lower  Devonian          f  Oriskanian. 
(300  to  2000  feet)   \Helderbergian. 

By  some  American  writers  the  Helderbergian  limestones  are 
referred  to  the  Upper  Silurian. 


348  A  TEXT-BOOK  OF  GEOLOGY. 

The  North  American  Devonian  rocks  are  mostly  sandstones, 
conglomerates,  shales,  quartzites,  and  limestones.  The  shales, 
limestones,  and  many  of  the  sandstones  are  marine.  Some  of  the 
red  sandstones,  red  shales,  and  conglomerates  are  estuarine  or 
lacustrine.  The  Catskill  Beds  of  New  York  and  Pennsylvania, 
which  represent  the  whole  of  the  Upper  Devonian,  belong  to  the 
continental  facies  of  rocks.  They  contain  only  a  few  freshwater 
and  brackish- water  forms. 

The  marine  faunas  possess  the  same  general  features  as  the 
European,  but,  unlike  the  European,  are  distinguished  by  a  re- 
markable revival  of  the  trilobites. 

Economic  Products. — The  Upper  Devonian  is  the  chief  source 
of  the  oil  and  gas  in  Pennsylvania  and  South- West  New  York, 
while  the  Middle  Devonian  is  the  oil-bearing  series  in  Ontario. 
The  Devonian  shales  of  Central  Tennessee  contain  valuable  deposits 
of  rock-phosphates.  In  Europe  and  the  other  continents  the 
Devonian  does  not  contain  ores  or  minerals  of  much  economic 
value. 


CHAPTER   XXVI. 
CARBONIFEROUS   SYSTEM. 

THIS  system  contains  the  principal  coal-deposits  of  the  globe,  and 
is  therefore  of  vast  economic  value  to  mankind.  The  name  Car- 
boniferous came  into  use  at  a  time  when  it  was  believed  that  no 
true  coal  existed  in  any  other  formation.  It  is  now  universally 
recognised  as  a  time-name  for  all  the  clastic  rocks  that  lie  between 
the  Devonian  and  Permian  systems. 

Distribution. — In  Europe  the  Carboniferous  System  occupies 
large  tracts  in  the  British  Isles,  North  France,  Belgium,  Westphalia, 
and  Russia,  where  they  lie  conformably  on  the  Devonian.  In  the 
Saarbriick  district,  Bohemia,  and  Russia,  they  pass  upward  without 
a  break  into  the  Permian  ;  but  as  the  result  of  local  earth-move- 
ment, a  break  is  found  in  some  regions  between  the  Lower  and  Upper 
Carboniferous. 

A  considerable  development  of  this  system  also  occurs  in  Southern 
Europe,  notably  on  the  south  border  of  the  Central  Plateaux  of 
France,  in  the  Pyrenees,  and  Alps.  The  Carboniferous  rocks  cross 
the  Mediterranean  basin  into  North  Africa  and  appear  over  a  wide 
extent  of  country  in  the  Western  Sahara,  in  the  hinterland  of 
Morocco,  in  Eastern  Egypt,  East  Sudan,  Arabia,  and  South 
Palestine. 

From  Eastern  Russia  the  Carboniferous  System  extends  into 
Siberia,  China,  and  Japan.  In  the  province  of  Shansi,  in  Eastern 
China,  the  productive  Coal-Measures  of  this  age  have  been 
estimated  by  Richthofen  to  occupy  an  area  of  35,000  square 
miles. 

Carboniferous  rocks  are  well  developed  in  Northern  India,  but 
their  greatest  development  in  the  Northern  Hemisphere  is  in  the 
United  States  and  Alaska. 

In  the  Southern  Hemisphere  Carboniferous  rocks  occupy  wide 
tracts  in  Eastern  Australia,  South  Africa,  Peru,  and  Bolivia.  They 
are  also  present  in  the  Antarctic  continent,  but  their  extent  in  that 
region  is  at  present  unknown. 

Rocks. — There  are  two  distinct  facies  of  rocks  represented  in 
the  Carboniferous  System  in  both  hemispheres,  namely,  a  marine 

349 


350 


A   TEXT-BOOK   OF   GEOLOGY. 


and  terrestrial.  The  marine  dominates  the  Lower  Carboniferous, 
and  the  terrestrial  the  Upper  Carboniferous. 

The  marine  facies  consists  mainly  of  limestones  and  shales  ;  the 
terrestrial  facies,  of  sandstones,  conglomerates,  grits,  and  shales 
with  seams  of  coal  and  ironstone. 

In  Great  Britain  and  Russia  the  Carboniferous  rocks  are  com- 
paratively undisturbed ;  but  in  North  France,  Belgium,  and 
United  States  they  are  frequently  sharply  folded.  In  almost  all 
the  great  coalfields  the  strata,  even  when  lying  horizontal,  are 
intersected  by  numerous  faults,  some  of  which  are  of  great 
magnitude. 

In  the  British  Isles,  Western  Europe,  North  India,  and  Australia, 
the  Carboniferous  rocks  are  intercalated  with  numerous  sheets  of 
lava  and  beds  of  tuff. 

Fauna    and    Flora. — The    fauna    of   the    Lower    Carboniferous 


FIG.  215A. — Showing  cross-section  of  the  New  Boston  anthracite  basin, 
Pennsylvania.     (Penn.  Geo.  Survey.) 


limestones  is  rich  in  corals,  crinoids  and  brachiopods.  Among 
the  corals  are  the  well-known  genera  LitJiostrotion,  Cyathophyllum. 
and  Syringopora.  The  crinoids  include  many  that  have  survived 
from  the  Devonian,  and  in  addition  we  have  the  new  genera  Actino- 
crinus  and  Woodocrinus,  both  confined  to  this  system. 

Echinoderms  are  still  plentiful  ;  and  brachiopods,  which  are 
numerous,  are  represented  by  Productus,  Spirifer,  Athyris,  Rhyn- 
chonella,  and  Terebratula.  The  punctate  Spiriferina  is  also 
common. 

Molluscs  are  abundant,  and  the  Cephalopods  include  Orthoceras 
and  Actinoceras. 

Trilobites  make  their  last  appearance  in  this  system,  and  are 
represented  by  Phillipsia  (Plate  XXXIII.  fig.  7)  and  other  genera. 

Sharks  and  other  fishes  are  numerous  and  important. 

Labyrinthodonts  appear  in  the  Lower  Carboniferous.  They  are 
the  earliest  of  the  amphibians. 

The  land  flora  of  the  Upper  Carboniferous  is  luxuriant  and  varied. 


/.-.,! 


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PLATE   XXXII. 

FOSSILS  OF  THE  CARBONIFEROUS  LIMESTONE. 

1.  Posidonomya  lateralis  (Sow.).     Carboniferous  Limestone.     Venn,  Trescot, 

etc.     North  Devon. 

2.  Posidonomya  Becheri   (Goldf.).     Carboniferous    Limestone.     Swimbridge. 

North  Devon. 

3.  Edmondia  sulcata  (Phill.).     Carboniferous  Limestone.     Yorkshire,  Derby- 

shire, Ireland,  etc. 

4.  Euomphalus    pentangulatus     (Sow.).     Carboniferous     Limestone.     York- 

shire, Northumberland,  etc. 
5a-d.  Pleurotoma  aspera  (Sow.).     Carboniferous  ? 

6.  Pleurotoma  carinata  (Sow.).    Carboniferous  Limestone.    Yorkshire,  Derby- 

shire, Ireland,  etc. 

7.  Tooth    of    Magalichthys   Uibberti   (Ag.).     Lower   Carboniferous.     Burdie 

House,  Leeds,  etc. 

8.  Orthoceras    undulatum    (Sow.).     Carboniferous    Limestone.     Derbyshire, 

Lancashire,  etc. 


To  face  page  350.] 


[ElSATifi.  XX'XH. 


7  2 

FOSSILS  OF~THE  CARBONIFEROUS  LIMESTONE. 


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PLATE   XXXIII. 

CARBONIFEROUS  FOSSILS. 

1.  Phanerotinus  cristatus  (Sow.).     Carboniferous   Limestone.     Derbyshire, 

Ireland,  etc. 

2.  Productus  giganteus  (Mart.),  var.     Carboniferous  Limestone — passim. 

3.  Productus  giganteus  (Mart.).     Carboniferous  Limestone, 

4.  Bellerophon  Urei  (Flemg.).     Carboniferous  Limestone.     Rutherglen. 

5.  Bellerophon  hiulcus  (Sow.).     Derbyshire,  etc. 

6.  Prestrichia  anthrax.     Coal-Measures.     Coalbrook  Dale. 

7.  Phillipsia.     Carboniferous  Limestone. 

8.  Brachymetopus  ouralicus  ?     Derbyshire. 

9.  Palcechinus  gigas  (M'Coy).     Carboniferous  Limestone.     Ireland. 

10.  Spirorbis  carbonarius  (Murch.).     Carboniferous  Limestone. 

11.  Leperditia  inflata.     Carboniferous  Limestone. 

12.  Chcetetes     (Alveolites)     depressus     (Flemg.).     Carboniferous     Limestone. 

Bristol,  Yorkshire,  Ireland,  etc. 


To  face  page  350.] 


[PLATE    XXXIII, 


CARBONIFEROUS  FOSSILS. 


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PLATE   XXXIV. 

CARBONIFEROUS  FOSSILS. 

1.  Stigmaria  ficoides  (Brong.).     Root  of  Sigillaria.     Common  in  every  coal- 

field.     World-wide. 

2.  Liihostrotion  basaltiforme  (Edw.).     L.  striatum  (Flemg.).     Carboniferous 

Limestone.     Everywhere. 
2a.  Enlarged  section  of  calice  of  single  corallite. 

3.  Clisiophyllum    turbinatum    (M'Coy).     Carboniferous    Limestone.     Scot- 

land, Derbyshire,  etc. 

4.  Aviculopecten  papyraceus  (Goldf.).    Carboniferous  (Coal-Measures).    York- 

shire, Lancashire,  etc. 

5.  Conocardium    minax    (Phill.).     Carboniferous    Limestone.     Lancashire, 

Ireland,  Yorkshire. 

6.  Conocardium   aliforme    (Sow.).     Carboniferous    Limestone.     Lancashire, 

Isle  of  Man,  Ireland. 

7.  Goniatites  Lister i  (Martin).     Carboniferous  Limestone.     Yorkshire.    Lan- 

cashire. 

8.  Goniatites  sphceroidalis  (M'Coy).     Carboniferous  Limestone.     Ireland. 

9.  Nautilus  sulcatus  (Sow.).     Carboniferous   Limestone.     Shropshire,   Ire- 

land, etc. 


To  face  page  350.] 


CARBONIFEROUS  FOSSILS. 


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PLATE   XXXV. 

FOSSILS  OF  THE  CARBONIFEROUS  LIMESTONE. 

1.  Orthis     resupinata     (Martin).     Carboniferous     Limestone.     Lancashire, 

Derbyshire,  Ireland,  etc. 

2.  Spirifer  (rotundata)  pinguis  (Sow.).     Carboniferous  Limestone.     Lanca- 

shire, Derbyshire,  Ireland,  etc. 

3.  Spirifer    trigonalis    (Martin).      Showing    spiral    appendages.      Carboni- 

ferous Limestone.     Derbyshire,  Lancashire,  Arran,  etc. 

4.  Spirifer  striata  (Martin).    Carboniferous  Limestone.    Lancashire,  Derby- 

shire, Ireland,  etc. 

5.  Spirifer  glabra  (Martin).     Carboniferous  Limestone — passim. 

6.  Spirifer  cuspidata  (Martin).     Carboniferous  Limestone.     Bristol,  York- 

shire, etc. 

7.  Rhynchonella  pleurodon  (Phill.).     Carboniferous  Limestone.     Lancashire, 

Ireland,  Derbyshire. 

8.  Bhynchonella  acuminata  (Mart.).     Carboniferous  Limestone.     Yorkshire, 

Derbyshire,  Ireland,  etc. 

9.  Terebratula  hastata  (Sow.).     Carboniferous  Limestone.     Common  every- 

where. 

10.  Productus    punctatas    (Martin).     Carboniferous    Limestone.     Yorkshire, 
Derbyshire,  etc. 


To  face  page  350/ 


FOSSILS  OF  THE  CARBONIFEROUS  LIMESTONE. 


«A          \   : 

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PLATE   XXXVI. 
FOSSILS  OF  THE  COAL-MEASURES. 

1.  Sigillaria  pachyderma  (Lindl.).     Coal-Measures.     Northumberland,  etc. 

2.  Pecopteris,  sp.     Coal-Measures — passim. 

3.  Neuropteris  Loshii  (Brong.).     Coal-Measures.     Newcastle,  Yorkshire,  etc. 

4.  Sphenopteris  HoningJiausii  (Brong.).     Coal-Measures.     Newcastle,  etc. 

5.  Annularia  ( Aster ophyllites)  brevifolia.     Coal-Measures. 

6.  Sphenopkyllum  dentatum  (Brong.).     Coal-Measures.     Newcastle,  etc. 

7.  Wakhia  hypnoides  ?    Trias. 


To  face  page  350.] 


6  7 

FOSSILS  OF  THE  CARBONIFEROUS  COAL-MEASURES. 


CARBONIFEROUS    SYSTEM.  351 

It  is  specially  characterised  by  the  prevalence  of  gigantic  Lycopods 
or  club-mosses,  ferns  and  fern  allies,  and  horse-tails,  all  of  which 
grew  to  the  size  of  forest  trees. 

Of  the  Lycopods  the  principal  genera  are  Lepidodendron  and 
Sigillaria  (Plate  XXXVI.  fig.  1),  both  of  which  attained  a  height 
of  100  feet.  Stigmaria,  at  one  time  believed  to  be  a  distinct  genus, 
is  the  name  applied  to  the  roots  of  various  Lycopods. 

Among  the  equisetums,  the  most  important  genera  are  Calamites 
and  Annularia. 

The  fern-like  forms  of  vegetation  include  Sphenopteris  (Plate 
XXXVI.  fig.  4),  Neuropteris  (Plate  XXXVI.  fig.  3),  Pecopteris 
(Plate  XXXVI.  fig.  2),  and  Alethopteris  Sphenophyllum  dentatum 
is  also  characteristic  (Plate  XXXVI.  fig.  6). 

The  gymnosperms  are  represented  by  Cordaites  and  Conifers. 

The  lower  portion  of  the  Upper  Carboniferous  is  frequently 
marine,  and,  in  some  regions,  marine  beds  are  intercalated  with  the 
terrestrial  beds.  The  fauna  in  these  marine  beds  is  mainly  composed 
of  molluscs,  among  which  Lamellibranchs,  Gasteropods,  and  Cepha- 
lopods  are  well  represented,  among  the  first  being  Nucula  oblonga, 
Nuculana  acuta,  and  Pterinopecten  papyraceus,  and  among  the 
last  Gastrioceras  carbonarium  and  Goniatiles  Listeri  (Plate  XXXIV. 
fig.  7). 

Conditions  of  Deposition. — In  the  British  Isles  a  downward 
movement  set  in  at  the  close  of  the  Devonian  ;  but  as  we  have 
already  observed,  the  uplift  of  the  preceding  period  was  differential, 
being  greatest  in  the  north  and  least  in  the  south,  so  that  when 
the  Carboniferous  period  began,  the  conditions  of  deposition  in 
the  south  of  England  were  still  marine. 

As  the  subsidence  progressed,  the  sea  once  more  began  to  advance 
slowly  northward  ;  but  the  basal  calcareous  deposits  of  the  Carboni- 
ferous were  laid  down  in  the  Devon  region  before  the  sea  had 
reached  the  Midlands,  or  deposition  had  commenced  in  the  north. 
Hence  the  basal  beds  of  the  Lower  Carboniferous  laid  down  in 
Devon  are  older  than  the  basal  beds  further  north  by  the  time  it 
took  the  sea  to  advance  to  the  northern  regions. 

When  traced  northward  the  Lower  Carboniferous  beds  of  the 
south  decrease  in  thickness  and  finally  disappear.  Thus,  in  the 
coalfields  of  South  Staffordshire  and  Warwickshire,  the  Lower 
Carboniferous  is  either  absent  or  but  feebly  represented  ;  and  where 
the  Upper  Carboniferous  rests  directly  on  the  older  rocks,  we  have 
a  striking  example  of  the  overlap  of  strata  due  to  subsidence. 

While  marine  limestones  were  being  deposited  in  the  south  of 
England,  deposition  was  still  in  progress  in  the  inland  basins  of  the 
north  ;  that  is,  in  the  Old  Red  Sandstone  basins.  Thus  it  happens 
that  while  the  Lower  Carboniferous  of  South  England  consists  of 


352  A  TEXT-BOOK  OF  GEOLOGY. 

marine  limestones,  the  contemporaneous  beds  in  Scotland  are 
mainly  red  and  yellow  sandstones  and  shales  with  coal-seams  ; 
that  is,  rocks  of  the  Old  Red  Sandstone  facies.  These  beds  are, 
in  their  upper  division,  intercalated  with  bands  of  limestone, 
some  of  which  are  marine  and  some  freshwater.  The  lowest  of 
the  marine  limestones  marks  the  date  when  the  advancing  sea 
invaded  the  Caledonian  basins  ;  while  the  underlying  lacustrine 
sandstones  and  shales  are  a  record  of  the  time  it  took  the  sea  to 
advance  to  that  region. 

As  soon  as  a  continuous  sea  was  established,  deposition  proceeded 
on  a  continuous  sea-floor  from  south  to  north,  the  deposits  being 
everywhere  marine  and  contemporaneous.  Hence  it  follows  that 
the  upper  portion  of  the  Lower  Carboniferous  of  South  England 
may  be  correlated  with  the  Calcareous  (Carboniferous  Limestone) 
Series  at  the  top  of  the  Lower  Carboniferous  in  Scotland. 

The  Millstone  Grit,  which  follows  the  Mountain  Limestone,  is 
a  littoral  deposit.  From  this  we  learn  that  the  downward  move- 
ment was  arrested  about  the  close  of  the  Upper  Carboniferous,  and 
immediately  followed  by  a  general  uplift,  which  not  only  affected 
the  British  Isles,  but  also  the  whole  of  Northern  Europe  and 
North  America. 

The  Millstone  Grit  is  succeeded  by  the  Coal-Measures  of  deltaic 
and  terrestrial  origin  from  which  we  further  gather  that  the  uplift 
continued  until  there  was  an  approach  to  the  continental  conditions 
of  the  Old  Bed  Sandstone  period.  That  is,  the  land  was  uplifted 
until  large  inland  basins  were  enclosed,  many  of  them  possessing 
connection  with  the  open  sea  in  some  direction. 

The  succession  of  coal-seams  that  exists  in  some  of  the  coal 
basins  tends  to  show  that  many  minor  oscillations  of  the  land  took 
place  towards  the  close  of  the  Carboniferous  period. 

The  close  of  the  Carboniferous  witnessed  great  crustal  movements 
throughout  Western  and  Central  Europe,  where  there  is  a  marked 
stratigraphical  break  between  the  Carboniferous  and  Permian. 

Summarising  the  above,  we  find  that,  at  the  beginning  of  the 
Carboniferous  period,  the  open  sea  lay  to  the  south  and  the  land 
to  the  north'.  The  radiolarian  cherts  of  Devon  were  doubtless  laid 
down  in  deep  water,  the  limestones  of  South  Wales  in  clear  water  of 
moderate  depth,  and  the  sandstones,  shales,  and  coals  of  the  north 
in  estuaries,  deltas,  and  freshwater  basins. 

Each  seam  of  coal  marks  an  old  land  surface  ;  therefore  the 
numerous  seams  that  occur  in  some  regions  are  an  evidence  of 
frequent  oscillations  of  the  land. 

General  uplift  and  crustal  deformation  began  at  the  close  of  the 
Carboniferous  period  throughout  the  Northern  Hemisphere,  pro- 
ducing the  stratigraphical  break  which  separates  the  Carboniferous 


CARBONIFEROUS    SYSTEM. 


353 


from  the  Permian  in  Britain  and  Germany.  In  the  Southern 
Hemisphere  the  Carboniferous  seems  to  pass  conformably  upward 
into  the  Permian,  from  which  we  are  led  to  assume  that  the  uplift 
did  not  affect  these  regions,  thereby  permitting  deposition  to  be 
continuous. 

In  North  America,  and  generally  throughout  the  Southern  Hemi- 
sphere, there  was  pronounced  uplift  in  the  Upper  Carboniferous, 
accompanied  by  continental  conditions  of  deposition  over  wide 
tracts. 

Subdivision. — In  the  British  Isles  the  Carboniferous  System  is 
divided  into  three  principal  groups  which  may  be  described  as 
typical  of  Western  and  Central  Europe,  and  North  America. 

Series.  Rocks. 

{4.  Coal-Measures.    Sandstones,   fireclays, 
ironstones,    and    coal- 
seams. 
3.  The  Millstone 
Grit.  Grits,  sandstones,  shales. 

•2.  The  Yoredale 

Beds.  Shales. 

Lower  Carboniferous-^  1.  The  Carboni- 
ferous (Moun- 
tain) Limestone.  Limestones. 

Generally  speaking,  the  Lower  Carboniferous  is  everywhere 
characterised  by  the  marine  facies  of  rocks,  and  the  Upper  Carboni- 
ferous by  the  deltaic  and  terrestrial. 

In  England  the  Carboniferous  System  is  mainly  developed  in 
Devon,  Somerset,  South  and  North  Wales,  Midlands,  and  on  both 
flanks  of  the  Pennine  Chain. 

In  Scotland  the  Carboniferous  rocks  stretch  from  south-west 
to  north-east,  crossing  the  country  from  sea  to  sea,  from  Ayr  to 
the  Firth  of  Forth,  and  occupying  the  great  trough  between  the 
slopes  of  the  Grampians  and  the  Southern  Uplands. 

The  Carboniferous  of  Great  Britain  was  at  one  time  a  continuous 
sheet,  but  it  now  occupies  a  number  of  disconnected  basins  that 
have  been  produced  by  two  systems  of  folds,  one  system,  the 
Pennine,  running  north  and  south,  the  other,  known  as  the 
Armorican  or  Hercynian,  running  from  the  south  of  Ireland  to 
Belgium  and  thence  to  Central  Europe.  The  Coal-Measures  have 
been  preserved  in  the  troughs  and  removed  by  denudation  from 
the  crests  of  the  folds.  Obviously,  the  preservation  of  the  English 
coals  is  due  to  the  depression  of  the  Coal-Measures  in  troughs  that 
now  form  disconnected  basins. 

23 


354  A  TEXT-BOOK  OF  GEOLOGY. 

The  Carboniferous  rocks  in  Devon  consist  of  shales  with  bands 
of  chert,  limestone,  and  seams  of  impure  coal  which  are  locally 
called  culm  ;  hence  the  name  Culm  Measures  frequently  applied 
to  the  whole  series.  The  strata  are  much  folded,  and  may  be 
divided  into  two  groups  of  beds  corresponding  to  the  Lower  and 
Upper  Carboniferous  further  north. 

Carboniferous  Limestone.— This  is  frequently  called  the  Mountain 
Limestone.  It  consists  of  massive  limestone  that  is  1600  feet 
thick  in  Derbyshire,  and  over  2000  feet  thick  in  Ireland,  where  it 
occupies  more  than  half  of  the  whole  island. 

The  Mountain  Limestone  is  mainly  composed  of  crinoids,  but 
corals  and  foraminifera  are  also  plentiful  in  it  in  many  places. 
It  also  contains  numerous  brachiopods,  the  most  common  genera 
being  Productus*  Spirifer,2  Athyris?  and  Terebratula* 

The  Gasteropods  are  represented  by  Euomphalus  and  Pleuroto- 
maria  ;  and  the  Cephalopods  by  Orthoceras,  Nautilus,  and  Gonia- 
tites,  the  last  being  very  abundant. 

The  trilobites  which  appear  for  the  last  time  in  the  British  Isles 
are  well  represented  by  the  genus  Phillipsia. 

When  traced  northward  into  Derbyshire,  Lancashire,  York,  and 
Northumberland,  the  limestone  becomes  interbedded  with  thin 
bands  of  shale  which  increase  in  thickness  going  northward  and 
begin  to  contain  thin  seams  of  coal. 

In  Scotland  the  Lower  Carboniferous  consists  mainly  of  red, 
white,  and  yellow  sandstones,  variously  coloured  shales,  limestones 
and  coal-seams,  which  are  the  equivalent  of  the  Mountain  Lime- 
stone of  the  south.  These  beds  are  divided  into  two  groups  : — 

/  2.  Calcareous  or  Carboniferous  Limestone  Series. 
Lower  /  (a)  Cement  Stone 

Carboniferous^    Calciferous  Sandstone  Senes.^   ^  RBed, 

stone  Beds. 

The  Calciferous  Sandstone  Series  is  intercalated  with  vast  sheets 
of  lava  and  tuffs. 

In  Ireland  the  Lower  Carboniferous  rocks  attain  their  greatest 
development  in  the  British  Isles.  They  stretch  as  a  continuous 
sheet  from  the  south  coast  northward  to  Donegal  Bay  and  Lough 
Foyle,  and  spread  eastward  to  the  Irish  Sea.  Altogether  they 
occupy  an  area  of  about  15,000  square  miles. 

In  the  south-west  they  resemble  the  Culm  Series   of   Devon  ; 

1  Lat.  productus=  lengthened. 

2  Lat.  spira=a,  coil,  and/ero  =  I  carry. 

3  Gr.  a = with  out,  and  thyris=a,  door. 

4  Lat.  terebratio  =  a,  hole  bored. 


CARBONIFEROUS    SYSTEM.  355 

but  in  the  north  the  beds  show  a  closer  relationship  to  the  Lower 
Carboniferous  of  Scotland.  In  Clare  and  Galway  massive  lime- 
stones appear  with  shales  at  the  base  and  a  few  lenticular  bands  of 
chert,  the  total  thickness  of  the  series  amounting  to  some  3000 
feet. 

Yoredale  Beds. — These  succeed  the  Mountain  Limestone  con- 
formably and  are  typically  developed  in  Yoredale,  in  Yorkshire, 
where  they  consist  of  flagstones,  gritstones,  shales,  and  limestones 
with  coal-seams. 

This  series  shows  an  approach  to  the  estuarine  and  terrestrial 
conditions  that  prevailed  in  Scotland  during  the  deposition  of  the 
Lower  Carboniferous  Coal-Measures  of  that  region,  with  which  they 
are  perhaps  contemporaneous. 

The  Millstone  Grit. — This  is  a  series  of  beds  consisting  of  massive 
grits  and  conglomerates  with  subordinate  bands  of  shale  and 
impure  limestones,  some  of  which  contain  marine  fossils.  The 
grits  are  mostly  composed  of  angular  fragments  of  quartz  and 
felspar  that  are  probably  the  waste  of  the  granitic  and  gneissic 
areas  of  North- West  Scotland  and  Norway. 

The  rocks  of  the  Millstone  Grit  are  frequently  current-bedded, 
which  shows  that  they  were  deposited  by  water  running  in  one 
direction.  The  presence  of  the  shales,  with  sometimes  marine 
shells,  would  lead  to  the  belief  that  this  group  of  beds  was  formed 
in  the  delta  of  a  large  river  coming  down  from  the  north-east. 
The  fossils  are  mostly  the  remains  of  land  plants,  but  even  these 
are  scarce. 

Coal-Measures. — -These  consist  of  a  great  succession  of  shales 
with  subordinate  beds  of  sandstone,  impure  limestone,  ironstone, 
fireclay,  and  coal-seams.  The  original  sediments  were  probably 
laid  down  in  a  great  delta  or  estuary  on  the  southern  margin  of 
the  great  Scandinavian  continent. 

The  shales  indicate  quiet  conditions  of  deposition,  and  the 
numerous  seams  of  coal  prove  that  luxuriant  land  floras  grew  on 
the  swampy  jungle-like  mud-flats  bordering  the  sea.  The  character 
and  rank  growth  of  the  vegetation  would  point  to  the  prevalence 
of  a  warm  moist  semi-tropical  climate. 

The  coal  is  mainly  composed  of  the  spores,  spore-cases,  and 
broken  remains  of  Lycopods,  ferns,  and  horse-tails  which  accumu- 
lated to  a  great  thickness  as  peat-like  sheets  on  the  steaming 
deltaic  mud-flats. 

The  Lycopods  which  were  allied  to  the  diminutive  club-mosses 
of  the  present  day,  grew  to  the  size  of  forest  trees,  and  their  trunks, 
roots,  foliage,  and  fruit  are  found  associated  with  the  coal.  The 
Catamites  or  horse-tails  also  grew  to  a  great  size  ;  and  the  ferns 
and  fern-allies  flourished  in  great  abundance. 


356  A  TEXT-BOOK  OF  GEOLOGY. 

The  coal  usually  rests  on  a  seam  of  fireclay  called  under-day, 
which  was  the  soil  in  which  the  coal- vegetation  grew,  and  which 
became  fire-resisting  through  the  exhaustion  of  the  lime  and 
alkalies  by  the  growing  vegetation.  In  these  fireclays  there  are 
not  infrequently  found  the  roots  and  prostrate  trunks  of  fossil 
trees.  In  some  places  the  upright  stumps  have  been  found  passing 
into  the  coal  or  even  reaching  into  the  roof,  which  is  usually  a 
stratum  of  sandstone. 

The  ironstone  found  in  the  Coal-Measures  occurs  mostly  as 
concretionary  lumps  embedded  in  clay.  Frequently  the  concre- 
tions are  so  close  together  as  to  form  an  almost  continuous  sheet. 
In  each  concretion  there  is  usually  enclosed  a  fossil  fern-leaf  or 
shell. 

The  ironstone  is  mostly  carbonate  of  iron.  When  associated 
with  clay  it  is  called  clay-band  ore,  and  when  mixed  with  Car- 
bonaceous matter,  black-band  ore. 

The  sandstones  of  the  Coal-Measures  Series  contain  many 
fossil  plants,  and  sometimes  thin  seams  of  coal.  When  highly 
siliceous  they  are  called  ganister,  which  is  extensively  used  as  a 
lining  for  furnaces. 

The  Coal-Measures  Series  contains  the  productive  coal-seams 
of  the  English  coalfields. 

In  Scotland  there  are  three  productive  groups  of  beds  in  the 
Carboniferous  System  : — 

rT          n-    ,      .,  f  4.  Coal-Measures  Series — Upper  coal-beds. 

Upper  Carboniferous^    Millstone  Grit  Series-Not  productive. 

/  2.  Carboniferous  Limestome  Series — Middle 
n    i      -t  1  coal-beds. 

Lower  Carboniferous. '       Calciferous     Sandstone    Series -Lower 
(  coal-beds. 

The  bulk  of  the  productive  coal-seams  of  Scotland  belong  to  the 
Carboniferous  Limestone  Series  or  Middle  Coal-beds.  Hence  we 
find  that  the  lower  coals  of  Scotland  are  older  than  those  of  England, 
and  we  know  that  this  has  arisen  from  the  circumstance  that 
terrestrial  conditions  existed  in  Scotland  during  the  time  that 
marine  conditions  prevailed  in  the  South. 

Productive  coal-seams  are  not  so  well  developed  in  the  Car- 
boniferous System  in  Ireland  as  in  England  and  Scotland. 

English  Coalfields. — The  largest  and  most  productive  coalfields 
in  England  are  as  follow  : — 

1.  Bristol. — Coal-Measures  5000  feet  thick,  with  51  seams  of  coal, 
of  which  20  are  over  2  feet  thick. 


CARBONIFEROUS    SYSTEM. 


357 


2.  South  Wales.— Coal-Measures  from  7000  to  10,000  feet  thick, 

with  25  seams  over  2  feet  thick. 

3.  North    Wales. — Occurs    in    2    coalfields — Denbighshire    and 

Flintshire — separated  by  the   great    Bala   Fault,   with  a 
displacement  of  10,000  feet. 

4.  South    Staffordshire   contains   the   famous    10-yard   Dudley 

seam. 

5.  North  Staffordshire  or  Pottery  coalfield.— The  Coal-Measures 

are  over  5000  feet  thick  and  contain  40  seams  of  coal. 

6.  Lancashire. — Coal-Measures,  6000  feet,  with  65  seams  of  coal. 

7.  Northumberland  and  Durham. — Coal-Measures  about  3000 

feet  thick,  with  15  seams  of  coal. 

8.  Yorkshire,   Nottingham,  and  Derbyshire. — Lies  east  of  the 

Pennine  Chain.     The  southern  extension  of  the  Northum- 
berland and  Durham  coalfield. 


Pennine  Cham 


W     h 


FIG.  216. — Showing  section  across  the  Pennine  Chain  from 
Lancashire  to  Yorkshire. 


(a)  Pre-Carboniferous  rocks. 

(b)  Basement  beds. 

(c)  Carboniferous  (Mountain)  lime- 

stone. 

(d)  Yoredale  Shales  Series. 


(e)  Millstone  Grit  Series. 
(/ )  Coal -Measures  Series. 
(g)  Magnesian  limestone — 

Permian. 
(h)  Permian  sandstone. 


Coalfields  of  Scotland. — The  principal  coalfields  in  Scotland  are 
as  follow  :— 

1.  Clyde  Basin. 

2.  Mid-Lothian,  Edinburgh,  and  Haddington. 

3.  Fifeshire. 

4.  Ayrshire. 

The  Clyde  Basin  contains  the  largest  and  most  valuable  coalfield 
in  Great  Britain.  It  occupies  the  greater  part  of  four  counties 
and  is  traversed  throughout  its  whole  length  by  the  Clyde.  In 
Lanarkshire  the  coal-bearing  measures  are  about  4000  feet  thick, 
and  contain  fifteen  seams  of  coal  and  six  rich  bands  of  ironstone, 
much  of  which  is  of  the  black-band  variety. 

Valuable  cannel  and  oil  shales  occur  west  and  south  of  Glasgow 
and  also  at  Torbane.  - 


358  A  TEXT-BOOK  OF  GEOLOGY. 

Coalfields  of  Ireland.— The  coalfields  of  Ireland  may  be  divided 
into  northern  and  southern  groups  as  under  : — 

XT    J.-U         f  !•  Leitrim. 

Northernl       Con         ht  and  T          . 

GrouP   \3.  Antrim 

a     ,  i  f  4.  Clare,  Limerick,  and  Kerry. 

Southern     |  6_  Queen>s  Cc-~.-  ™~. 

6.  Tipperary. 


^  5.  Queen's  County,  Kilkenny. 


Denudation  has  removed  the  greater  portion  of  the  productive 
Coal-Measures  in  the  north  of  Ireland. 

Contemporaneous  Volcanic  Rocks. — The  early  part  of  the  Lower 
Carboniferous  period  was  disturbed  by  local  volcanic  outbursts 
in  the  south  of  Ireland,  Devon,  Isle  of  Man,  and  Derbyshire  ;  but 
it  was  in  Scotland  that  the  eruptions  attained  their  greatest 
intensity.  The  eruptions  began  at  the  close  of  the  Old  Red  Sand- 
stone period,  and,  with  intervals  of  rest,  continued  till  the  beginning 
of  the  Coal-Measures.  In  the  earlier  stages  the  outbursts,  according 
to  Sir  Archibald  Geikie,  were  characterised  by  the  quiet  outpouring 
of  great  floods  of  lavas  of  the  plateau  type,  and  in  the  waning 
phases  by  the  emission  of  piles  of  ashes  and  streams  of  lava  from 
prominent  volcanic  vents. 

The  plateau-lavas  covered  a  large  area  in  the  Mid-Lothians,  and 
in  places  reached  a  thickness  of  3000  feet.  They  are  mostly 
andesitic.  The  lavas  of  the  volcanic  type  are  mainly  basalts. 

In  Derbyshire  the  Carboniferous  Limestone  is  associated  with 
sheets  of  basalt  and  olivine-dolerite,  some  of  which  are  apparently 
contemporaneous,  while  others  are  probably  intrusive  sills. 

Beds  of  volcanic  tuff  are  interstratified  with  the  Lower  Carboni- 
ferous rocks  in  the  Isle  of  Man,  where  intrusive  sills,  dykes,  and 
agglomerates  also  occur. 

The  Carboniferous  rocks  in  Ireland  are  remarkably  free  from 
contemporaneous  volcanic  outbursts,  except  at  Limerick,  where 
there  are  two  series  of  volcanic  rocks  separated  by  a  great  thickness 
of  sedimentary  rocks.  The  lower  series  is  mainly  composed  of 
andesites  and  basalts  of  the  plateau  type  with  beds  of  tuff,  and 
the  upper  of  basaltic  lavas. 

It  is  notable  that  the  Carboniferous  centres  of  activity  are 
situated  in  the  same  regions  as  those  of  the  Old  Red  Sandstone 
period  ;  and  it  is  significant  that  while  the  rocks  of  the  earlier 
period  in  the  Midland  of  Scotland  are  calcic,  those  of  the  Carboni- 
ferous in  the  same  region  are  of  a  distinctly  alkali  type. 

In  the  North  England  coalfields,  the  Coal-Measures  are  intruded 
by  many  igneous  dykes  of  probably  Tertiary  date.  Some  of^these 


CARBONIFEROUS    SYSTEM.  359 

dykes,  like  the  well-known  Cockfield  Dyke  of  Cleveland,  traverse 
Carboniferous,  Permian,  Triassic,  and  Jurassic  rocks,  and  displace 
the  coal-seams  like  faults. 

North  America.— The  rocks  of  the  Carboniferous  System  cover 
an  area  of  approximately  200,000  square  miles  in  the  United 
States  and  British  North  America.  They  are  divided  into  two 
great  sub-systems,  comprising  eight  series,  as  follow  : — 


Pennsylvanian 

(Littoral  and 

lacustrine  facies) 


8.  Monongahela    Series  —  Upper  ^ 

Productive  measures. 
7.  Conemaugh     Series  —  Barren 


measures. 


Coal- 
Measures. 


6.  Allegheny  Series — Lower  Pro- 
ductive measures. 

5.  Potts ville  Series —  .         .  Millstone  Grit, 

f  4.  Kaskaskia  Series.  \ 

Mississippian        I  3.  St  Louis  Series.  [  Carboniferous 

(Marine  facies)      I  2.  Osage  or  Augusta  Series.  I     Limestone. 

[l.  Kinderhook  Series. 

The  early  stages  of  the  Mississippian  in  Michigan  were  littoral 
and  terrestrial,  but  as  the  result  of  a  general  subsidence  which 
affected  almost  the  whole  of  the  Northern  Hemisphere,  the  deposi- 
tion of  the  marine  facies,  mainly  characterised  by  limestones,  soon 
followed. 

At  the  close  of  the  Mississippian,  there  began  a  general  uplift 
which  led  to  a  return  of  the  terrestrial  and  continental  conditions 
which  characterised  the  Old  Bed  Sandstone  period.  During  this 
time  of  uplift,  the  Upper  Carboniferous  Pennsylvanian  beds  were 
laid  down  partly  on  a  sea-littoral  and  partly  in  estuaries  or  en- 
closed basins.  The  uplift,  as  in  Europe,  continued  well  into  the 
Permian. 

The  anthracitic  and  bituminous  coals  of  Pennsylvania,  Illinois, 
Ohio,  and  neighbouring  States  are  of  vast  extent  and  great  value. 

India. — In  Northern  India,  in  the  Spiti  Valley,  there  is  a  pile  of 
shales  4000  feet  thick  which  is  believed  to  represent  the  whole  of 
the  Carboniferous  System.  The  lower  half,  known  as  the  Lipak 
Series,  is  mainly  composed  of  calcareous  shales  that  contain  a 
rich  marine  fauna,  including  Productus,  many  molluscs,  and  the 
trilobite  Phillipsia. 

The  upper  half,  about  2000  feet  thick,  called  the  Po  Series, 
consists  of  quartzites  and  shales,  the  lower  portion  of  which  contains 
a  few  fossil  plants  that  seem  to  be  identical  with  plants  in  the 
Culm  of  Europe  and  Australia.  The  upper  subdivision  contains 
many  marine  forms,  among  which  bryozoans  are  plentiful,  including 
the  genus  Fenestella,  which  has  given  its  name  to  this  group. 


360  A  TEXT-BOOK  OF  GEOLOGY. 

Throughout  the  whole  length  of  the  Himalayas  and  in  the  Chinese 
provinces  beyond  the  eastern  limits  of  India,  there  is  a  vast  develop- 
ment of  volcanic  rocks  which  may  perhaps  be  of  Lower  Carboni- 
ferous age. 

The  Carboniferous  succession  in  Northern  India  is  as  follows  : — 


Upper  Carboniferous — Po  Series  <  \  | 

Lower  Carboniferous — Lipak  Series — Marine  facies. 

It  will  be  seen  from  this  succession  that  we  have  in  India,  at 
the  close  of  the  Lower  Carboniferous,  the  same  break  as  in  Northern 
Europe  and  North  America ;  which  demonstrates  that  the  uplift 
and  crustal  disturbance  of  the  northern  continents  was  general 
throughout  the  whole  of  the  Northern  Hemisphere.  The  relation- 
ship between  this  uplift  and  the  volcanic  activity  which  disturbed 
the  Lower  Carboniferous  is  not  very  clear,  for  it  would  appear  that 
the  volcanic  outbursts  everywhere  preceded  the  uplift. 

Australasia.  —  Hocks  of  Carboniferous  age  occupy  extensive 
tracts  in  New  South  Wales,  Queensland,  Victoria,  Western  Australia, 
and  Tasmania.  In  New  South  Wales  the  Upper  Carboniferous 
passes  upward  into  the  Permian  without  any  evidence  of  a  strati- 
graphical  break.  The  subdivisions  of  the  Carboniferous  recognised 
in  that  State  are  as  follow  : — 

Permo- Carboniferous   f  c,      ,  ,  .,1 

(11,000  to  13,000  feet)\8andstones  and  shales  Wlth  coal-seams- 

Lower    Carboniferous /Sandstones  and  conglomerates  with  bands 
(11,000  feet)         ^     of  shale  and  limestone. 

The  Lower  Carboniferous  rocks  occur  chiefly  between  the  Hunter 
and  Manning  Rivers.  The  sandstones  contain  the  gigantic  club- 
moss,  Lepidodendron  austmle,  which  is  also  found  in  Queensland. 
It  would  thus  appear  that  in  Eastern  Australia  the  Carboniferous 
was  ushered  in  with  terrestrial  conditions  of  deposition. 

The  Permo-Carboniferous  is  the  productive  coal-series  of  New 
South  Wales.  It  is  displayed  over  an  area  of  25,000  square  miles 
in  the  Port  Macquarie  and  Newcastle  districts.  Going  southward, 
the  Coal-Measures  disappear  below  the  Triassic  Hawkesbury 
Sandstone,  and  extend  along  the  coast  to  Sydney,  where  they  have 
been  proved  at  a  depth  of  3000  feet  below  sea-level. 

Among  the  plants  associated  with  the  coals  are  several  species 
of  the  genus  Glossopteris,  which  is  characteristic  of  the  Mesozoic 
Gondwana  System  of  India,  and  was  formerly  believed  to  be  con- 
fined to  the  Mesozoic.  Its  range  is  now  known  to  extend  from  the 
Carboniferous  to  the  Jurassic, 


CARBONIFEROUS    SYSTEM.  361 

The  marine  beds  of  this  series  contain  a  rich  Carboniferous 
fauna  which  includes  the  brachiopods  Athyris,  Orthis,  and  Pro- 
ductus,  and  the  bryozoan  Fenestella  plebeia.  The  coal  occurs  in 
three  horizons  as  determined  by  Professor  David : — 

6.  Upper  or  Newcastle  Coal-Measures. 

5.  Dempsey  Series. 

4.  Middle   or   Tomago,    or   East   Maitland 


Permo-Carbonif  erous  -< 


Series. 

3.  Upper  Marine  Series. 
2.  Lower  or  Greta  Coal-Measures. 
1.  Lower  Marine  Series. 


There  was  intense  volcanic  activity  in  the  north-east  portion  of 
New  South  Wales,  where  the  Carboniferous  strata  are  intercalated 
with  many  sheets  of  lava,  mostly  rhyolite,  as  well  as  with  thick 
beds  of  tuff.  The  Permo-Carboniferous  was  freer  from  disturbance, 
but  the  strata  of  this  period  are  intercalated  with  beds  of  tuffs 
and  contemporaneous  sheets  of  andesite  and  basalt- 
Associated  with  the  Upper  Coal-Measures  there  are  massive 
beds  of  conglomerate  containing  scratched  boulders  which  are 
believed  to  have  been  transported  by  ice. 

In  Queensland  the  Carboniferous  rocks  have  been  divided  into 
five  distinct  series  of  beds  as  follow : — 

5.  Upper  Bowen  Series— Terrestrial  beds  with   coal-seams  and 


4.  Middle  Bowen  Series — Partly  marine  and  partly  terrestrial, 

with  Productus  and  Glossopteris. 

3.  Lower  Bowen  Series — Partly  terrestrial  and  partly  volcanic. 
2.  Star    Series  —  Partly    freshwater    and    partly    marine,    with 

Lepidodendron . 
1.  Gympie  Series — Marine,  with  Productus,  Fenestella,  etc. 

The  productive  Permo-Carboniferous  rocks  of  New  South  Wales 
have  not  been  discovered  in  Victoria,  but  Lower  Carboniferous 
beds  are  well  developed  in  Central  Gippsland,  where  they  contain 
characteristic  Lower  Carboniferous  fishes. 

The  Carboniferous  rocks  of  Western  Australia  mainly  belong  to 
the  lower  or  marine  facies.  The  Permo-Carboniferous  type  is 
present  in  the  Irwin  and  Collie  coalfields. 

In  Tasmania  Carboniferous  rocks  occupy  a  large  tract  in  the 
south-east  portion  of  the  island.  The  Lower  Carboniferous  is 
typically  marine  ;  and  the  upper  Carboniferous,  terrestrial  or 
estuarine,  consisting  mainly  of  grits  and  shales  with  Gangamopteris, 
Glossopteris,  and  Nceggerathiopsis.  The  seams  of  coal  are  thin. 


362  A  TEXT-BOOK  OF  GEOLOGY. 

Carboniferous  rocks  of  the  marine  and  estuarine  types  are 
present  in  New  Zealand,  and  contain  Productus,  etc. 

South  Africa. — No  Carboniferous  rocks  have  so  far  been  dis- 
tinguished in  South  Africa  ;  but  it  is  not  improbable  that  the 
upper  portion  of  the  Cape  System  may  be  the  equivalent  of  the 
European  Carboniferous. 

Economic  Products. — The  supreme  importance  of  the  Carboni- 
ferous System  lies  in  the  abundance  of  coal  which  it  contains. 
Economically  this  system  is  more  important  than  any  other,  and 
the  value  of  the  coal  annually  produced  from  it  is  greater  than  the 
total  value  of  the  mineral  production  of  all  the  other  systems  put 
together. 

The  annual  production  of  coal  amounts  to  about  1,000,000,000 
tons,  valued  at  £500,000,000,  which  exceeds  the  value  of  the  annual 
output  of  iron,  gold,  silver,  tin,  copper,  lead,  diamonds,  and  all 
other  minerals  more  than  twofold. 

The  ironstones  produced  from  the  Coal-Measures  of  Great  Britain 
and  Western  Europe  are  still  of  great  value. 

The  limestones  of  this  system  are  useful  as  building- stone  and 
for  the  production  of  lime  for  mortar  and  agricultural  purposes. 


CHAPTER   XXVII. 
PERMIAN   SYSTEM, 

THE  Permian  is  the  youngest  of  the  Palaeozoic  systems.  In 
Southern  Europe,  Russia,  India,  Australia,  and  South  Africa,  it 
follows  the  Carboniferous  quite  conformably  ;  but  in  the  British 
Isles  and  Germany  it  is  separated  from  the  Carboniferous  by  a 
well-marked  physical  break,  and  in  these  regions  is  more  closely 
related  to  the  Mesozoic  than  to  the  underlying  Palaeozoic  formations. 

In  Europe  the  series  of  red  sandstones,  marls,  conglomerates, 
breccias,  limestones,  and  dolomites  which  follows  the  Carboni- 
ferous was  formerly  known  in  England  as  the  New  Red  Sandstone 
to  distinguish  it  from  the  Old  Red  Sandstone  which  underlies  the 
Carboniferous. 

The  lower  portion  of  the  New  Red  Sandstone  was  subsequently 
found  to  contain  fossils  related  to  those  in  the  Carboniferous, 
and  the  upper  portion  fossils  related  to  Mesozoic  forms.  This  led 
to  the  division  of  the  New  Red  Sandstone  into  two  distinct 
systems,  the  lower,  called  the  Permian  System,  being  placed  in 
the  Palaeozoic ;  and  the  upper,  called  the  Triassic  System,  being 
referred  in  the  Mesozoic. 

The  name  Permian  was  first  suggested  by  Sir  Roderick  Murchison 
in  1841  for  a  great  development  of  these  rocks  in  the  old  kingdom 
of  Perm  in  Eastern  Russia. 

The  Permian  System  of  England  is  the  Dyas  of  German  geolo- 
gists. 

Distribution. — The  Permian  System  attains  a  considerable 
development  in  Russia,  Germany,  France,  Alps,  Sicily,  Armenia, 
India,  Australia,  South  Africa,  and  South  America.  The  area  it 
covers  in  the  British  Isles  is  comparatively  insignificant. 

Rocks. — In  Europe  the  Permian  consists  of  two  distinct  facies 
of  rocks,  namely,  the  Dyas  type  of  Germany,  and  the  Russian 
type. 

In  the  Dyas  type  there  are,  as  the  name  implies,  two  divisions 
or  groups  of  beds  :  (1)  a  lower  terrestrial  series  consisting  mainly 
of  red  sandstones  and  conglomerates  ;  and  (2)  an  upper  marine 
series  of  limestones  and  dolomites. 

363 


364  A  TEXT-BOOK  OF  GEOLOGY. 

In  the  Russian  type  the  same  strata  are  represented,  but  they  are 
interstratified  in  such  a  way  as  to  preclude  a  twofold  subdivision  ; 
that  is,  there  is  an  alternation  of  terrestrial  and  marine  beds  through- 
out the  whole  system. 

The  prevailing  rocks  of  the  European  Permian  are  red  sandstones, 
in  many  places  interbedded  with  bands  of  conglomerate,  fine  shales, 
or  marls.  The  basal  beds  are  frequently  conglomerates,  which  in 
places  pass  into  coarse  angular  breccias. 

The  sandstones  are  typically  brick-red,  and  the  so-called  marls 
are  even  deeper  red.  In  many  parts  of  Germany  the  marl-slate  is 
impregnated  with  a  small  percentage  of  copper-ore. 

The  limestone,  which  may  be  regarded  as  characteristic  of  the 
Dyas  type,  is  well  bedded,  often  clayey,  and  usually  more  or  less 
dolomitic. 

On  the  flanks  of  the  Harz  Mountains  the  sandstones  contain 
seams  of  coal  with  which  are  associated  bituminous  shales  and 
ironstones. 

In  Western  Europe  the  lower  portion  of  the  Permian  is  inter- 
calated with  masses  of  contemporaneous  igneous  rocks. 

Relationship  to  Carboniferous. — Throughout  Western  and  Central 
Europe  the  Permian  rests  on  the  denuded  folds  of  the  Carboniferous 
System  and  on  older  rocks.  Obviously  the  folding  and  denudation 
took  place  after  the  deposition  of  the  Carboniferous  and  before  the 
Permian  period  began.  In  the  interval  that  separates  these  two 
systems  the  greater  portion  of  Northern  Europe  must  have  been 
dry  land. 

But  in  portions  of  North  America,  Eastern  and  Southern  Europe, 
Central  Asia,  and  Australia,  the  Carboniferous  seas  still  existed,  and 
on  the  floor  of  these  there  was  laid  down  a  continuous  succession  of 
marine  sediments,  covering  the  interval  which  separates  the  Car- 
boniferous and  Permian  in  Western  and  Central  Europe.  That  is, 
the  interval  representing  the  unconformity  in  Europe  is  bridged 
over  by  what  may  be  described  as  transition  beds.  Therefore,  in 
the  regions  where  a  continuous  sea  existed  throughout  the  Upper 
Palseozoic,  we  get  the  following  succession  : — 

f  Permian. 

Upper  Palaeozoic^  Permo-Carboniferous  (Transition  Beds). 
(^Carboniferous. 

Conditions  of  Deposition.— The  distribution  and  character  of 
the  Permian  in  Europe  afford  conclusive  proof  that  the  great 
Scandinavian  continent  of  North- West  Europe,  which  played  so 
important  a  role  in  the  formation  of  the  Carboniferous  Coal-Measures 
of  Western  Europe,  gradually  increased  in  size  until  its  shores 
encroached  on  Southern  Europe.  The  late  Carboniferous  uplift 


PERMIAN    SYSTEM.  365 

of  the  northern  lands,  with  the  simultaneous  retreat  of  the  sea  to 
the  south,  was  merely  an  expression  of  a  great  crustal  disturbance 
which  affected  a  wide  zone  of  country  which  can  be  traced  from 
the  southern  extremity  of  Ireland  eastward  through  the  southern 
promontories  of  South  Wales  and  Mendip  Hills  to  Belgium  and 
Central  Germany.  This  belt  of  intense  folding  everywhere  followed 
an  approximate  WNW.-ESE.  course,  and  raised  a  system  of  moun- 
tain folds,  known  as  the  Armorican  or  Hercynian  Chain,  which 
extended  from  the  Atlantic  eastward  to  Central  Europe,  but  of 
which  only  the  worn-down  stumps  now  remain,  mostly  buried 
beneath  the  later  rock-formations. 

The  name  Armorican  is  derived  from  Armorica,  the  ancient  name 
of  Brittany,  where  the  chain  attained  a  great  height ;  and  the  name 
Hercynian  from  the  Hercynian  Forest,  of  which  portions  still 
remain  in  Swabia  and  the  Harz  Mountains. 

The  productive  coal-basins  of  Ireland,  Great  Britain,  North 
France,  and  Belgium  lie  in  the  northern  folds  of  the  Armorican 
Chain. 

The  general  uplift  we  have  spoken  of  caused  the  sea  to  retreat 
southward,  and  at  the  same  time  it  established  continental  condi- 
tions in  Northern  Europe,  where  great  inland  basins  of  the  Caspian 
type  and  seas  of  the  Mediterranean  type  were  formed.  In  these 
Permian  basins  and  land-locked  seas,  to  some  of  which  the  ocean  still 
had  access,  was  laid  down  a  great  succession  of  sandy,  pebbly,  and 
clayey  deposits,  alternating  with  calcareous  sediments.  Some  of 
the  sandstones  present  the  aspect  of  consolidated  sands  that  may 
have  accumulated  in  desert  conditions  not  unlike  those  prevailing 
at  the  Isthmus  of  Suez,  where  we  have  a  tract  of  more  than  10,000 
square  miles  of  wind-blown  desert  sand,  salt-water  lagoon,  and 
swamp,  lying  a  few  yards  above  sea-level. 

In  the  land-locked  basins  the  fauna  was  meagrely  represented  by 
forms  descended  from  the  inhabitants  of  the  Carboniferous  seas. 
Amphibians  crawled  about  the  marshy  shores,  and  the  land 
supported  a  vegetation  closely  related  to  the  Carboniferous. 

Fauna.— The  conditions  of  life  and  the  environment  that  pre- 
vailed in  Northern  Europe  were  not  favourable  for  the  development 
of  a  prolific  fauna.  The  majority  of  the  Carboniferous  genera 
disappeared  before  the  continental  conditions  became  general, 
and  the  forms  that  survived  were  mostly  small  and  frequently 
of  abnormal  type.  Moreover,  the  increasing  salinity  of  the  en- 
closed basins  was  not  favourable  for  the  introduction  of  new 
genera. 

The  forms  that  were  least  affected  by  the  changed  conditions 
were  the  Polyzoans,  which  flourished  in  such  abundance  as  to 
constitute  the  bulk  of  the  limestones. 


366  A  TEXT-BOOK  OF  GEOLOGY. 

Among  the  Polyzoans  Fenestella  retiformis  is  a  characteristic 
species. 

The  corals,  echinoderms,  and  Cephalopods  that  were  so  promi- 
nent in  the  Carboniferous  seas  have  almost  disappeared.  Trilobites 
are  unknown  in  the  British  Permian,  and  are  but  feebly  represented 
elsewhere. 

Generally  the  fauna  of  the  lower  division  of  the  Permian  possesses 
a  terrestrial  facies,  and  consists  of  insects,  molluscs,  crustaceans, 
a  few  fish,  and  amphibians,  the  last  represented  by  Labyrinthodonts. 

In  the  limestones  and  dolomites  of  the  Upper  Permian  are  found 
a  few  stunted  brachiopods,  Lamellibranchs,  Gasteropods,  and 
Cephalopods. 

While  the  fauna  entrapped  in  the  Caspian-like  seas  show  unmis- 
takable evidence  of  decadence,  the  genera  living  in  the  open  seas 
continued  to  flourish  and  follow  the  normal  processes  of  develop- 
ment. In  Sicily,  Armenia,  and  India  there  lived  a  rich  and  varied 
marine  fauna,  in  which  new  genera  came  in  to  take  the  place  of 
the  old. 

The  normal  marine  Permian  fauna  is  therefore  not  found  in 
Western  or  Central  Europe,  but  in  Southern  Europe  and  Asia, 
where  it  is  represented  by  corals,  bryozoans,  brachiopods,  and 
numerous  molluscs. 

Ammonites,  which  are  so  characteristic  of  the  Mesozoic  forma- 
tions, began  to  appear  for  the  first  time  in  the  Permian  seas  ;  and 
the  earliest  reptiles,  represented  by  the  genera  Proterosaurus  and 
Palceohatteria,  are  present  in  the  continental  Permian  of  Germany. 

The  Brachiopods  include  Productus,  Spirifer,  Spiriferina,  Tere- 
bratula,  and  Rhynchonella  ;  the  Gasteropods,  Bellerophon,  Pleuroto- 
maria,  and  Naticopsis ;  the  Lamellibranchs,  Avicula,  Pecten, 
Schizodus  (allied  to  Trigonia)  ;  and  the  Cephalopods,  the  ammon- 
oids  Medlicottia  and  Popanoceras,  and  a  whole  series  of  Ortho- 
ceratites. 

Plants  are  represented  by  many  survivors  from  the  Car- 
boniferous, and  include  the  familiar  Calamites,  also  Walchia  (Plate 
XXXVIII.  fig.  2)  and  Callipteris. 

In  America,  as  in  Europe,  the  close  of  the  Carboniferous  witnessed 
great  changes  in  the  distribution  of  the  land.  Freshwater  deposits 
continued  to  be  laid  down  in  the  Coal-Measure  basins  in  Pennsyl- 
vania, Ohio,  West  Virginia,  and  Maryland  ;  and  a  vast  sheet  of 
Permian  of  the  Mediterranean  type  of  deposits  was  laid  down  in 
Texas,  Kansas,  and  Nebraska. 

The  dominant  feature  of  the  Southern  Hemisphere  in  this  period 
is  a  vast  pile  of  shales  and  sandstones  of  a  deltaic  and  terrestrial 
facies,  comprising  what  is  typically  known  in  India  as  the  Gond- 
wana  System,  which  ranges  in  age  from  the  Permo-Carboniferous 


To  face  page  366.] 


[PLATE    XXXVII. 


REPRESENTATIVE  TYPES  OF  GLOSSOPTERIS  FLORA 


(a)  Glossopteris  communis. 

(6)  G.  angustifolia. 

(c)  Gangamopteris  cydopteroides. 

(d)  Nceggerathiopsis  hislop. 


(e)   Neuropteris  valida. 

(/)    Schizoneura  gondwanensis, 

(g)  Phyllotheca  indica. 

(h)   Voltzia  heterophylla. 


(After  Chamberlin  and  Salisbury.) 


V.I. 


.  .:->  ,  ij.i  rariH      .«- 

'  , 


.- 


PLATE   XXXVIII. 

PERMIAN  FOSSILS. 

1.  Voltzia  heterophylla.     Permian. 

2.  Walchia  Schlotheimii.     Permian. 

3.  Synocladia  virgulacea  (Phill.).     Permian.     Humbleton,  Tunstall,  etc. 

4.  Fenestella    retiformis    (Schloth.).     Permian.     Tynemouth,    Humbleton, 

etc. 
4a.  Enlarged  portion  of  Fenestella  retiformis.     Permian.     Humbleton,  etc. 

5.  Productus    horridus    (Sow.).     Permian.     Humbleton,    Tunstall,    Tyne- 

mouth, etc. 

6.  Strophalosia    Morrisiana    (King).     Permian.     Tynemouth,    Humbleton, 

etc. 


To  face  page  366.] 


PERMIAN  FOSSILS. 


PERMIAN  SYSTEM.  367 

to  the  Jurassic.  This  system  is  well  developed  in  Australia,  New 
Zealand,  South  America,  South  Africa,  and  Antarctic  continent  ; 
and  is  everywhere  characterised  by  the  presence  of  a  peculiar 
terrestrial  vegetation  usually  called  the  Glossopteris  flora  (Plate 
XXXVII.).  The  most  distinctive  types  of  this  are  the  ferns 
Glossopteris  and  Gangamopteris,  and  the  horse-tail  Schizoneura 
(Plate  XXXVII.). 

The  wide  distribution  of  the  Glossopteris  flora  in  India,  Australia, 
South  Africa,  and  South  America  has  given  rise  to  the  belief  that 
the  sediments,  in  which  the  remains  of  this  flora  are  preserved,  were 
laid  down  on  the  margin  of  a  great  continent  which  occupied  the 
site  of  the  existing  Indian  Ocean,  and  extended  in  the  Australian 
and  American  quadrants  to  the  Antarctic  region.  The  ancient 
continent  has  been  called  Gondwana  Land~  It  attained  its  greatest 
size  and  height  at  the  close  of  the  Carboniferous  period,  and  existed 
till  near  the  close  of  the  Mesozoic  era. 

The  existence  of  an  almost  identical  flora  in  regions  so  far  apart 
would  tend  to  show  that  the  climatic  conditions  prevailing  around 
the  littoral  of  Gondwana  Land  were  everywhere  about  the  same. 
The  great  thickness  of  water-borne  sediments,  the  Alpine  glacia- 
tion,  and  the  dense  deltaic  jungle  vegetation  whose  buried  remains 
now  form  valuable  seams  of  coal,  all  bear  evidence  of  an  abundant 
rainfall  and  temperate  coastal  climate. 

British  Isles. — A  considerable  development  of  Permian  rocks 
occurs  in  England  on  both  sides  of  the  Pennine  Chain,  in  the 
Midlands,  and  in  Devon.  In  Scotland  small  areas  of  Permian  strata 
are  found  in  Ayrshire,  Dumfriesshire,  and  Isle  of  Arran  ;  and  in 
Ireland  a  few  small  patches  crop  out  in  County  Tyrone,  and  on  the 
south  shore  of  Belfast  Lough. 

The  subdivision  of  the  Permian,  as  it  occurs  on  the  Yorkshire 
side  of  the  Pennines,  is  as  follows  : — 

TT          T>       •  f  4.  Red  Sandstone  and  Marl. 

Upper  Permian     |3    Magnesian  Limestone. 


T->       .  .  Marl-slate. 

Lower  Permian     <  i    xr  n       a      j 
Yellow  Sands. 


f2. 
<  i 

\1. 


The  Magnesian  Limestone  is  the  most  conspicuous  member  of 
the  succession.  In  Durham  it  is  over  600  feet  thick. 

On  the  Lancashire  side  of  the  Pennine  Chain  the  Magnesian  Lime- 
stone is  subordinate  in  extent  ;  and  the  place  of  the  Yellow  Sands  is 
taken  by  sandstones,  with  which  are  associated  breccias  and  con- 
glomerates, the  total  thickness  of  this  basal  series  in  places  being 
1500  feet. 

The  breccias  were  at  one  time  believed  to  be  of  glacial  origin,  but 


368  ATEXT-BOOK    OF    GEOLOGY. 

the  present  view  is  that  they  are  ancient  screes  of  frost-shattered 
rock  that  descended  into  the  Permian  basins. 

Germany. — The  Dyas  type  of  the  Permian  System  is  well 
developed  in  the  Rhine  Province,  Thuringia,  Saxony,  Bavaria, 
and  Bohemia.  It  is  typically  displayed  in  the  flanks  of  the  Harz 
Mountains. 

The  two  great  divisions  of  the  Dyas  are  : — 

2.  Zechstein  x — Limestones  and  clayey  beds. 

1.  Rothliegende  2 — Bed  sandstones  and  pebbly  beds. 

These  two  divisions  are  quite  distinct,  and  on  account  of  overlap 
arising  from  subsidence,  the  Zechstein  is  found  covering  regions  far 
beyond  the  limits  of  the  underlying  Rothliegende. 

The  copper-bearing  shales  at  Mansfeld,  on  the  south  flank  of  the 
Harz  Mountains  in  Upper  Saxony,  have  been  famous  as  a  source 
of  copper  for  many  centuries.  They  occur  at  the  base  of  the 
Zechstein. 

Russia. — Rocks  of  Permian  age  cover  an  enormous  tract  in 
Eastern  Russia,  principally  in  the  province  of  Perm,  which  is 
bounded  on  the  east  by  the  Ural  Mountains.  They  follow  the 
Carboniferous  conformably,  and  consist  of  sandstones,  marls, 
shales,  conglomerates,  and  limestones,  the  latter  usually  dolomitic. 
Intercalated  with  these  rocks  there  are  beds  of  rock-salt,  gypsum, 
and  thin  seams  of  coal. 

The  terrestrial  beds  at  the  base  contain  many  land  plants,  includ- 
ing Catamites  and  Pecopteris,  also  fish  and  labyrinthodont  remains. 
The  limestone  bands  are  marine,  and  contain  several  brachiopods, 
among  which  Productus  is  represented  by  a  few  species. 

The  Russian  type  of  Permian  is  also  found  in  Armenia. 

India. — In  the  Salt  Range  of  the  Punjab  the  Permian  Damuda 
Series  attains  a  thickness  of  about  10,000  feet,  and  contains, 
among  other  plants,  the  genera  Glossopteris,  Gangamopteris,  and 
Schizoneura. 

The  coarse  Talchir  conglomerates  at  the  base  of  the  Permo- 
Jurassic  Gondwana  System  contain  striated  and  smoothed  boulders 
which  are  believed  to  be  of  glacial  origin. 

South  Africa.— At  the  base  of  the  Permo-  Jurassic  Karoo  System, 
which  occupies  a  prominent  place  in  the  geological  structure  of 
South  Africa,  there  occurs  a  series  of  shales  and  conglomerates 
called  the  Dwyka  Series. 

The  Dwyka  conglomerate  of  this  series  contains  striated  stones 
and  boulders  that  are  now  generally  believed  to  be  glacial. 

1  Zechstein  =  solid  or  tough  rock,  referring  to  the  character  of  the  limestone. 

2  Rothliegende = red-dead-layer,  referring   to  the  sudden  disappearance  of 
the  copper  in  passing  downward  into  the  sandstones  of  this  group. 


PERMIAN    SYSTEM.  369 

This  remarkable  conglomerate  occupies  an  extensive  tract  in 
North  and  South  Cape  Colony,  Orange  River  State,  and  the  Trans- 
vaal, where  it  forms  an  encircling  sheet  around  the  margin  of  the 
Karoo  Basin.  In  the  south  it  is  about  1000  feet  thick,  but  going 
northward  it  diminishes  in  thickness.  It  is  devoid  of  all  organic 
remains  except  at  Vereeniging,  where  the  overlying  shales  contain 
the  remains  of  many  varieties  of  plants  and  coal-seams. 

The  subdivisions  of  the  Dwyka  Series  are  as  follow : — 

p.  Upper  Shales      .         .         .         600  feet. 
Dwyka  Series      •<  2.  Dwyka  Conglomerate          .       1000    „ 
1^1.  Lower  Shales      .         .         .         700    „ 

North  America. — The  older  Permian  beds  in  the  State  of  Kansas 
are  marine.  They  are  followed  by  sandstones  containing  beds  of 
gypsum  and  salt,  which  would  indicate  that  the  Permian  uplift 
enclosed  salt- water  basins  in  an  arid  region  where  the  evaporation 
was  greater  than  the  precipitation. 

The  subdivision  of  the  Permian  in  Kansas,  where  the  develop- 
ment of  the  system  may  be  taken  as  typical,  is  as  follows  : — 

4.  Kiger  Stage       1  ^.  «    . 

Q    a  u  T    i     Q4.        >Cimarron  Series. 
p       •  o.  Salt  Lake  Stage  j 

'    2.  Summer  Stage  \w    _      Q    . 
1.  Chase  Stage       /Bl§  Blue  Senes' 

In  Texas,  where  the  development  of  the  Permian  System  is 
greater  than  in  any  other  State,  the  strata  attain  a  thickness  of 
7000  feet. 

Permian  Glaciation. — Notable  evidences  of  Permian  glaciation 
are  found  in*  South  Africa,  where  the  famous  Dwyka  Conglomerate, 
lying  near  the  base  of  the  Karoo  System,  contains  striated  boulders, 
and  possesses  many  of  the  characteristics  of  a  consolidated  glacial 
till ;  hence  the  name  tillite  which  has  been  applied  to  it  by  Davis. 
In  some  places  it  rests  on  a  striated  platform  of  older  quartzite. 

In  Victoria  the  well-known  Upper  Carboniferous  or  Permian 
glacial  deposits  at  Bacchus  Marsh,  Bendigo,  and  other  places, 
contain  smoothed  and  striated  boulders,  and  rest  on  striated, 
grooved,  ice- worn  surfaces. 

The  Bacchus  Marsh  glacial  conglomerate  is  believed  by  some 
writers  to  occupy  a  position  equivalent  to  the  Talchir  glacial  con- 
glomerate at  the  base  of  the  Indian  Gondwana  System. 

The  Lyons  Glacial  Conglomerate  in  Western  Australia  has  been 
shown  by  Gibb  Maitland  to  be  associated  with  strata  containing  a 
marine  fauna  which  seems  to  fix  its  age  as  Permo-Carboniferous. 
It  can  be  traced  without  a  break  in  a  NNW.-SSE.  course  from 
latitude  23°  south  to  latitude  26°  south. 

24 


370  A  TEXT-BOOK  OF  GEOLOGY. 

Glacial  conglomerates  are  associated  with  rocks  of  Permo-Car- 
boniferous  age  in  many  parts  of  South  America,  typically  in  Brazil 
and  Argentina.  Near  Minas,  Brazil,  in  latitude  28°  30'  south, 
there  is  a  glacial  boulder-bed  known  as  the  Orleans  Conglomerate 
lying  below  beds  containing  Glossopleris  and  Gangamopteris. 

Near  San  Luis  in  Argentina  a  similar  glacial  conglomerate  is 
associated  with  strata  containing  Glossopteris. 

The  whole  of  the  southern  portion  of  East  Falkland  Island  is 
composed  of  Permo-Carboniferous  strata  characterised  by  the 
typical  Glossopteris  flora  ;  and  beneath  these  there  is  a  clayey 
bed  containing  boulders  and  blocks  of  apparently  glacial  origin. 

A  coarse  conglomerate  containing  many  large  angular  blocks  of 
granite  occurs  at  the  base  of  the  Permo-Jurassic  Hokonui  System 
of  New  Zealand,  and  is  believed  to  be  of  fluvio-glacial  origin. 

There  appears  to  be  overwhelming  evidence  of  widespread 
Permian  glaciation  in  India,  Eastern  Australia,  South  Africa,  and 
South  America,  in  regions  both  north  and  south  of  the  equator, 
that  now  enjoy  tropical  and  semi-tropical  climates. 

It  is  almost  certain  that  the  uplift  which  began  in  the  Car- 
boniferous culminated  in  the  early  stages  of  the  Permian  ;  and 
we  are  led  to  the  belief  that  the  glaciation  of  that  period  was 
essentially  of  the  Alpine  type. 

In  India,  Gondwana  glacial  beds  occur  in  the  Talchir  district  and 
the  Salt  Kange,  at  places  from  700  to  800  miles  apart.  In  Australia, 
the"  glacial  conglomerates  have  been  traced  through  20  degrees  of 
latitude  ;  while  in  South  Africa  the  Dwyka  Conglomerate  has  a 
horizontal  range  of  800  miles.  These  are  significant  facts,  and  seem 
to  support  the  view  that  the  Gondwana  continent  was  traversed 
by  gigantic  ice-covered  Alpine  chains.  The  widespread  glacia- 
tion of  the  regions  bordering  this  ancient  land  is  one  of  the  most 
interesting  features  of  the  Permo-Carboniferous  period  in  the 
Southern  Hemisphere. 

It  is  not  improbable  that  the  height  of  the  mountain-chains 
and  the  amount  of  precipitation  were  sufficient  to  favour  the 
accumulation  of  great  valley-glaciers  that  descended  to  the  foot- 
hills, where  they  deployed  as  wide  piedmont  sheets  of  ice  on  the 
shores  of  the  inland  basins  and  land-locked  seas. 

Economic  Products. — Bocks  of  Permian  age  produce  vast 
quantities  of  rock-salt  and  gypsum,  and  also  some  copper. 

The  German  Zechstein  or  Upper  Permian  is  celebrated  for  its 
extensive  beds  of  rock-salt  which  occur  on  the  north  of  the  Harz 
Mountains.  The  rock-salt  at  Strassfurt  in  Prussia  is  1200  feet 
thick,  and  is  followed  by  a  zone  150  feet  thick  of  potassium  and 
magnesium  salts.  At  Sperenberg,  south  of  Berlin,  the  bed  of  salt 
is  1100  feet  thick. 


To  face  page  370.] 


[PLATE  XXXVIIlA. 


Photo,  by  E.  F.  Pittman.]  [Lent  by  Geo.  Survey  of  New  South  Wales. 

DOLERITE  DYKE  INTERSECTING  THE  PERMO-CARBONIFEROUS  COAL-MEASURES, 
NOBBYS,  NEWCASTLE,  N.S.W. 

The  course  of  the  dyke  can  be  seen  in  the  foreground,  together  with  some  masses 
of  coal  which  have  been  cindered  by  the  heat  of  the  intrusive  lava. 


PERMIAN    SYSTEM.  371 

The  beds  of  rock-salt  and  gypsum  in  Kansas  are  of  great  extent 
and  value. 

The  copper-bearing  shales  at  Mansfeld,  in  Saxony,  have  been  a 
source  of  copper  for  many  centuries.  At  Kokand,  in  Turkestan, 
the  Permian  sandstones  contain  about  one  per  cent,  of  copper  in 
certain  zones. 


CHAPTER   XXVIII. 

MESOZOIC  ERA:    TRIASSIC   SYSTEM. 

THE  Mesozoic  era  comprises  that  portion  of  the  geological  record 
lying  between  the  Palaeozoic  and  Cainozoic  eras,  and  its  deposits 
contain  a  fauna  and  flora  that  form  the  connecting-link  between  the 
ancient  and  existing  life;  hence  the  origin  of  the  name,  which 
signifies  middle  life. 

The  sedimentary  rocks  of  this  era  are  usually  divided  into 
three  great  systems,  namely  : — 

3.  Cretaceous. 
2.  Jurassic. 
1.  Triassic. 

In  Northern  India,  South  Africa,  and  Australia  there  is  a 
continuous  conformable  succession  ol  strata  ranging  from  the 
Permian  to  the  Jurassic.  In  England  and  Eastern  States  of  North 
America  the  Trias  rests  unconformably  on  the  Permian,  and  in 
other  regions  there  are  breaks  in  the  Mesozoic  succession  arising 
from  warping  and  differential  crustal  movements. 

The  dominant  rocks  of  the  Mesozoic  formations  are  sandstones, 
shales,  and  conglomerates  of  the  continental  facies,  with  which  are 
often  associated  lenticular  beds  of  rock-salt  and  gypsum,  and 
limestones  and  marls  of  the  marine  facies.  The  limestones  are 
usually  more  or  less  dolomitic. 

-  Generally  speaking,  the  Mesozoic  rocks  are  (1)  more  calcareous 
than  the  Palaeozoic ;  (2)  less  metamorphosed ;  and  (3)  less  disturbed, 
except  where  they  have  been  entangled  in  the  folds  of  mountain- 
chains. 

As  they  have  suffered  less  metamorphism,  such  altered  rocks  as 
slates,  schists,  and  quartzites  are  relatively  scarce,  and  seldom  or 
never  seen  except  in  regions  of  intense  folding  and  tectonic  dis- 
turbance. 

Mesozoic  rocks  take  a  prominent  place  in  the  geological  structure 
of  the  Pyrenees,  Alps,  Apennines,  Carpathians,  Urals,  Himalayas, 

372 


MESOZOIC  ERA:    TRIASSIC  SYSTEM.  373 

New  Zealand  Alps,  Andes,  Sierras,  Rocky  Mountains,  and  all  the 
great  mountain-chains  of  the  globe,  the  age  of  which  is  therefore 
post-Mesozoic. 

The  Mesozoic  formations  are  frequently  invaded  by  igneous 
dykes  and  intrusive  sills  of  Tertiary  date,  but  except  in  a  few 
isolated  places  of  limited  extent  they  are  singularly  free  from  inter- 
calations of  contemporaneous  volcanic  rocks  until  the  close  of  the 
Cretaceous,  from  which  it  would  appear  that  the  Mesozoic  era 
enjoyed  almost  complete  immunity  from  volcanic  activity  through- 
out the  whole  globe. 

The  close  of  the  Carboniferous  period,  as  previously  described, 
witnessed  widespread  crustal  movements  and  uplift,  which 
eventually  led  to  the  continental  conditions  of  deposition  so  char- 
acteristic of  the  Permian.  The  continental  conditions  that  pre- 
vailed in  Western  and  Central  Europe  and  other  regions  during 
the  early  Mesozoic  were  merely  a  continuance  of  the  Permian 
conditions. 

But  although  there  is  no  evidence  of  contemporaneous  folding, 
volcanic  activity,  or  intense  disturbance  of  any  kind  until  the 
closing  stages  of  the  Mesozoic,  the  character  of  the  sediments  prove 
conclusively  that  there  were  minor  oscillations  of  the  land,  and  that 
in  the  Northern  Hemisphere  there  was  a  general  downward  move- 
ment which  culminated  in  the  Jurassic. 

In  the  Mesozoic  there  was  a  marked  decline  of  the  brachiopods. 
The  graptolites,  trilobites,  armoured  fishes,  Lepidodendron,  and 
Catamites  which  characterise  the  Palaeozoic  era,  are  entirely 
absent.  On  the  other  hand,  there  is  a  great  development  of 
Saurians,  Ammonites,  and  Belemnites.  But  the  feature  which 
specially  characterises  the  Mesozoic  is  the  appearance  of  the  earliest 
birds,  mammals,  leaved  trees,  and  flowering  plants. 

The  life  of  the  Lower  Mesozoic  is  related  to  that  of  the  Palaeozoic, 
and  of  the  Upper  Mesozoic  to  that  of  the  Cainozoic. 

Triassic  System. 

The  Triassic  is  the  oldest  of  the  Mesozoic  systems,  and  it  owes 
its  name  to  the  three  groups  or  series  into  which  it  is  divided  in 
Germany,  where  it  is  typically  developed,  and  where  it  was  first 
studied  in  detail. 

Rocks  and  Distribution. — Throughout  the  globe  there  are  two 
dominant  facies  of  Triassic  deposits,  the  Continental  and  Marine, 
each  occupying  a  well-defined  geographical  province. 

In  Europe,  where  the  two  facies  of  the  Trias  was  first  recognised, 
the  Continental  facies,  which  is  mainly  developed  in  the  great 
Germanic  Basin  of  Central  Europe,  is  called  the  German-,  and 


374  A    TEXT-BOOK    OF    GEOLOGY. 

the  Marine  facies,  which  is  typically  developed  in  the  Maritime 
Alps,  the  Alpine. 

The  Continental  Trias  consists  mainly  of  red  sandstones,  shales, 
and  conglomerates,  with  beds  of  rock-salt  and  gypsum. 

The  Alpine  Trias  is  mainly  composed  of  thick  masses  of  marine 
limestone,  with  beds  of  marls  and  shales  containing  marine  shells. 

The  fossil  remains  found  in  the  rocks  of  the  German  facies  are 
chiefly  those  of  land  plants  and  land  animals.  The  sandy  beds 
frequently  exhibit  current-bedding,  are  often  ripple-marked, 
sun-cracked,  and  imprinted  with  the  tracks  of  land  animals.  These 
features,  when  taken  in  conjunction  with  the  prevailing  red  colour 
of  the  sediments  and  the  intercalated  beds  of  gypsum  and  rock- 
salt,  seem  to  show  that  the  rocks  of  this  facies  of  the  Trias  originated 
in  continental  conditions,  perhaps  not  unlike  those  now  prevailing 
in  the  Caspian  Basin,  where  the  deposits  are  partly  desert  sands 
and  partly  fluvio-lacustrine. 

The  inland  basins  in  which  these  continental  deposits  accumu- 
lated were  probably  situated  in  maritime  regions  where  slight 
oscillations  of  the  land  permitted  occasional  invasions  of  the  sea. 

The  meagre  land  flora,  the  scanty  fauna  and  the  existence  of  the 
beds  of  gypsum  and  rock-salt  indicate  the  prevalence  of  arid 
climatic  conditions  in  a  wide  zone  passing  through  Western  and 
Central  Europe.  Desert  conditions  also  prevailed  in  the  Eastern 
States  and  Western  Interior  Basin  of  North  America. 

The  fluvio-lacustrine  or  continental  Trias  of  South  Africa,  with 
its  numerous  carnivorous  and  herbivorous  saurians  and  amphibians, 
and  the  complete  absence  of  intercalated  deposits  of  rock-salt  and 
gypsum,  clearly  points  to  the  prevalence  of  luxuriant  jungle 
conditions  somewhat  similar  to  those  now  prevailing  in  the  great 
lake-basins  at  the  sources  of  the  Nile. 

The  German  Trias  is  extensively  developed  in  Germany,  where 
it  occupies  a  larger  area  than  any  other  formation.  It  also  occurs 
in  North-East  Russia,  in  the  British  Isles,  in  the  Eastern  States, 
and  Interior  Basin  of  North  America. 

The  Alpine  Trias,  which  is  believed  to  be  the  time-equivalent  of 
the  Continental  facies,  is  extensively  developed  in  Southern  Europe, 
Asia  Minor,  Northern  India,  South-East  Asia,  New  Zealand,  Peru, 
Mexico,  and  Western  States  of  North  America. 

GERMAN  OR  CONTINENTAL  FACIES. 

Subdivisions  in  Germany. — There  are  three  main  divisions  of  the 
Trias  recognised  in  Germany,  namely  : — 

3.  Keuper — Red  sandstones,  conglomerates,  and  shales. 
2.  Muschelkalk — Massive  dolomitic  limestones. 


MESOZOIC    ERA  :     TRIASSIC    SYSTEM.  375 

1.  Bunter — Bed   sandstones,   conglomerates,    and   shales,    with 
beds  of  gypsum  and  rock-salt. 

The  Bunter  Series. — This  series  was  deposited  partly  in  inland 
seas  and  partly  on  the  dry  land  as  wind-blown  sands.  The  sand- 
stones are  frequently  current-bedded,  and  the  tracks  of  land  reptiles 
are  sometimes  found  in  both  the  shales  and  sandstones,  which  tends 
to  show  that  they  were  deposited  in  shallow  bays,  estuaries,  or 
deltas.  In  many  cases  the  tracks  occur  in  muddy  sediments  that 
are  sun-cracked,  which  would  indicate  tidal  conditions  of  deposition, 
or  the  existence  of  marshy  swamps  subject  to  occasional  inundations. 
The  prevailing  colour  of  the  rocks  is  red,  which  is  characteristic  of 
desert  sands  that  have  been  subject  to  the  oxidising  influence  of 
the  atmosphere. 

The  Bunter  Sandstone,  as  this  series  is  frequently  called,  follows 
the  Permian  quite  conformably. 

Among  the  few  fossils  found  in  the  lower  part  of  the  series  are 
the  characteristic  Triassic  species  Estheria  minuta,  a  diminutive 
crustacean,  and  Gervillia  Murchisoni.  About  the  middle  of  the 
series  the  sandstones  contain  the  footprints  of  the  amphibian 
Cheirotherium  and  the  remains  of  the  Labyrinthodont,  Tremato- 
saurus  Brauni. 

In  the  upper  part  of  the  series  the  sandstones  are  in  some  regions, 
notably  in  Thuringia,  intercalated  with  mud-beds  of  dolomitic 
limestone  containing  many  fossils,  among  which  is  the  character- 
istic Lamellibranch,  Myophoria  costata.  Other  common  forms  are 
Myophoria  vulgaris,  Gervillia  socialis,  Pecten  discites,  and  Lingula 
tenuissima,  all  found  in  the  overlying  Muschelkalk. 

The  fine-grained  micaceous  sandstones  of  the  Eifel  contain 
numerous  plant  remains,  among  which  occur  the  peculiar  conifer 
Voltzia  of  world-wide  distribution,  and  a  species  of  Equisetum. 

The  Muschelkalk  Series. — This  attains  a  maximum  thickness  of 
1000  feet,  and  is  mainly  calcareous  and  marine.  It  follows  the 
Bunter  Sandstone  quite  conformably,  and  its  presence  is  an  evidence 
of  subsidence  followed  by  the  trespass  of  the  sea  into  the  Bunter 
basins,  which  were  obviously  situated  in  maritime  regions. 

The  lower  and  upper  portions  of  the  Muschelkalk  consist  of  thin 
bedded  limestones  and  marls,  and  the  middle  portion,  of  dolomites 
with  beds  of  gypsum  and  salt-bearing  marls. 

The  series  contains  many  bands  that  are  richly  fossiliferous.  The 
lower  beds  contain  numerous  fine  examples  of  Natica  gregaria  and 
Dentalium  torquatum  ;  and  these  are  followed  by  beds  crowded 
with  Myophoria  orbicularis.  Among  other  common  forms  in  the 
Lower  Muschelkalk  are  Terebratula  vulgaris,  Aihyris  trigonella, 
Spiriferina  gracilis,  Myophoria  vulgaris,  M.  elegans,  Gervillia 


376  A  TEXT-BOOK  OF  GEOLOGY. 

costata,  Monotis  Alberti,  Lima  lineata,  Pecten  discites,  and  many 
Ammonites. 

The  Upper  Muschelkalk  is  the  most  prolific  in  fossils,  and  among 
the  species  that  occur  in  vast  numbers  are  Terebratula  vulgaris, 
Myophoria  vulgaris,  Pecten  discites,  Gervillia  socialis,  and  Encrinus 
liliiformis.  The  large  Nautilus  N.  bidorsatus  is  common,  as  also  are 
the  brachiopods  Spiriferina,  Athyris,  and  Terebratula, 

The  Keuper  Series. — This  consists  of  various  coloured  clays, 
mostly  red,  and  sandstones,  which  contain  thick  beds  of  gypsum 
and  thin  beds  of  rock-salt.  Fossils  occur  in  all  the  divisions  of  the 
series,  but  are  never  abundant. 

The  general  character  of  the  rocks  and  fossils  show  that  the 
Keuper  sediments  were  laid  down  in  shallow  estuaries  or  continental 
basins  to  which  the  sea  had  occasional  access. 

The  estuarine  conditions  of  the  Lower  Keuper  are  characterised 
by  the  presence  of  Myophoria  Goldfussi,  M.  costata,  Lingula 
tenuissima,  Estheria  minuta,  and  the  fishes  Acrodus,  Hybodus,  and 
Ceratodus  ;  and  the  terrestrial  conditions  by  amphibians  and 
saurians,  the  latter  including  the  genera  Mastodonsaurus  and 
Nothosaurus.  Plant  remains  are  also  common. 

The  Middle  or  Main  Keuper  is  a  group  of  gypsum-bearing  shales 
and  marls,  which  passes  upward  into  sandstones  with  Equisetum 
arenaceum.  Still  higher  is  the  famous  Stuben  Sandstone  which, 
near  Stuttgart,  has  yielded  numerous  saurian  remains,  including 
those  of  the  crocodile  Belodon,  which  is  also  found  at  Elgin  in 
Scotland. 

The  Upper  Keuper,  or  Rhsetic  as  it  is  sometimes  called  from  its 
occurrence  in  the  Rhsetian  Alps,  contains  the  characteristic  species 
Avicula  contorta,  which  is  limited  to  this  stage  and  is  therefore  of 
zonal  value.  Among  other  forms  that  occur  in  these  beds  are 
Modiola  minuta  and  Protocardia  Rhcetica,  but  they  are  nowhere 
abundant.  The  coal-bearing  sandstones  of  this  stage  contain  the 
remains  of  many  cycads,  ferns,  and  horse-tails. 

The  Upper  Keuper  is  also  celebrated  for  its  Bone-bed,  from  which, 
near  Stuttgart,  were  obtained  the  teeth  of  the  small  marsupial-like 
quadruped  Microlestes  antiquus,  which  is  the  oldest  known  mammal. 
The  remains  of  this  mammal  have  since  been  found  in  England 
and  United  States. 

Great  Britain. 

In  England  Triassic  rocks  are  present  in  Devon,  whence  they 
extend  into  Somerset,  South  Wales,  and  the  Midlands,  where  they 
spread  out  considerably  and  divide  into  two  main  arms  that  extend 
northwards,  one  passing  on  one  side  and  the  other  on  the  other  side 
of  the  Pennine  Chain  like  the  prongs  of  a  hay-fork. 


MESOZOIC    ERA  :     TRIASSIC    SYSTEM.  377 

The  Trias  of  England  belongs  to  the  German  or  Continental 
facies,  and  closely  resembles  the  rocks  and  succession  in  Central 
Germany  with  one  important  exception.  The  Muschelkalk 
limestone  series  which  so  completely  dominates  the  Middle  Trias 
of  Central  Europe  is  entirely  absent. 

The  Keuper  follows  the  Bunter  series  in  England  with  little  or 
no  stratigraphical  break,  and  hence  we  may  assume  that,  while 
the  Muschelkalk  was  being  deposited  in  Germany,  deposits  of  a 
Continental  facies  continued  to  be  deposited  in  England  due  to  a 
continuance  of  the  continental  conditions  which  prevailed  in  the 
Bunter  period.  The  German  Muschelkalk  may  possibly  be 
represented  in  England  by  beds  that  now  form  a  part  of  the  Keuper 
and  Bunter  in  that  region.  If  this  view  be  correct,  then  we  must 
conclude  that  the  subsidence  which  permitted  1000  feet  of  marine 
beds  to  be  deposited  in  Germany  did  not  affect  the  British  Isles. 

The  three  main  divisions  of  the  Trias  recognised  in  the  British 
Isles  are  : — 

3.  Khsetic — Maximum  thickness,  150  feet. 
2.  Keuper—         „  „         3000     „ 

1.  Bunter—          „  „         2000     „ 

The  Bunter  consists  of  red  and  variously  hued  sandstones  and 
conglomerates  or  pebble  beds  of  fluviatile  or  fluvio-lacustrine 
origin.  The  fossils  comprise  the  remains  of  land  plants,  among 
which  are  the  cypress-like  conifers  Voltzia  and  Walchia. 

The  Keuper  consists  mainly  of  marls  and  sandstones  ;  but  north 
of  the  Mendip  Hills  it  has  a  remarkable  littoral  conglomerate  at  its 
base  from  150  to  250  feet  thick,  chiefly  composed  of  pebbles  of 
Carboniferous  limestone  ranging  from  a  few  inches  to  three  feet  in 
diameter,  set  in  a  dolomitic  limestone  matrix.  Hence  the  name 
Dolomitic  Conglomerate.  This  conglomerate  is  quite  local  in 
distribution,  and  is  obviously  a  shore  deposit  formed  at  the  foot  of 
a  steep  range  rising  abruptly  from  the  shore  of  an  enclosed  sea. 

All  through  the  south-west  of  England  the  Keuper  beds  overlap 
the  Bunter  ;  and  even  the  Lower  Keuper  is  overlapped  by  the 
Upper  Keuper,  which  is  conclusive  evidence  of  subsidence  accom- 
panied by  a  fairly  rapid  advance  of  the  waters  of  the  inland  basin. 
Such  conspicuous  overlap  as  may  be  seen  on  the  Welsh  borders 
could  only  have  taken  place  where  the  land  fringing  the  basin  of 
deposition  sloped  gently  down  to  the  edge  of  the  water. 

The  Rhsetic  of  England  forms  the  summit  of  the  Keuper  in 
Germany,  where  it  is  not  recognised  as  a  separate  series.  It  is  of 
marine  origin  ;  although  relatively  thin,  it  is  widespread  and  forms 
the  closing  stage  of  both  the  German  and  Alpine  Trias  throughout 
Western,  Central,  and  Southern  Europe.  From  this  we  learn  that 


378  A  TEXT-BOOK  OF  GEOLOGY. 

the  general  subsidence  which  took  place  in  Continental  Europe  at 
the  close  of  the  Trias  also  affected  the  British  Isles  ;  and  the 
conspicuous  overlap  of  the  different  beds  of  the  Keuper,  to  which 
we  have  referred  above,  shows  that  the  downward  movement 
commenced  in  early  Triassic  times.  This  subsidence  eventually 
led  to  the  introduction  of  marine  conditions  of  deposition  in  the 
English  area. 

The  Rhsetic  in  a  way  comprises  passage-beds  connecting  the 
Triassic  and  Jurassic  systems.  It  is  characterised  in  England, 
as  also  in  Germany,  Southern  Europe,  Indo-China,  and  Shan 
States,  by  the  presence  of  Avicula  (Pteria)  contorta,  which  was 
first  described  by  Portlock  from  examples  found  near  Port  Hush, 
in  Ireland. 

Among  other  fossils  found  in  the  English  Bhsetic  are  Protocardia 
rlicetica  and  Estheria  minuta,  both  of  world- wide  distribution.  At 
the  base  of  the  Rhsetic  there  is  a  bone-bed  which  has  yielded  the 
teeth  of  a  small  mammal,  which  has  been  referred  to  the  genus 
Microlestes. 

A  small  patch  of  Triassic  sandstone  near  Elgin,  in  Scotland,  has 
been  a  prolific  source  of  saurian  remains.  Among  the  genera 
found  there  are  Gardonia,  Elinia,  Hyperodapedon,  and  many 
others. 

ALPINE  TRIAS. 

Triassic  formations  of  the  Alpine  or  Marine  facies  are  widely 
distributed  in  Southern  Europe,  particularly  in  the  Maritime  Alps, 
Apennines,  Sicily,  Balearic  Isles,  Spain,  Balkan  Peninsula,  Carpa- 
thians ;  and  in  Turkestan,  Central  Asia,  Northern  India,  Burma, 
Japan,  Northern  Siberia,  Spitzbergen,  New  Guinea,  Australia, 
New  Zealand,  South  Africa,  Peru,  Mexico,  California,  Nevada, 
British  Columbia,  and  Alaska. 

The  Alpine  Trias  is  typically  developed  in  the  Maritime  Alps, 
where  Mojsisovics  has  recognised  five  divisions  or  groups  of  beds  : — 

5.  Rhsetic. 

4.  Carinthian. 

3.  Norian. 

2.  Alpine  Muschelkalk. 

1.  Alpine  Bunter  Sandstone. 

The  Alpine  Bunter  Sandstone  consists  of  a  series  of  red  sandy 
micaceous  slates  containing  beds  of  gypsum  and  rock-salt  in  the 
lower  stages,  and  bands  of  impure  limestone  in  the  upper. 

The  series  is  quite  conformable  to  the  Permian,  and  hence  it  is 
difficult  to  fix  the  boundary  between  the  two.  The  typical  fossils 


MESOZOIC    ERA  :     TRIASSIC    SYSTEM.  379 

are  Avicula  Clarai,  Naticella  costata,  and  Ceratites  cassianus,  with 
Myophoria  costata  in  the  upper  calcareous  layers. 

The  Alpine  Muschelkalk  consists  mainly  of  limestones  with 
clayey-bands  that  contain  an  abundance  of  marine  fossils,  among 
which  Brachiopods  are  conspicuous,  including  Aihyris  trigonella, 
Spiriferina  Mentzeli,  and  Terebratula  vulgaris.  Other  abundant 
forms  are  Gervillia  socialis,  Myophoria  vulgaris,  Pecten  discites, 
and  Encrinus  liliiformis.  The  Ammonites  Ceratites,  Arcestes,  and 
Pinacoceras  are  common. 

The  massive  bands  of  limestone  are  believed  to  be  old  coral 
reefs. 

The  Norian  Series  comprises  rocks  laid  down  in  two  distinct 
biological  provinces,  one  called  the  Juvavian  Province,  reaching 
eastwards  from  Salzburg  to  the  Carpathians  ;  the  other,  known  as 
the  Mediterranean  Province,  comprising  the  remainder  of  the  Alpine 
region. 

Each  province  is  characterised  by  its  own  fauna,  and  singularly 
enough  the  numerous  Ammonites  of  these  two  adjacent  marine 
areas  belong  to  different  species,  showing  that  each  province  had 
its  own  peculiar  biological  development. 

The  Mediterranean  Province  is  typically  developed  in  the  Southern 
Tyrol,  where  the  rocks  consist  of  massive  dolomitic  limestones 
and  marly  sandstones,  the  former  probably  of  old  coral  reef  forma- 
tion. In  the  Bavarian  Alps  the  dolomites  are  mainly  composed 
of  rock-building  marine  algae.  The  distinguishing  fossils  of  this 
province  are  the  characteristic  Ammonite  Trachyoceras  Archelaus 
and  the  Lamellibranch  Daonella  ( Halobia)  Lommeli  (Plate 
XXXVIIIs.  fig.  2),  which  has  a  world- wide  distribution,  being 
abundant  even  in  the  Trias  of  New  Zealand. 

The  Juvavian  Province  consists  mainly  of  massive  dolomitic 
limestones  and  marly  beds  containing  a  rich  fauna,  which  includes 
the  Cephalopods  Pinacoceras,  Arcestes,  Trachyoceras,  Nautilus,  and 
Orthoceras  dubium  ;  also  the  Lamellibranchs  Halobia  and  Monotis 
salinaria,  the  last  abundant  in  the  Alpine  Trias  of  Siberia,  Japan, 
Australia,  New  Zealand,  North  and  South  America. 

The  Garinthian  Series  consists  chiefly  of  coral  limestones  and 
calcareous  marls,  the  latter  best  known  as  the  St  Cassian  Beds, 
being  crowded  with  a  well-preserved  mixed  molluscous  fauna 
among  which  Gasteropods  predominate  ;  but  Brachiopods,  Lamel- 
libranchs, and  Ammonites  are  well  represented. 

The  zonal  fossil  of  the  St  Cassian  Beds  is  Trachyoceras  aon. 
It  is  notable  that,  although  the  St  Cassian  fauna  is  so  prolific  and 
varied,  the  forms  are  mostly  stunted  and  small. 

The  Great  Dolomite  or  Lower  Dachstein  Limestone  which  closes 
the  Trias  proper  in  Southern  Europe,  consists  of  several  thousand 


380  A  TEXT-BOOK  OF  GEOLOGY. 

feet  of  thin-bedded,  platy,  and  imbedded  limestone,  and  is  dis- 
tinguished by  the  zonal  fossils  Avicula  exilis  and  Turbo  solitarius. 
f^The  Rhsetic  Series  consists  principally  of  dolomitic  limestones, 
marls,  and  sandstones.  It  forms  a  thin  but  continuous  sheet, 
which  is  present  in  Germany  and  England  as  well  as  in  the  Alpine 
region,  thereby  proving  that  the  partial  separation  of  the  Germanic 
Basin  and  the  seas  of  Southern  Europe  ceased  at  this  stage  of  the 
Trias. 

The  Trias  in  Other  Countries. 

India. — The  Continental  and  Marine  facies  of  the  Trias  are  well 
represented  in  Northern  India,  and,  as  in  Europe,  they  occur  in 
separate  geographical  provinces. 

The  Triassic  division  of  the  Permo-Jurassic  Gondwana  System 
contains  a  scanty  flora  which  includes  several  species  of  Glossopteris, 
and  the  beautiful  fern  Panacopteris  Hughesi,  found  also  in  Tonkin 
and  China.  Besides  these  it  contains  the  remains  of  amphibians  ; 
and  among  the  fish  remains  there  is  a  form  related  to  the  Cemtodus 
of  the  Trias  of  Europe. 

The  Trias  of  the  Gondwana  System  is  obviously  of  continental 
origin,  and  corresponds  to  the  German  facies  of  Central  Europe. 
The  Indian  Gondwana,  as  previously  described,  includes  three 
main  divisions,  the  Lower,  Middle,  and  Upper,  which  correspond 
with  the  Permian,  Trias,  and  Jurassic.  The  main  divisions  are 
as  follow  : — 

f  Jubbulpore  Series^  Jurassic 

|  Rajmahal  Series    /       ' 


f  Maleri  Stage 
11 


Gondwana  System^  Mahadeva  Series    ^  Kamthi  Stage  ^Triassic. 

I^Panchet  StageJ 

Damuda  Series      ~\  -p 

m  i  •!_•«•  r       •         •         •     .rermian. 

Series         f 


The  Gondwana  System  contains  the  most  important  coal-bearing 
formations  in  India.  Valuable  coal-seams  occur  in  both  the 
Jurassic  and  Permian  divisions,  but  at  the  present  time  the  domestic 
supplies  of  India  are  mostly  drawn  from  the  Damuda  Series. 

The  Marine  or  Alpine  Trias  of  India  is  splendidly  developed  in 
the  Salt  Range  of  the  Punjab,  in  Baluchistan,  and  Tibetan  Plateaux 
in  Western  Tibet.  The  rocks  are  mainly  limestones  interbedded 
with  marly  and  shaly  rocks.  Fossils  are  exceedingly  abundant 
and  generally  well  preserved.  Among  the  fossils  recognised  in 
these  beds  are  the  European  species  Ammonites  floridus,  A.  diffusus, 
Halobia  Lommeli,  Monotis  salinaria  (Plate  XXXVTIlB.  fig.  1), 
and  Megalodon  scutatus. 


To  face  page  380.] 


[PLATE  XXXVIIIs. 


FIG.  1. 


FIG.  2.  FIG.  3. 

TYPICAL  TRIASSIC  FOSSILS  or  ALPINE  FACIES. 

FIG.  1.   Monotis  salinaria  var.  Richmondiana  (Zittel). 
FIGS.  2  and  3.    Halobia  Lommeli  (Wissm.). 


MESOZOIC  ERA:    TBIASSIC  SYSTEM.  381 

Australasia. — Triassic  rocks  are  well  developed  in  Queensland, 
New  South  Wales,  and  New  Zealand. 

The  Trias  of  New  South  Wales  consists  of  three  distinct 
groups : — 

3.  The^Wianamatta  Shales. 
2.  The  Hawkesbury  Sandstone. 
1.  Narrabeen  Shales. 

These  rocks  are  of  continental  origin  and  hence  belong  to  the 
German  facies  of  the  Trias.  They  contain  thin  seams  of  coal 
and  numerous  plant  remains,  among  which  are  found  the 
characteristic  genera  Tcenopteris,  Thinnfeldia,  and  Sphenopteris. 
Fish  remains  are  not  uncommon. 

In  New  Zealand  the  Trias  forms  the  middle  division  of  the 
Hokonui  System.  It  consists  of  a  vast  pile  of  sandstones,  shales, 
and  conglomerates  of  estuarine  and  fluviatile  origin  intercalated 
with  marine  beds  which  contain  many  of  the  characteristic  species 
of  the  Alpine  Trias  of  Southern  Europe  and  India,  notably  Halobia 
Lommeli  and  Monotis  salinaria.  The  limestones  which  dominate 
the  marine  Trias  of  the  Northern  Hemisphere  are  entirely  unknown, 
and  calcareous  rocks  of  all  kinds  are  conspicuously  absent. 

Among  the  fossils  present  in  the  marine  beds  are  the  Brachiopods 
Athyris,  Spiriferina,  Terebratula,  Rhynchonella ;  the  Lamellibranchs 
Trigonia,  Megalodon,  Ostrea,  and  Mytilus  ;  and  the  Gasteropods 
Murchisonia  and  Pleurotomaria.  Cephalopods  are  represented 
by  a  large  Nautilus  and  a  few  species  of  Ammonites,  but  they  are 
scarce  and  badly  preserved.  There  are  no  corals  and  bryozoans. 

The  beds  of  conglomerate  contain  pebbles  of  granites  and  other 
igneous  rocks  that  are  unknown  in  the  present  land  surface  of 
New  Zealand. 

The  New  Zealand  area  in  the  Triassic  period  appears  to  have 
formed  the  southern  coasts  of  a  continental  region  that  lay  to  the 
north-west  in  the  present  Tasman  Sea,  and  was  drained  by  large 
rivers  that  discharged  their  load  of  sand  and  mud  in  shallow 
deltaic  seas.  The  prevalence  of  muddy  sediments  was  probably 
responsible  for  the  absence  of  coral-building  polyps  and  the  scarcity 
of  Ammonites  and  deep-water  Cephalopods. 

Some  of  the  clayey  and  sandy  beds  are  crowded  with  broken 
plant  remains  ;  and  in  the  shallow- water  marine  beds  there  are 
sometimes  found  saurian  remains,  among  which  the  genus  Ich- 
thyosaurus has  been  doubtfully  identified. 

Among  the  plants  recognised  by  Dr  Arber  from  the  late  Trias 
(Rhsetic)  or  very  early  Jurassic  rocks  of  New  Zealand,  are  the 
genera  Thinnfeldia,  Cladophlebis,  Tceniopteris,  Sphenopteris,  Ptero- 
phyllum,  and  Palissya. 


382  A  TEXT-BOOK  OF  GEOLOGY. 

Antarctic  Continent. — Sandstones  and  shales  of  Trias-Jura  age 
occur  in  the  north-east  end  of  Graham's  Land,  in  the  Antarctic 
region.  They  contain  a  rich  and  varied  flora  embracing  ferns, 
cycads,  and  conifers.  Among  the  plants  are  the  genera  Sageno- 
pteris,  Thinnfeldia,  Cladophlebis,  Pterophyllum,  and  Otozamites, 
all  of  which  are  found  in  the  older  Mesozoic  rocks  of  Northern 
India  and  Eastern  Australia,  and  some  are  found  in  the  Argentine. 

The  presence  of  these  plants  would  indicate  a  mild  and  moist 
climate  during  the  Jurassic  period,  where  now  the  land  is  covered 
with  permanent  ice  and  snow. 

In  Victoria  Land  the  Beacon  Sandstone  Series  covers  a  large 
tract  of  country.  The  sandstones  are  horizontal  and  intercalated 
with  shales  and  seven  coal-seams  capped  with  a  thick  flow  of 
dolerite,  and  intruded  by  sills  of  the  same  rock.  The  plant 
remains  found  in  the  sandstones  are  too  fragmentary  for  critical 
determination.  Hence  the  age  of  this  formation  may  be  older  or 
younger  than  Trias. 

South  Africa. — The  Karoo  System  of  South  Africa  appears  to 
be  approximately  the  equivalent  of  the  Gondwana  System  of  India. 
It  consists  of  a  great  succession  of  sandstones,  shales,  and  con- 
glomerates, with  coal-seams  and  plant  beds.  All  the  known  fossils 
are  land  or  freshwater  forms,  and  nowhere  do  the  rocks  contain 
evidence  of  marine  conditions  of  deposition. 

The  lower  divisions  contain  a  Glossopteris  flora,  and  the  middle 
a  flora  related  to  that  of  the  Middle  Gondwana,  as  well  as  a  remark- 
able assemblage  of  reptiles  and  lizards.  The  rock-salt  and  gypsum 
beds  which  characterise  the  continental  conditions  of  deposition 
in  the  Northern  Hemisphere  are  entirely  absent.  It  would  there- 
fore appear  that  the  Karoo  deposits  were  laid  down  in  great  inland 
freshwater  basins  fringed  with  mud-flats  that  swarmed  at  certain 
stages  with  reptiles  and  amphibians .  The  climate  was  probably  tropi- 
cal or  semi-tropical,  but  the  numerous  reptilians  and  coal-seams 
would  seem  to  indicate  the  absence  of  arid  desert  conditions. 

The  Karoo  System  comprises  four  main  divisions,  namely  : — 

4.  Stormberg  Series. 
3.  Beaufort        ,, 
2.  Ecca 
1.  Dwyka  „ 

The  Dwyka  Series,  as  already  described,  is  dominated  by  glacial 
conglomerates.  The  Ecca  Series  contains  a  Glossopteris  flora 
related  to  the  Permian  facies  of  the  Lower  Gondwana.  Among 
the  genera  of  land  plants  common  to  the  two  systems  are  Glosso- 
pteris, Gangamopteris,  Nceggerathiopsis,  Schizoneura,  Phyllotheca, 
and  Sphenopteris,  many  of  which  are  also  found  in  the  Lower  and 


MBSOZOIC    ERA  I     TEIASSIC    SYSTEM.  383 

Upper  Coal-Measures  of  New  South  Wales,  the  Bowen  River  Series 
of  Queensland,  the  Lower  Coal-Measures  of  Tasmania,  and  also  in 
the  Trias  of  Brazil  and  Argentina. 

The  Beaufort  Series  is  mainly  characterised  by  the  occurrence  in 
it  of  a  number  of  reptilian  remains,  including  representatives  of  the 
Anomodontia  and  Theriodontia,  which  are  almost  limited  to  this 
series  and  the  Panchet  Beds  of  the  Middle  Gondwana.  Of  the 
Anomodontia  there  is  the  peculiar  genus  Dicynodon  ;  and  of  the 
Theriodontia,  the  genera  Placodus  and  Galesaurus.  Among  the 
plants  in  this  series  are  Glossopteris  and  Schizoneura. 

The  Stormberg  Series  contains  fish  remains,  including  those  of 
Ceratodus  and  some  Deinosaurs,  but  is  chiefly  distinguished  by  a 
fairly  abundant  flora,  which  includes  representatives  of  Thinnfeldia, 
Tceniopteris,  and  Sphenopteris,  which  are  also  found  in  the  Upper 
Gondwana  of  India,  the  Triassic  Hawkesbury  Sandstone  of  New 
South  Wales,  and  the  Upper  Coal  Series  of  Tasmania.  But  correla- 
tions based  on  the  fragmentary  remains  of  a  scanty  land  flora, 
indicating  a  continuance  of  the  same  physical  conditions  and 
showing  little  progressive  development  through  a  long  period  of 
time,  are  never  trustworthy  or  satisfactory. 

North  America. — The  Continental  and  Marine  facies  of  the  Trias 
are  typically  developed  in  North  America,  the  former  in  the  Eastern 
States  and  Central  Basin,  and  the  Marine  in  the  Pacific  States. 

Continental  Facies. — In  the  Eastern  States  a  chain  of  discon- 
nected patches  of  the  Trias  extend  from  Nova  Scotia  to  South 
Carolina,  running  parallel  with  the  present  coast-line  and  the 
Appalachian  Chain.  The  largest  developments  are  found  about 
the  Bay  of  Fundy,  in  Connecticut  River  Valley,  and  in  a  belt 
extending  southward  from  South  New  York  to  New  Jersey,  Pennsyl- 
vania, Maryland,  and  Virginia. 

Everywhere  the  Trias  lies  unconformably  on  the  underlying 
formations.  The  rocks  are  mainly  sandstones  and  shales,  with 
which  are  interbedded  massive  bands  of  conglomerate  and  breccia. 

In  New  Jersey,  where  the  continental  Trias  is  well  developed, 
the  system  has  been  divided  into  three  distinct  groups  : — 

f"3.  Brunswick  ~"| 

Newark  System  ^  2.  Lockatong     ^Triassic. 
[I.  Stockton      J 

The  rocks  of  the  Newark  System  are  poor  in  fossils,  which  com- 
prise land  plants,  fresh-  and  brackish-water  fishes,  and  the  teeth 
of  reptiles.  The  prevailing  colour  of  the  sandstones  and  shales  is 
red.  The  character  of  the  sediments  and  their  fossils  clearly  show 
that  deposition  took  place  in  large  inland  basins. 

Triassic  rocks  of  a  similar  character,  but  containing  beds  of  rock- 


384  A  TEXT-BOOK  OF  GEOLOGY. 

salt  and  gypsum,  occupy  large  tracts  in  Texas,  South  Dakota,  and 
Wyoming;  and  a  belt  of  the  same  rocks  extends  along  the  eastern  base 
of  the  Rocky  Mountains  from  Western  Canada  to  New  Mexico. 

The  plant  life  of  this  period  contained  numerous  cycads,  ferns, 
and  conifers,  the  latter  including  the  genera  Palissya,  Albertia,  and 
Ullmania.  The  ginkgos  were  represented  by  Baiera,  which  also 
occurs  in  the  Upper  Karoo,  and  first  appeared  in  the  Permian  of 
Central  Germany.  The  Calamites  are  replaced  by  true  horse-tails  ; 
but  the  Glossopteris  flora,  which  is  so  characteristic  of  this  period 
in  India,  Australia,  and  South  Africa,  is  entirely  absent,  as  it  also 
is  from  Western  Asia  and  Europe. 

Land  animals  were  represented  by  numerous  Labyrinthodonts 
and  the  curious  deinosaurs,  a  group  of  reptiles  which  was  at  one 
time  believed  to  possess  bird-like  affinities.  The  flying  saurian, 
Pterosaurus,  appeared  for  the  first  time  at  the  close  of  the  period. 

Marine  Trias. — The  greatest  development  of  the  North  American 
marine  Trias  is  on  the  Pacific  watershed,  in  the  Sierras,  California, 
West  Humboldt  Range  of  Nevada,  Oregon,  British  Columbia,  and 
Alaska.  In  Nevada  the  maximum  thickness  of  the  system  is  17,000 
feet,  the  lower  division,  known  as  the  Koipeto  Series,  consisting 
mainly  of  sandstones  and  shales,  and  the  upper  division,  the  Star 
Series,  of  sandstones,  quartzites,  and  limestones.  In  the  West 
Humboldt  Range  the  rocks  of  the  Star  Series  are  sharply  folded 
and  highly  metamorphosed. 

The  marine  fauna  of  this  system  includes  many  European  genera, 
among  which  we  have  the  Cephalopods  Trachyoceras,  Ceratites 
nodosus,  and  Orthoceras  ;  the  Lamellibranchs  Corbula,  Myophoria, 
and  Pecten  ;  and  the  Brachiopods  Terebratula,  Rhynchonella,  and 
Spiriferina. 

Surface  Features. — In  England,  where  the  Triassic  rocks  are 
mostly  sandstones  and  shales,  the  Triassic  areas  usually  exhibit 
gentle  undulating  contours.  In  Germany  the  argillaceous  Keuper 
also  forms  outlines  of  low  relief  ;  but  in  the  South  Tyrol,  where  the 
Muschelkalk  is  strongly  developed,  the  softer  marls  have  been  worn 
away,  and  the  limestone  bands  stand  up  as  high,  tent-shaped, 
craggy  ridges  and  bold  escarpments  that  combine  to  form  the 
picturesque  beauty  for  which  the  mountains  of  that  region  are 
famous. 

Economic  Products. — In  England  the  Triassic  rocks  contain 
valuable  beds  of  rock-salt,  which  are  important  as  a  source  of  the 
British  salt  supplies.  The  principal  salt-producing  areas  are 
situated  in  Cheshire,  Worcestershire,  and  North  Yorkshire. 

The  rock-salt  and  gypsum  beds  in  Texas  and  South  Dakota  are 
extensively  worked  and  of  considerable  value. 

The  red  sandstone   and  massive   dolomitic   limestones   of  the 


MESOZOIC  ERA:    TRIASSIC  SYSTEM.  385 

Triassic  System  are  used  for  building  purposes  in  many  parts  of 
Europe,  America,  and  Australia. 

Triassic  rocks  do  not  contain  metalliferous  deposits  of  any 
moment.  Coals  of  Devonian,  Carboniferous,  and  Triassic  age 
occur  in  Turkestan,  but  so  far  only  the  Triassic  coals  have  been 
worked  on  a  commercial  scale. 


25 


CHAPTER   XXIX. 
JURASSIC    SYSTEM. 

THE  Jurassic  is  the  upper  division  of  the  great  succession  of 
conformable  strata  of  which  the  Triassic  forms  the  middle  division 
and  the  Permian  the  lower.  It  represents  the  continuation  of  the 
marine  conditions  of  deposition  ushered  in  by  the  Rhsetic  as  the 
result  of  the  subsidence  which  set  in  towards  the  close  of  the 
Triassic  period.  The  Rhsetic  acts  the  part  of  passage  or  transition 
beds  connecting  the  two  systems,  and  is  sometimes  placed  at  the 
close  of  the  Triassic  and  sometimes  at  the  base  of  the  Jurassic. 

The  name  Jurassic  was  derived  from  the  Jura  Mountains  in  West 
Switzerland,  where  rocks  of  this  age  are  typically  developed. 

Rocks. — The  rocks  comprising  the  Jurassic  System  in  the 
Northern  Hemisphere  are  mainly  marine  marls  and  limestones, 
with  subordinate  beds  of  sandstones  and  shales,  with  which  seams 
of  coal  are  sometimes  associated.  The  conditions  of  deposition 
were  mainly  marine,  but  estuarine  and  terrestrial  conditions  were 
introduced  in  some  regions  through  slight  oscillations  of  the  land. 

In  the  Southern  Hemisphere  the  rocks  are  mainly  sandstones 
and  shales  of  the  continental  facies  ;  and  limestones  are  con- 
spicuously absent,  except  perhaps  in  some  parts  of  South  America. 

Different  Facies  of  Deposits. — It  should  always  be  remembered 
that  the  lithological  character  of  the  rocks  comprising  a  formation 
bears  no  relation  whatever  to  the  age  of  the  formation,  but  is  merely 
an  expression  and  record  of  the  physical  geography  of  the  region 
in  which  the  deposition  of  the  sediments  took  place. 

Let  us  once  more  briefly  summarise  the  physical  conditions  in 
which  clastic  rocks  are  formed. 

In  past  geological  ages,  as  now,  there  always  existed  deep  seas 
fringed  with  shallow  estuaries  and  deltas,  open  bays  and  land- 
locked harbours,  seas  bounded  by  high,  rugged  coasts  and  by  wide 
maritime  plains,  Mediterranean  seas  and  inland  salt-water  seas  of 
the  Dead  Sea  type,  and  great  inland  freshwater  lakes,  some  situated 
in  humid,  others  in  arid  deserts.  In  all  the  seas,  estuaries,  and 
inland  lakes  existing  at  the  same  time,  sediments  were  laid  down 


MESOZOIC  ERA  :    JURASSIC  SYSTEM.  387 

contemporaneously,  the  various  deposits  enclosing  representatives 
of  the  faunas  and  floras  that  peopled  the  waters  and  clothed  the 
neighbouring  lands. 

We  have  already  seen  that  in  the  Devonian,  Carboniferous, 
Permian,  and  Triassic  periods  there  were  laid  down  two  facies  of 
deposits — the  marine  and  continental — each  characterised  by  the 
sediments  and  life  peculiar  to  the  conditions  of  deposition.  These 
two  facies  are  also  found  in  the  Jurassic,  and  they  have  doubtless 
been  formed  throughout  all  geological  time,  or  ever  since  the  great 
continents  came  into  existence,  just  as  they  are  forming  at  the 
present  time. 

Coral  reefs,  coralline  sands,  and  muds  are  now  forming  on  the 
north-east  coast  of  Australia,  and  vast  sheets  of  estuarine  sands  and 
muds  are  accumulating  in  the  shallow  seas  fringing  the  northern 
coasts  ;  while  in  the  centre  of  that  ancient  and  worn-down  continent 
we  find  great  lake-basins  which  are  now  completely  filled  with 
brick-red,  wind-borne  sands  and  desert  soils,  or  the  remains  of 
basins  now  marked  by  chains  of  brackish- water  lagoons  and  swamps, 
frequently  encrusted  with  layers  of  rock-salt. 

The  coral  reefs,  coralline  sands,  and  muds  represent  the  marine 
facies  of  warm  seas ;  the  tidal  mud-flats  of  the  north,  the  estuarine 
facies  ;  and  the  red  desert  sands  and  clays  filling  the  inland  basins, 
the  continental  facies  of  an  arid  interior. 

In  later  times  the  marine  facies  will  be  represented  by  massive 
limestones  and  marls,  with  corals,  echinoderms,  and  other  dis- 
tinctive life  of  clear  sea-water  ;  and  the  continental  facies,  by  red 
sandstones  and  shaly  clays,  current-bedded  and  sun-cracked, 
enclosing  the  remains  of  the  fishes  and  molluscs  that  inhabited  the 
lakes,  also  land  plants  and  the  carcases  of  land  animals  swept  into 
the  basins  by  the  overwhelming  inundations  that  characterise  semi- 
arid  regions.  In  arid  regions,  where  the  inland  basins  were  portions 
of  the  sea  isolated  by  uplift,  the  sandstones  and  shales  may  be  inter- 
calated with  lenticular  beds  of  rock-salt  and  gypsum,  but  in  humid 
regions  favourable  for  the  growth  of  a  rank  vegetation  and  the 
accumulation  of  peaty  deposits,  the  continental  beds  may  be 
associated  with  seams  of  coal. 

The  estuarine  and  deltaic  sediments  are  mainly  fluvio-marine. 
On  the  seaward  side  they  pass  into  marine  deposits,  and  on  the 
landward  into  terrestrial.  They  will  contain  the  remains  of  the 
crustaceans  and  molluscs  that  find  a  congenial  habitat  on  tidal  mud- 
flats and  sandy  shell-banks,  mingled  with  the  leaves,  twigs,  and 
trunks  of  land  plants,  the  carcases  of  land  animals  carried  down  by 
rivers,  and  the  shells  of  marine  molluscs  cast  up  by  high  tides  and 
storms.  On  the  wide  mud-flats  lying  above  the  influence  of  the 
tides,  the  accumulation  of  peaty  matter  may  form  the  material  for 


388  A    TEXT-BOOK    OF    GEOLOG^. 

extensive  seams  of  coal.  If  the  land  subsides,  the  sea  will  encroach 
on  the  swampy  lands,  whereby  the  peats  may  become  covered  with 
a  protecting  sheet  of  sands  and  muds,  and  thereby  be  preserved 
from  destruction.  If  further  subsidence  takes  place,  the  estuarine 
sands  may  become  covered  with  marine  deposits.  If  an  uplift  now 
takes  place,  the  sea  will  once  more  retreat,  and  we  may  get  a  repeti- 
tion of  the  first  conditions,  with  a  new  growth  of  vegetation  on  the 
former  site,  but  on  a  new  soil  separated  from  the  old  by  the  layers 
of  sand  and  mud  previously  laid  down. 

Hence,  when  we  take  a  general  view  of  a  world-wide  formation, 
such  as  the  Jurassic,  we  must  expect  to  find  considerable  diversity 
in  the  character  of  the  rocks  and  fossil  remains,  notwithstanding 
that  they  may  be  contemporaneous ;  and  since  the  estuarine  is 
merely  a  pathological  phase  of  the  marine,  the  two  distinctive 
genetic  types  of  deposits  must  always  be  the  marine  and  continental. 

Distribution. — The  Jurassic  System  is  extensively  developed  in 
England,  France,  Germany,  European  and  Asiatic  Russia,  Asia 
Minor,  India,  Japan,  Borneo,  New  Guinea,  Australia,  New  Zealand, 
South  Africa,  Chile,  Peru,  Bolivia,  Western  States  of  America,  and 
Alaska. 

Although  the  Jurassic  was  not  a  period  of  mountain-building, 
we  know  that  widespread  land  movements  took  place  in  the  eastern 
side  of  the  North  American  continent,  in  European  and  Asiatic 
Russia,  and  India. 

The  entire  absence  of  Jurassic  rocks  in  the  Eastern  States  of 
North  America  shows  that  the  uplift  of  that  region  lasted  through- 
out the  whole  of  this  period,  but  such  uplift  did  not  affect  the  western 
portion  of  the  continent,  where  sediments  continued  to  be  laid 
down  in  the  Triassic  areas  up  to  the  close  of  the  Jurassic,  which 
means  that  the  uplift  of  the  rocks  and  retreat  of  the  sea  on  the 
east  coast  was  balanced  by  subsidence  of  the  rocks  and  advance 
of  the  sea  on  the  west  coast.  Here  the  axis  of  the  tilting  move- 
ment followed  a  north  and  south  direction,  and  passed  through 
the  great  Western  Interior  Basin,  bounded  on  the  east  by  the 
Rocky  Mountains. 

In  England,  France,  Germany,  and  Southern  Europe,  the  whole 
of  the  Jurassic  is  present,  but  in  the  Baltic  area  and  European 
Russia  the  lower  half  of  the  system  is  absent,  thereby  proving  that 
the  continuous  subsidence  in  Southern  Europe  was  compensated 
in  the  northern  region  by  uplift  during  the  lower  half  of  the  period, 
and  by  subsidence  during  the  upper  half.  Here  the  axis  of  the 
tilting  movement  followed  an  approximately  north-west  and  south- 
east direction. 

In  India,  where  the  marine  Jurassic  rocks  are  distributed  in  two 
distinct  geographical  areas,  each  characterised  by  a  peculiar  facies, 


MESOZOIC  ERA:    JURASSIC  SYSTEM.  389 

the  one  in  the  Inner  Himalayas  and  Tibetan  region,  the  other  in 
the  coastal  region,  the  canting  movement  was  the  converse  of  that 
in  Europe  and  Western  Asia. 

In  the  coastal  or  southern  area  the  lower  half  of  the  system  is 
absent,  while  in  the  Himalayan  area  the  lower  is  present  and  the 
upper  absent  or  greatly  interrupted.  Here  we  have  evidence  that 
the  uplift  in  the  south  during  the  lower  half  of  the  Jurassic  was 
balanced  by  subsidence  in  the  north,  and  that  the  uplift  in  the 
north  during  the  upper  half  of  the  period  was  balanced  by  sub- 
sidence in  the  south.  The  axis  of  this  rhythmical  see-saw  move- 
ment was  about  north-west  and  south-east,  or  parallel  with  the 
tilt-axis  of  Europe. 

This  singular  contrariwise  tilting  in  Europe  and  India  must 
have  caused  warping  and  enormous  crustal  stress  in  Western 
Asia. 

Fauna  and  Flora. — Since  the  rocks  of  the  Jurassic  System,  as 
developed  in  Europe,  Asia  Minor,  Himalayan  region,  and  Pacific 
States  of  North  America,  are  essentially  composed  of  marine  sedi- 
ments, the  fossils  which  they  contain  for  the  most  part  represent 
the  marine  life  of  that  period. 

Corals  are  abundant  in  the  limestones,  and  belong  to  the  aporose 
and  perforate  types  which  have  now  replaced  the  rugose  and 
tabulate  corals  of  the  Palaeozoic  era. 

Sponges,  foraminifera,  and  radiolarians  are  plentiful,  the  former 
in  most  cases  well  preserved. 

Crinoids  have  become  scarce,  but  sea-urchins,  which  become  so 
prominent  in  the  succeeding  Cretaceous  period,  are  represented  by 
Cidaris,  Hemicidaris,  Echinobrissus,  and  Pygaster,  and  other 
echinoids. 

The  brachiopods  are  mostly  of  the  Terebratula  type,  the  straight- 
hinged  Spiriferinas  being  less  common  than  in  the  Trias. 

Of  the  molluscs,  Lamellibranchs,  Gasteropods,  and  Cephalopods 
are  abundant.  The  Jurassic  is  specially  characterised  by  the  great 
development  of  Ammonites,  which  are  so  numerous  and  important 
that  this  period  has  not  inappropriately  been  called  The  Age  of 
Ammonites.  Many  of  the  species  of  Ammonites  are  world- wide  in 
distribution,  and  so  limited  in  vertical  range  that  they  serve  to 
divide  the  system  into  palseontological  zones,  each  distinguished 
by  a  characteristic  species.  The  Ammonite  zones  follow  the  same 
order  of  succession  in  all  parts  of  the  globe. 

The  Belemnites,  a  somewhat  peculiar  type  of  Cephalopods,  make 
their  first  .appearance  in  the  Jurassic,  and  attain  their  maximum 
development  before  the  close  of  the  period. 

Fishes  are  numerous,  and  among  the  genera  that  appear  for  the 
first  time  are  the  skates  and  rays,  gar-pikes,  sturgeons,  and  cat- 


390  A    TEXT-BOOK    OF    GEOLOGY. 

fish.  The  bony  fishes,  the  Teleosts,  which  are  the  dominant 
existing  type,  are  now  represented  by  several  species. 

Reptilians  which  were  prominent  in  the  Middle  and  Upper  Trias 
occur  in  such  extraordinary  numbers  that  the  Jurassic  is  familiarly 
called  The  Age  of  Reptiles.  The  seas,  the  estuaries  and  deltas,  the 
dry  land  and  the  air,  swarmed  with  reptilians,  many  of  them  of 
huge  size  and  peculiar  form. 

The  marine  types  are  represented  by  Ichthyosaurus  a  and  Plesio- 
saurusP  (Plate  XXXIX..  figs.  1  and  2) ;  the  land  reptiles  by  the 
herbivorous  deinosaurs,?  some  of  which  grew  to  a  length  of  80  feet, 
and  a  height  of  over  20  feet ;  and  the  flying  saurians  by  Pterosaurns.s 

The  earliest  known  birds  lived  in  the  Jurassic  lands.  Archceop- 
teryx,e  found  in  the  lithographic  shales  of  Solenhofen  in  Bavaria, 
was  provided  with  true  teeth  and  a  pair  of  feathers  on  each  caudal 
vertebra. 

The  Jurassic  is  further  distinguished  by  the  presence  of  the 
earliest  mammals,  the  remains  of  which  have  been  found  in  England 
and  America.  These  primitive  mammals  are  believed  to  belong  to 
the  marsupial  type,  which  still  survives  in  the  Australian  continent. 

Plant  remains  are  found  in  great  abundance  in  the  terrestrial  and 
estuarine  beds,  and  tend  to  show  that  the  Jurassic  lands  were 
clothed  with  a  luxuriant  vegetation.  Cycads  attain  their  maximum 
development ;  hence  the  name  Age  of  Cycads  sometimes  applied  to 
the  Jurassic. 

Ferns  and  conifers  are  also  conspicuous,  among  the  latter  appear- 
ing examples  of  the  ancestral  forms  of  the  modern  pines,  cypresses, 
and  yew. 

The  remains  of  insects,  including  those  of  beetles,  moths,  butter- 
flies, and  flies,  are  abundant  in  the  estuarine  muds,  having  doubtless 
been  blown  seaward  by  strong  winds.  Many  of  the  beetles  belong 
to  the  tree-boring  kinds,  which  is  further  evidence  of  the  existence 
of  forest  trees  on  the  lands  fringing  the  Jurassic  sea  coasts. 

Subdivisions. — For  our  first  knowledge  of  the  subdivisions  of 
the  Jurassic  System  we  are  indebted  to  William  Smith,  the  father 
of  English  geology,  who  in  the  first  decade  of  the  nineteenth  century 
determined  the  chronological  succession  of  the  Middle  Mesozoic 
rocks  of  England.  This  work  was  afterwards  supplemented  by  the 
investigations  of  Conybeare,  Phillips,  and  others,  and  so  important 
a  part  has  the  researches  of  English  geologists  played  in  the  history 
of  the  Jurassic  that  many  of  the  names  used  by  them  have  passed 

a  Gr.  ichthys  =  &  fish,  and  sauros  =  a,  reptile. 
/3  Gr.  plesios  =  near  to,  and  sauros  =  a,  reptile. 
7  Gr.  deinos  =  terrible,  and  semros  =  a  reptile. 
5  Gr.  pteron  =  a  wing,  and  sauros  =  &  reptile, 
e  Gr.  arche  =  &  beginning,  and  pteryx  —  a,  wing. 


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PLATE   XXXIX. 

FOSSILS  OF  THE  LOWER  AND  MIDDLE  LIAS. 

1.  Ichthyosaurus  communis  (Conyb.).     Order  Ichthyopterygia,  Owen  (=Ich- 

thyosauria,    Huxley).      Lower    Lias.      Lyme     Regis,     Barrow-on-Soar, 
Street,  etc. 

2.  Plesiosaurus  dolichodeirus  (Conyb.).     Order  Sauropterygia,  Owen  (  =  Ple- 

siosauria,  Huxley).     Lower  Lias.     Lyme  Regis,  Watchet. 

3.  Ophioderma  Milleri  (Phill).     Middle  Lias.     Staithes,  Yorkshire. 

4.  Ophioderma  Egertoni  (Broderip).     Middle  Lias.     Golden  Cap,  near  Char- 

mouth. 


To  face  page  390.] 


[PLATE  XXXIX. 


FOSSILS  OF  THE  LOWER  AND  MIDDLE  LIAS. 


MESOZO1C    ERA  :     JURASSIC    SYSTEM. 


391 


into  general  use  throughout  the  globe.  And  since  the  Ammonite 
zones  of  England  have  been  found  to  be  almost  world- wide  in  dis- 
tribution, the  subdivisions  of  the  Jurassic,  as  determined  in 
England,  may  be  described  as  typical  of  the  system  for  all  other 
regions. 

The  Jurassic  System  is  very  fully  developed  in  England,  France, 
and  Germany.  In  other  countries  the  succession  is  incomplete 
or  not  sufficiently  worked  out  for  comparative  purposes. 

BRITISH  ISLES. 


England. 

Germany. 

f 

o 

Purbeckian 

"^ 

2.  Oolite 

Upper  - 

cy 

~'2. 

Portlandian 
Kimeridgian 
Corallian 

Upper  or 
>  White  Jura. 

(Upper  - 
Jurassic) 

Middle 

1. 

Oxfordian  < 

Oxford 
Callovi* 

Clay 
in 

< 

"2. 

Bathonian  (( 

keat  Oolite) 

Middle  or 

Lower  - 

1. 

B^cian    {JJ£ 

Earth 
•  Oolite 

Brown  Jura 

1.  Lias    ("Upper, 
(Lower  -I  Middle,     . 
Jurassic)  I^Lower 

• 

• 

• 

1  Lower  or 
[Black  Jura. 

The  two  great  divisions  of  the  Jurassic  System  in  England  are 
the  Lias  or  Lower  Jurassic  and  the  Oolite  or  Upper  Jurassic,  which 
correspond  to  the  Black  Jura,  Brown  Jura,  and  White  Jura  of 
Germany. 

The  Jurassic  of  England  occupies  a  broad  zone  extending  from 
the  coast  of  Dorset  to  the  coast  of  Yorkshire.  There  are  small 
patches  in  South  Wales,  on  the  border  of  Cheshire,  and  further 
north  in  Cumberland,  Inner  Hebrides,  and  east  coast  of  Sutherland. 
In  Ireland  small  outcrops  occur  on  the  borders  of  the  Antrim 
plateau. 

Lias. 

The  Lias  is  essentially  an  argillaceous  formation.  In  England, 
and  also  in  France  and  Germany,  it  consists  mainly  of  clays  and  soft 
shales  that  vary  in  colour  from  grey  to  black.  The  clays  are  sandy 
in  places,  and  in  the  lower  part  of  the  series  in  England  contains 
bands  of  limestone  that  are  sometimes  shelly,  but  most  frequently 
sedimentary,  being  composed  of  calcareous  muds  derived  from  the 
denudation  of  Palaeozoic  limestones  in  the  neighbourhood. 

In  the  middle  or  Marlstone  division  the  clays  are  interbedded 


392  A  TEXT-BOOK  OF  GEOLOGY. 

with  bands  of  limestone  and  ironstone,  the  last  a  valuable  source  of 
iron  ore  in  the  Cleveland  district  of  Yorkshire  and  in  the  Midland 
counties. 

The  shaly  clays  of  the  Upper  Lias  contain  a  considerable  quantity 
of  the  marcasite  form  of  pyrite.  The  decomposition  of  this  ore 
produces  sulphuric  acid,  which  combines  with  the  alumina  of  the 
clay  and  forms  alum,  which  appears  as  an  efflorescence  on  the 
surface  of  the  rock. 

The  remains  of  insects  are  plentiful  in  the  clays  and  shales.  The 
muddy,  shallow  seas  in  which  the  Lias  was  deposited  did  not  favour 
the  growth  of  corals  and  bryozoans  or  the  existence  of  echinoderms, 
all  of  which  are  rare. 

The  characteristic  brachiopods  are  Spiriferina  Walcottii  (Plate 
XLI.,  fig.  4),  and  Rhynchonella  tetrahedra. 

Among  the  abundant  Lamellibranchs  are  Gryphcea  arcuata  (Plate 
XL.,  fig.  3),  Lima  gigantea  (Plate  XL.,  fig.  2),  and  Hippopodium 
ponderosum  (Plate  XLI.,  fig.  2). 

Fishes  are  well  represented,  but  the  most  important  vertebrates 
are  the  reptilians,  which  included  the  marine  saurians,  Ichthyo- 
saurus and  Plesiosaurus,  as  well  as  flying  Pterodactyls* 

Ammonites  are  numerous  and  have  been  used  to  divide  the  layers 
into  zones,  each  characterised  by  a  particular  species,  as  shown 
below  : — 

{Zone  of  Ammonites  jurensis. 
„  „  communis. 

„  ,,  serpentinus. 


Middle  Lias<^  " 

\          „  „  margantatus. 


„  capncornus. 

„  jamesoni. 


Lower  Lias^  ,,  ,,  oxynotus. 

„  „  bucklandi. 

„  ,,  planorbis. 

Lower  Oolite. 

The  various  divisions  of  the  Lower  Oolite  are  local  in  distribution 
and  variable  in  thickness.  Most  of  them  are  well  developed  in 
Dorset  and  South- West  District,  but  they  thin  out  rapidly  going 
toward  the  north-east,  and  many  of  them  disappear  entirely  before 
the  borders  of  Oxfordshire  are  reached.  Only  one  bed,  the 
Cornbrash,  the  closing  member  of  the  Lower  Oolite,  is  persistent 
from  the  south-west  coast  to  the  coast  of  Yorkshire. 

The  name  Oolite  applied  to  the  limestones  of  the  Upper  Jurassic 


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PLATE   XL. 

FOSSILS  OF  THE  LOWER  AND  MIDDLE  LIAS, 

1.  Gryphcea  cymbium  (Lam.).     Lower  Lias. 

2.  Lima  gigantea  (Sow.).     Lower  Lias — passim. 

3.  Gryphcea   incurva    (Sow.)    (arcuata).     Lower   Lias      Everywhere    in    the 

Bucklandi  Zone. 

4.  Plicatula  spinosa  (Sow.).     Lower  and  Middle  Lias — passim. 

5.  Avicula  incequivalvis  (Sow.).     Lias  to  Kellaways  Rock. 

6.  Unicardium  cardioides  (Phill.).     Lower  Lias.     Yorkshire,  Gloucestershire, 

etc. 

7.  Modiola  scalprum.     Middle  Lias.     Lyme  Regis,  Cheltenham. 


To  face  page  392.  J 


[PLATE    XL. 


FOSSILS  or  THE  LOWER  AND  MIDDLE  LIAS. 


"Jr 

I 

' 


PLATE   XLI. 

FOSSILS  OF  THE  LOWER  AND  MIDDLE  LIAS. 

1.  Extracrinus  briar  eus.     Lower  Lias.     Lyme  Regis,  etc.     Common 

2.  Hippopodium  ponderosum.     Lower  Lias. 

3.  Avicula  cygnipes.     Lower  Lias.     Bristol,  Yorkshire,  etc. 

4.  Spiriferina  Walcottii.     Lower  and  Middle  Lias.     Lyme  Regis,  etc. 

5.  Hybodus  reiic.ulatus.     Fin  Spine  and  Tooth.     Lower  Lias — passim. 

6.  Tooth  of  Acrodus.     Lower  Lias.     Bucklandi  beds. 


'To  face  page  392. J 


[PLATE    XLI. 


FOSSILS  OF  THE  LOWER  AND  MIDDLE  LIAS. 


• 

I 

" 
' 


PLATE   XLII. 

CHARACTERISTIC  JURASSIC  AMMONITES. 

1.  Ammonites  (Arietites)  obtusus  (Sow.).     Lower  Lias. 

Group  Arietites.     Fam.  Mgoceratidce. 

2.  Ammonites  (Harpoceras)  serpentinus  (Rein.).     Upper  Lias. 

Group  Harpoceras.     Fam.  Harpoceratites. 

3.  Ammonites  (Amaltheus)  cordatus  (Sow.).     Oxford  Clay  and  Coral  Rag,  etc. 

Group  Amaltheus.     Lias  to  Coral  Rag. 

4.  Ammonites  (Cosmoceras)  Duncani  (Sow.).     Oxford  Clay  and  Kellaways 

Rock. 

Group  Ornati.     Fam.  Steph-anoceratites. 

5.  Ammonites  {Mgoce/ras]  armatus  (Sow.).     Lower  Lias. 

Group  Mgoc&ras.     Fam.  Stephanoceratites. 

6.  Ammonites  (JEgoceras)  capricornus  (Schloth.).     Middle  Lias. 

Group  Mgoceras.     Fam.  dEgoceratites. 

7.  Ammonites     (Phylloceras)     heterophyllus     (Sow.).     Upper     Lias.     Chiefly 

Whitby. 

Group  Heterophylli.     Fam.  Phylloceras. 

8.  Ammonites  (Stephanoceras)  annulatus  (Sow.).     Upper  Lias — passim. 

Group  Planulati.     Fam.  Stephanoceratites. 


PLATE   XLTI. 


To  face  page  392 


A    R    I    E  T    I   T    E   S. 


F  A  L  C    I    F  E  R  I  . 


A   R    M  A  T  I  . 


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CHARACTERISTIC  JURASSIC  AMMONITES. 


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PLATE  XLIII. 
JURASSIC  AMMONITES  AND  STRUCTURAL  PARTS. 

1.  Ammonites   (Stephanoceras)    Bechei   (Sow.)-     Middle   Lias.     Cheltenham 

and  Lyme  Regis. 

Group  Ooronarii.     Fara.  Stephanoceratites. 

2.  Ammonites    (Stephanoceras)    macrocephalus    (Schloth.).     Cornbrash    and 

Oxford  Clay. 

Group  Macrocephali.     Fam.  Stephanoceratites. 

3-4.  Ammo.  (Stephanoceras)  Braikenridgii  (Sow.).     Inferior  Oolite. 
Group  Coronarii.     Fam.  Stephanoceratites. 

These  two  figures  show  the  side  and  front  view  of  the  so-called 
labial  prolongations,  or  complete  mouth  or  aperture. 

5.  Section  showing  the  aperture  or  end  view  of  Ammo.  Heterophyllus,  and 

position  of  the  lobes  and  saddles  (L.  =  Lobes,  S.  =  Saddles,  D.  =  Dorsal, 
and  V.  =  Ventral  lobes). 

6.  Forms  and  disposition  of  the  lobes  and  saddles. 

7.  Disposition  of  the  lobes — front  view   of  base   of  body-chamber    (D.  = 

Dorsal  lobe,  V.  =  Ventral  lobe,  L.  =  Lateral  lobe,  L1.  =  Inferior  lateral 
lobe). 

The  intermediate  spaces  are  occupied  by  the  saddles. 

8.  Ammonites  (Harppceras)  serpentinus.     Showing  extension  of  the  siphona 

or  ventral  area,  and  sigmoidal  folds  or  successive  growth  of  the  shell. 


PLATE   XLIII. 


392. 


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JURASSIC   AMMONITES  AND   STRUCTURAL   PARTS. 


To  face  page 


[PLATE    XLIV. 


LOWER  JURASSIC  FOSSILS. 


PLATE   XLIV. 
LOWER  JURASSIC  FOSSILS. 

1.  Isocardia    (Ceromya)    concentrica    (Sow.).     Inferior    Oolite.     Gloucester- 

shire and  Yorkshire. 

2.  Pernn  quadrata  (Sow.).     Cornbrash  and  Great  Oolite.     Gloucestershire, 

etc. 

3.  Astarte   elegans   (Sow.).     Inferior    Oolite.     Somerset,    Cotteswold    Hills, 

Yorkshire. 
3a.  Astarte  Voltzii.     Inferior  Oolite. 

4.  Pseudodiadema  seriale  ? 

5.  Pygaster  semisulcatus  (Phill.).     Inferior  Oolite.     Leckhampton,  Crickley, 

Stroud,  etc. 

6.  Echinobrissus    cluniculans    (Llhwyd.).     Inferior    Oolite.     Wilts,    York- 

shire, Dorset,  Northampton. 

7.  Collyrites  bicordatus  (Desor).     Dorse,  Somerset,  etc. 

8.  Rhynchonella  cynocephala  (Richard).     Inferior  Oolite.     Gloucestershire. 

9.  Belemnites  sulcatus  (Miller).     Inferior  Oolite.     Dundry,  etc. 


.VIJX    3TAJ<I 


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MESOZOIC    ERA :     JURASSIC    SYSTEM.  393 

in  England  is  derived  from  the  peculiar  roe-like  grains  of  calcite  of 
which  the  limestones  are  composed. 

The  characteristic  limestone  bands  of  the  south  are  replaced, 
going  northward,  by  marine  sandy  beds,  which  in  Yorkshire  become 
typically  estuarine,  and  are  locally  called  the  Estuarine  Series, 
which  is  the  equivalent  of  the  Inferior  Oolite. 

On  account  of  the  change  in  the  character  of  the  sediments  and 
fossils,  the  Ammonite  zones  of  the  south  cannot  be  traced  into  the 
north. 

Bajocian. — This  stage  name  is  derived  from  Bayeux,  in  the 
Norman  Department  of  Calvados  in  France,  where  the  Lower  Oolite 
is  well  developed.  In  England  it  is  divided  into  two  sub-stages  : — 

2.  Fuller's  Earth. 
1.  Inferior  Oolite. 

Inferior  Oolite. — This  follows  the  Lias  conformably,  and  extends 
from  Dorset  north-east  to  Yorkshire.  In  the  South- West  District 
it  consists  mainly  of  shelly  marine  limestones  interbedded  with 
clays  and  sandstones,  but  going  northward  the  deposits  in  Lincoln- 
shire contain  an  estuarine  facies  in  which  freshwater  genera,  such 
as  Unio  and  Ct/rena,  replace  the  Ammonites  so  common  in  the 
south.  Still  further  north,  in  Yorkshire,  the  strata  consists  mainly 
of  estuarine  sandstones  and  shales,  with  bands  of  ironstone  and  coal 
together  with  several  calcareous  beds  of  marine  origin.  These 
estuarine  beds  comprise  the  well-known  Estuarine  Series  of  York- 
shire. 

The  marine  beds  of  the  south-west  district,  which  attain  a  thick- 
ness of  264  feet  at  Cheltenham,  contain  an  abundant  fauna,  which 
includes  several  genera  of  corals,  the  crinoid  Pentacrinus,  and  a 
few  starfish,  including  Goniaster  and  Stellaster,  as  well  as  the  sea- 
urchin  Cidaris,  distinguished  by  its  club:like  spines. 

Of  Brachiopods,  Terebratula  and  Rhynchonella  (Plate  XLIV.) 
are  fairly  abundant ;  and  among  the  Lamellibranchs,  Lima,  Ostrea, 
Pecten,  Pinna,  Astarte  (Plates  XLIV.  and  XLVII.),  Cucullcea, 
Mytilus,  Pholadomya,  and  Trigonia  are  common.  Gasteropods 
are  numerous,  especially  the  genera  Cerithium,  Pleurotoma,  and 
Turbo.  The  Cephalopods  include  many  genera  of  Ammonites, 
Nautili,  and  the  peculiar  dart-shaped  Belemnites.1 

The  palaeontological  zones  of  the  marine  facies  of  the  Inferior 
Oolite  are  in  descending  order  : — 

4.  Zone  of  Ammonites  Parkinsoni. 
3.        ,,  ,,  Humphriesianus. 

2.        ,,  ,,  Murchisonce. 

1.        „  „  opalinus. 

1  Gr.  belemnon  =  &  dart. 


394  A  TEXT-BOOK  OF  GEOLOGY. 

The  estuarine  facies  of  the  Inferior  Oolite  in  Yorkshire  contains 
an  abundant  fossil  flora  which  comprises  many  ferns,  cycads,  and 
conifers. 

The  ferns  include  Pecopteris,  Sphenopteris,  and  Tceniopteris, 
(Plate  XLV.). 

The  cycads  include  Zamites,  Otozamites,  and  Cycadites. 

The  conifers  include  Walchia,  Araucarites,  and  Taxites. 

The  three  calcareous  beds  intercalated  in  the  estuarine  series  are 
the  Dogger  at  the  base,  with  valuable  bands  of  concretionary 
iron-stone ;  the  so-called  Millepore  Limestone,  so  named  from  the 
abundance  of  Millepora  straminea ;  and  the  Scarborough  Limestone. 

Fuller's  Earth. — This  bed  extends  from  Dorset  to  Bath  and 
Cheltenham,  but  it  is  absent  in  the  north-east  countries.  Its  thick- 
ness nowhere  exceeds  150  feet.  It  contains  numerous  fossils,  which 
include  many  examples  of  Ostrea,  Rhynchonella,  Magellania,  and 
Ammonites.  The  clays  of  this  sub-stage  are  commercially  useful 
for  the  fulling  of  cloth  ;  hence  the  origin  of  the  name. 

Bathonian  (Great  Oolite).— This  consists  of  a  series  of  thin- 
bedded  limestones  and  clays,  which  have  been  divided  into  three 
well-marked  sub-stages  : — 

C(c)   Cornbrash. 
Bathonian^  (b)  Forest  Marble. 
[(a)  Great  Oolite. 

At  the  base  of  the  Great  Oolite  there  is  what  is  known  as  the 
Stonesfield  Slate,  which  is  of  peculiar  geological  interest.  It  is 
developed  in  parts  of  Gloucestershire  and  Oxfordshire,  and  contains 
a  remarkable  mixture  of  marine  and  estuarine  forms  mingled  with 
the  remains  of  land  plants  and  animals.  Among  the  most  prevalent 
fossils  are  the  following  genera  : — 

Brachiopods  include  Terebratula  and  Rhynchonella  (Plate  XLIV.). 
Lamellibranchs  ,,        Gervillia,    Ostrea,    Lima,    Pecten,     Astarte, 

Modiola,  and   Trigonia. 

Gasteropods        ,,        Natica,  Patella,  and  Trochus. 
Cephalopods        ,,        Ammonites   gracilis    and   Belemnites  fusi- 

formis. 

Fishes  ,,        Ceratodus,  Hybodus,  and  Ganodus. 

Reptiles  ,,        Plesiosaurus,  Cetiosaurus,  Teleosaurus,  and 

Megalosaurus. 
Mammalia  ,,       the  marsupials   Amphilestes  and  Phascolo- 

therium. 
Plants  ,,      Pecopteris,  TcBniopteris,  and  Sphenopteris. 

Crustaceans  and  insects  are  numerous,  the  latter  including  many 
examples  of  beetles,  moths,  butterflies,  dragon-flies,  etc. 


,9qioif;tehQ 


oriioO  ic 

. 


PLATE   XLV. 

LOWER  JURASSIC  FOSSILS. 

(Inferior  Oolite.) 

1.  Equisetites  columnaris  (Brong.).     Inferior  Oolite  shale.     Gristhorpe,  etc., 

Yorkshire. 

2.  Pterophyllum.     Inferior  Oolite  shale.     Yorkshire  coast. 

3.  Pterophyllum  comptum  (L.  &  H.).     Inferior  Oolite  shale.     Yorkshire. 

4.  Tceniopteris  vittata  (Brong.).     Inferior  Oolite  shale.     Whitby,  Gristhorpe, 

etc. 


To  face  page  394.] 


[PLATE    XLV. 


LOWER  JURASSIC  FOSSILS. 
(Inferior  Oolite.) 


MESOZOIC    ERA  :     JURASSIC    SYSTEM.  395 

The  Great  Oolite  proper  was  laid  down  in  a  shallow  sea  swarming 
with  a  prolific  marine  life,  which  included  corals,  bryozoans,  sea- 
urchins,  and  starfish,  all  the  inhabitants  of  clear  sea- water  ;  also  : — 

Brachiopods,  including  Rhynchonella,    Terebratida,    Mqgellania, 

and   Crania. 
Lamellibranchs,     „         Pecten,   Lima,   Ostrea,  Avicula,  Astarte, 

Modiola,  Pholadomya,  Trigonia,   Car- 

dium,  Area,  etc. 

Gasteropods,         ,,          Nerita,  Nerincea,  Patella,  etc. 
Cephalopods,         ,,          Ammonites  arbustigerus,    A.  gracilis,    A. 

subcontractus,  etc. 
Eeptilians,  ,,          most  of  those  in  the  Stonesfield  Slate. 

The  Forest  Marble  attains  a  thickness  of  several  hundred  feet  in 
Dorset,  but  thins  out  rapidly  going  northward.  It  is  chiefly  notable 
for  its  echinoderms,  which  include,  among  other  species,  the  dis- 
tinctive form  Apiocrinus  elegans. 

The  Cornbrash  receives  its  name  from  the  abundant  crops  of  grain 
which  are  produced  on  its  soils.  It  is  a  thin  bed  of  earthy  lime- 
stone varying  from  5  to  40  feet  thick,  which,  notwithstanding  its 
insignificant  dimensions  that  rarely  exceed  20  feet,  runs  across  the 
country  from  Devonshire  to  Yorkshire,  and  is  therefore  the  most 
persistent  member  of  the  Lower  Oolite  Series  or  even  of  the  Jurassic 
System. 

Among  the  characteristic  fossils  of  the  Cornbrash  are  Echino- 
brissus  clunicidaris  (Plate  XLIV.),  Hinnites  gradus,  Cardium  latum, 
and  the  Cephalopods,  Ammonites  discors  and  A.  macrocephalns. 

Middle  Oolite. 

This  division  of  the  Jurassic  System  comprises  two  groups  of 
beds  : — 

f2.  Corallian. 

Middle  OolitJ  L  Oxfordian  f  (6)  Oxford  Clay. 
L  \(a)  (Jallovian. 

Oxfordian. — This  stage  consists  of  two  sub-stages,  namely,  the 
Callovian  or  Kellaways  Rock,  which  is  the  lower  sub-stage,  .and  the 
Oxford  Clay,  the  upper  sub-stage. 

The  Callovian,  also  known  as  Kellaivays  Rock,  which  derives  its 
name  from  the  village  of  Kellaways,  in  Wiltshire,  where  this 
important  subdivision  was  first  described,  is  a  calcareous  sand- 
stone, varying  from  5  to  80  feet  thick.  It  can  be  traced  from 
Wiltshire  to  Lincolnshire,  and  northward  into  Yorkshire,  where 
it  is  well  developed. 

The  Callovian  is  chiefly  notable  for  its  fish  remains,  of  which  over 


396  A  TEXT-BOOK  OF  GEOLOGY. 

200  species  have  been  identified  ;  of  these  about  one-third  are 
found  in  the  underlying  Jurassic  rocks,  and  about  one-third  pass 
upward  into  the  overlying  beds. 

The  fauna  indicates  a  revival  of  the  estuarine  conditions  which 
characterised  the  Estuarine  Series  of  the  northern  counties  in  the 
Lower  Oolite  times  ;  and  contains,  besides  the  fishes  mentioned 
above,  a  considerable  number  of  molluscs,  among  which  we  have 
Ammonites,  Belemnites,  the  widely  distributed  Gryphcea  bilobata, 
Ostrea,  Lima,  Avicula,  Lucina,  Trigonia  complanata,  Cerithium 
abbreviatum,  and  Pleurotoma  arenosa. 

The  characteristic  Ammonite  of  Kellaways  Eock  is  Ammonites 
calloviensis ;  hence  the  name  Callovian  by  which  this  zone  is  so 
commonly  known  outside  the  British  Isles. 

Oxford  Clay. — This  great  argillaceous  deposit  ranges  throughout 
England  from  the  coast  of  Dorset  to  Scarborough  on  the  Yorkshire 
coast.  It  consists  essentially  of  stiff  clays  and  bituminous  shales, 
and  varies  from  170  to  600  feet  thick. 

The  muddy  conditions  of  deposition  of  these  sediments  were 
obviously  unfavourable  for  the  growth  of  corals  and  bryozoans. 
which  are  rare,  as  also  are  echinoderms,  which  usually  frequent 
clear  water.  Brachiopods  and  Gasteropods  are  not  common,  but 
the  shallow-water  Lamellibranchs  which  congregate  in  shell- 
banks  are  very  abundant,  and  include  Gryphcea  dilatata,  Ostrea, 
Lima,  Pecten,  Avicula,  Astarte,  Trigonia,  etc. 

Ammonites  are  numerous  and  Belemnites  not  uncommon,  of  the 
latter  B.  Oweni  being  the  best  known.  The  reptilian  genera 
Ichthyosaurus,  Plesiosaurus,  and  Megalosaurus  are  also  present- 
Crustaceans  and  insects  also  occur,  but  plant  remains  are  compara- 
tively scarce. 

Palseontologically  the  Oxfordian  Series  is  divided  into  four 
Ammonite  zones  : — 

4.  Zone  of  Ammonites  cordatus. 
3.        ,,  „  Lamberti. 

2.        „  „  Jason. 

1.        „  „  calloviensis. 

The  Corallian. — This  is  the  upper  division  of  the  Middle  Oolite. 
It  consists  mainly  of  shelly  and  oolitic  limestones  and  calcareous 
sandstones,  and  is  chiefly  characterised  by  the  presence  of  many 
corals.  It  extends  from  the  coast  of  Dorset  to  Yorkshire.  In  some 
parts  of  Wiltshire  the  upper  limestones  have  been  replaced  by 
valuable  deposits  of  ironstone. 

The  corals  mostly  belong  to  the  reef-building  kinds,  and,  notably 
in  Yorkshire,  are  found  forming  coral  reefs  in  the  positions  in  which 
they  grew. 


To  face  page  397.] 


[PLATE    XLVT. 


MIDDLE  JURASSIC  FOSSILS. 


PLATE   XLVI. 
MIDDLE  JURASSIC  FOSSILS. 

1.  Trigonia  clavellata  (Park.).    Kimineridge  Clay  and  Portland  Sand.     Wey- 

mouth,  Swindon,  etc. 

2.  Ostrea    gregaria    (Sow.).     Corallian    beds.     Yorkshire,    Wiltshire,    Cam- 

bridgeshire. 

3.  Nerincea    Goodhallii    (Sow.).     Corallian    beds.     Dorsetshire    (Osmington, 

etc.). 

4.  Gryphcea  dilatata  (Sow.).     Corallian  and  Oxfordian  rocks.     Wilts,  York- 

shire, Oxfordshire,  etc. 

5.  Littorina  muricata  (Sow.).     Corallian  beds.     Wiltshire,  Yorkshire,  Cam- 

bridgeshire. 

6.  Cidaris  florigemma  (Phill.).     Coral  Rag.     Calne,  Yorkshire,  etc. 


-.  -'.'"•  '      ',.-•> 


- 

V>   . 

,• 

-•    .. 


To  face  page  397.] 


[PLATE    XLVIL 


UPPER  JURASSIC  FOSSILS. 
(Kimmeridgian,  Portland,  Purbeck.) 


PLATE   XLVII. 
UPPER  JURASSIC  FOSSILS. 

(Kimmeridgian,  Portland,  Purbeck.) 

1.  Mantellia  nidiformis  (Brong.).     Dirt  bed,  Purbeck  beds.     Isle  of  Purbeck 

2.  Pecten  lamellosus  (Sow.).     Portland  Oolite,  Wiltshire,  Dorset.  Oxfordshire, 

etc. 

3.  Cerithium    portlandicum    (Sow.).     Portland    Oolite.     Portland,    Vale    of 

Wardour. 

4.  Ostrea  expansa  (Sow.).     Portland  Oolite.     Portland,  Swindon,  Quainton, 

etc. 

5.  Ostrea  deltoidea  (Sow.).     Kimmeridge  Clay.     Portland,  Weymouth,  and 

passim. 

6.  Thracia    depressa    (Sow.).     Kimmeridge     Clay.     Weymouth,     Hartwell, 

Brill,  etc. 

7.  Exogyra    virgula    (Def.).       Kimmeridge    Clay.      Aylesbury,    Weymouth, 

etc. 

8.  Modiola,  sp.     Portland  Oolite. 

9.  Trigonia  gibbosa  (Sow.).     Portland  Oolite.     Portland,  Swindon,  Vale  of 

Wardour,  etc. 


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MESOZOIC    ERA  :     JURASSIC    SYSTEM.  397 

Sea-urchins  are  numerous  and  include  Cidaris,  Hemicidaris 
intermedia,  and  Pygaster  umbrella.  Among  the  Lamellibranchs, 
Gryphcea,  Ostrea,  Lima,  Pecten,  and  Avicala  are  plentiful  ;  also 
Trigonia  clavellata,  which  is  characteristic.  Ostrea  gregaria  occurs 
in  great  numbers  (Plate  XLVI.,  fig.  2). 

The  principal  Ammonites  are  A.  perarmatus  and  A.  plicatilis, 
both  of  zonal  importance : — 

2.  Zone  of  Ammonites  plicatilis — Upper  Corallian. 
1.        „  „          perarmatus — Lower  Corallian. 

Upper  Oolite. 

The  three  main  subdivisions  of  the  Upper  Oolite  are  : — 

3.  Purbeckian — Limestone. 
2.  Portlandian — Limestone. 
1.  Kimeridgian — Clays. 

The  Kimeridgian. — The  Kimeridge  Clay  is  one  of  the  most  per- 
sistent subdivisions  of  the  Jurassic  in  England.  It  usually  consists 
of  dark-grey  or  black  shaly  clays,  with  frequent  layers  of  septarian 
concretions.  Occasionally  the  shales  are  calcareous  and  pass 
into  bands  of  limestone.  In  the  north  of  Scotland,  on  the  east 
coast  of  Sutherlandshire,  the  beds  are  mostly  sands,  grits,  and 
limestones. 

The  Kimeridge  Clay  is  well  developed  around  Kimeridge  on  the 
coast  of  Dorset,  whence  it  extends  northward  to  the  Yorkshire 
coast.  The  thickness  varies  from  1200  feet  in  the  South- West 
District  to  100  feet  in  Oxfordshire. 

The  Kimeridgian  is  everywhere  richly  fossiliferous,  and  was 
obviously  laid  down  in  muddy  seas  swarming  with  the  peculiar 
marine  life  of  shallow  waters.  Hence  corals  and  echinoderms  are 
rare,  and  brachiopods  are  not  common  ;  but  Lamellibranchs  and 
Cephalopods  are  numerous,  the  former  being  represented  by  the 
characteristic  species  Exogyravirgula  (Plate  XLVII.,  fig.  7),  Ostrea 
deltoidea  (Plate  XLVII.,  fig.  5),  and  Astarte  supracorallina ;  and  the 
latter  by  Ammonites  alternans,  A.  mutabilis,  and  A.  biplex. 

The  seas  and  estuaries  still  swarmed  with  reptilians,  which 
included  the  marine  plesiosaurs  Plesiosaurus  and  Pliosaurus.  The 
unwieldy  and  uncouth  deinosaurs  Cetiosaurus,  Gigantosaurus, 
Camptosaurus  (Iguanodon],  and  Megalosaurus  still  herded  in  the 
marshy  lands,  while  the  crocodiles  Teleosaurus,  Steneosaurus,  and 
others  frequented  the  estuaries  and  deltas.  The  fish-like  ichthyo- 
saurs  still  inhabited  the  neighbouring  seas,  and  the  flying  pterosaur 
Pterodactylus  continued  to  dominate  the  air. 


398  A  TEXT-BOOK  OF  GEOLOGY. 

Palseontologically  the  Kimeridgian  is  divided  into  two  Ammonite 
zones  : — 

2.  Zone  of  Ammonites  biplex  =  Upper  Kimeridgian. 
1.        „  ,,  alternans  =  Lower  Kimeridgian. 

The  Portlandian. — This  series  takes  its  name  from  the  Isle  of 
Portland,  where  it  is  typically  developed.  It  follows  the  Kime- 
ridgian conformably,  but  has  a  narrower  surface  exposure,  this,  in 
the  south  of  England,  being  mainly  due  to  the  overlap  of  the  Upper 
Cretaceous.  Further  north  from  Bedfordshire  to  Norfolk,  and  in 
Yorkshire,  no  Portlandian  beds  are  known,  which  may  be  due  either 
to  local  uplift  after  the  Kimeridgian  period  or  to  denudation  before 
the  Cretaceous. 

The  Portlandian  consists  typically  of  marine  limestones  and 
sands,  and  the  former  encloses  a  rich  fauna. 

Corals  are  rare,  and  represented  by  one  species,  Isastrcea  oblonga. 
Brachiopods  are  also  scarce.  The  most  abundant  fossils  are 
Lamellibranchs,  Gasteropods,  and  Cephalopods.  In  this  formation 
the  Ammonites  attain  a  remarkable  size. 

Among  the  best-known  molluscs  in  the  Portlandian  are  the 
following  : — 

Lamellibranchs — Trigonia  gibbosa  (Plate  XLV1L,  fig.  9). 
Gasteropods — Gerithium  portlandicum  (Plate  XLVIL,  fig.  3). 
Cephalopods —  Ammonites  giganteus. 

Fishes  are  represented  by  the  persistent  Hybodus  and  Gyrodus, 
while  most  of  the  reptilians  of  the  Middle  Jurassic  are  also  present. 

In  the  Isle  of  Portland  and  South- West  England  the  Portlandian 
presents  two  distinct  subdivisions,  namely  : — 

2.  Portland  Stone — Upper  Portlandian. 
1.  Portland  Sand — Lower  Portlandian. 

The  Portland  Sand  consists  of  yellow,  brown,  and  greenish  sands, 
with  occasional  layers  of  clay  and  limestone  ;  and  the  Portland 
Stone,  of  white  shelly  or  oolitic  limestone,  with  layers  and  nodules 
of  chert. 

The  Purbeckian. — This  series  is  typically  developed  in  the  Isle 
of  Purbeck  and  in  South- West  England,  and  generally  its  dis- 
tribution is  coextensive  with  that  of  the  underlying  Portlandian 
with  which  it  is  everywhere  closely  associated.  In  most  places 
it  follows  the  Portlandian  quite  conformably,  but  in  some  localities 
there  is  evidence  of  uplift  at  the  close  of  that  stage  whereby  some 
portions  of  the  sea-floor  became  dry  land  and  other  portions 
shallow  estuaries.  This  uplift  by  introducing  terrestrial  and 


MESOZOIC   ERA  I     JURASSIC   SYSTEM. 


399 


estuarine  conditions  caused  the  migration  from  these  areas  of  the 
marine  life  and  reptilian  forms  which  characterised  the  preceding 
Jurassic  seas  and  shores. 

t^  The  Purbeckian  consists  mainly  of  shales,  marls,  and  limestones, 
with  occasional  beds  of  dark  sandy  clays  containing  much  carbon- 
aceous matter.  These  clays  probably  represent  the  soils  of  old 
land  surfaces.  They  are  locally  called  Dirt  Beds,  and  in  some 
places  contain  the  trunks  of  cycads  and  conifers. 

There  is  also  a  marine  bed  intercalated  in  the  series  composed 
almost  entirely  of  the  shells  of  the  oyster  Ostrea  distorta.  These 
shells  impart  a  rough  surface  to  the  outcrops  of  the  bed,  which  is 
in  consequence  frequently  called  the  Cinder  Bed. 

Among  the  freshwater  shells  found  in  the  Purbeckian  Series, 
Unio,  Paludina,  Physa,  and  Limncea  are  abundant.  Insects 
are  also  plentiful  and  often  beautifully  preserved.  Fishes  are 

Malver,n  Hills 
a 


NW 


FIG.  216A. — Showing  arrangement  of  Mesozoic  formations 
from  Malvern  Hills  to  coast  of  Essex. 


(o)  Pre-Cambrian  gneiss. 

(a)  Trias. 

(6)  and  (c)  Lias. 


(d)  and  (e)  Oolites  of  Cotteswold  Hills. 
(/)  Chalk  of  Chiltern  Hills. 
(g)  Eocene  of  Essex. 


numerous,  but  reptiles  are  not  common,  and  the  forms  present 
are  mostly  of  the  crocodilian  type. 

The  Purbeckian  is  chiefly  celebrated  for  its  mammalian  remains, 
which  are  found  at  Durleston  Bay  in  a  stratum  five  inches  thick 
in  the  middle  of  the  series.  The  mammals  belong  to  the  marsupial 
order  and  include  the  genera  Plagiaulax  and  Triconodon. 

Jurassic  in  other  Countries. 

France. — Lithologically  and  palseontologically,  the  Jurassic   in 
this  region  does  not  differ  greatly  from  that  of  England  except 
the   Mediterranean  Province.       The    subdivisions    recognised 


111 


in  France  are  as  follow  : — 


Oolite 


9.  Portlandian. 
8.  Kimeridgian. 
7.  Corallian. 
6.  Oxfordian. 
5.  Bathonian. 
4.  Bajocian. 


400  A  TEXT-BOOK  OF  GEOLOGY. 

{3.  Toarcian. 
2.  Liassian. 
1.  Sinemurian. 

Infra-Lias  —  Hettangian — Lies  conformably  on 
the  Avicula  contorta  zone  of 
the  Khsetic. 

Germany. —  The  subdivisions  of  the  Jurassic  recognised  in 
North- West  Germany  are  : — 

3.  Upper  or  White  Jura  (  =Malm). 
2.  Middle  or  Brown  Jura  (  =  Dogger). 
1.  Lower  or  Black  Jura  (  =Lias). 

The  Black  Jura  is  essentially  an  argillaceous  formation  and 
closely  resembles  the  English  Lias.  It  derives  its  name  from  the 
prevailing  black  colour  of  the  shales  which  in  the  upper  part  of 
the  series  are  so  bituminous  as  to  be  a  source  of  mineral  oil. 

The  Brown  Jura  or  Dogger  corresponds  to  the  English  Lower 
Oolite  division,  with  the  exception  of  the  Callovian  sub-stage 
which  German  geologists  include  in  the  White  Jura.  It  consists 
mainly  of  brown  and  yellow  sandstones,  dark  clays,  and  shales 
with  bands  of  oolitic  ironstone. 

The  Malm  or  White  Jura  receives  its  name  from  the  prevailing 
colour  of  the  rocks,  which  consist  mainly  of  dolomitic  limestones 
and  marls. 

The  Jurassic  rocks  in  Germany  exhibit  a  close  -faunal  relation- 
ship to  the  Jurassic  of  England,  a  correspondence  which  doubtless 
arises  from  the  circumstance  that  England  and  Germany,  in  common 
with  North  France,  lie  in  the  Central  European  Biological  Province, 
to  which  reference  will  be  made  later. 

Russia. — Jurassic  rocks  cover  a  larger  area  in  Russia  than  in 
any  other  part  of  Europe  notwithstanding  that  the  Lias  and  the 
Lower  Oolite — that  is,  all  below  the  Callovian — are  absent.  From 
this  it  would  appear  that  Russia  was  dry  land  during  the  lower 
half  of  the  Jurassic  period. 

The  Jurassic  fauna  of  Russia  does  not  bear  a  close  resemblance 
to  that  of  England  or  Germany,  which  may  arise  from  the  fact 
that  the  Jurassic  rocks  of  that  region  were  deposited  in  a  more 
northerly  biological  province  or  zone. 

India. — There  are  two  types  of  Jurassic  rocks  in  India,  namely, 
the  Marine  and  the  Continental. 

The  Marine  type  has  an  extraordinary  development  in  Northern 
India,  where  it  is  represented  by  two  facies  in  point  of  age,  the 
Alpine  and  Coastal,  the  former  comprising  the  Lower  Jurassic 
rocks,  the  latter  the  Upper  Jurassic. 


MESOZOIC  ERA:   JURASSIC  SYSTEM.  401 

The  Alpine  facies  consists  of  massive  beds  of  limestone  which 
are  developed  on  a  vast  scale  in  Baluchistan,  in  the  Inner  Himalayas, 
and  in  Tibet.  The  Coastal  facies  is  met  with  in  the  Cutch,  and 
Salt  Range  in  the  Punjab. 

The  succession  of  the  Alpine  Jurassic  is  interrupted  by  a  break 
at  the  close  of  the  Callovian  stage  arising  from  uplift ;  and  deposition 
did  not  again  begin  in  this  region  until  about  the  end  of  the  Jurassic 
or  beginning  of  the  Cretaceous  period.  The  Liassic  and  Lower 
Oolitic  rocks  are  well  represented  ;  and  the  principal  Ammonite 
zones  of  Central  Europe  and  England  have  been  identified  in  the 
black  limestones  of  Baluchistan,  in  which  they  follow  the  same 
chronological  succession. 

In  the  Coastal  facies  the  Lias  and  Lower  Oolite  are  absent, 
showing  that,  at  the  time  a  sea-floor  existed  in  the  North  Himalayan 
and  Tibetan  areas,  dry  land  occupied  the  Cutch.  When  deposi- 
tion began  in  the  Coastal  area,  the  Alpine  area  emerged  from 
the  sea. 

In  the  matter  of  age,  the  Jurassic  rocks'of  the  North  Himalayan 
and  Tibetan  regions  exhibit  a  singular  contrast  with  those  of 
the  Cutch,  the  stages  present  in  the  coastal  region  being  absent 
in  the  extra-peninsular,  and  the  converse.  Obviously  subsidence 
in  the  one  region  was  compensated  by  uplift  in  the  other. 

The  Continental  type  of  the  Jurassic  System  of  India  is  repre- 
sented by  the  Upper  Gondwana,  which  consists  mainly  of  sandstones 
and  shales  with  coal-seams  and  bands  of  limestone.  In  the 
Rajmahal  Hills  the  rocks  are  intercalated  with  massive  sheets 
of  basalt. 

The  Rajmahal  Shales  contain  a  rich  fossil  flora  which  includes 
an  abundance  of  ferns  and  cycads,  the  latter  being  represented 
by  Ptilophyllium,  and  the  former  by  Twniopteris  and  Dicksonites. 

During  the  Jurassic  period  the  Himalayan  region  of  Northern 
India  was  a  sea-floor  ;  and  so  far  as  the  available  evidence  will 
permit  us  to  judge,  it  would  appear  that  the  continent  from  which 
the  Jurassic  sediments  were  derived  lay  to  the  south  and  south-east. 
This  continent,  of  which  the  Peninsular  area  formed  a  part,  was  the 
Gondwana  Land  of  Indian  geologists. 

North  America. — Jurassic  rocks  are  not  found  in  the  Eastern 
States,  but  are  well  developed  in  California,  Sierra  Nevada,  and 
Alaska  ;  and  also  in  the  States  of  Wyoming,  Utah,  Dakota,  and 
Colorado,  where  they  have  yielded  a  remarkable  group  of  deino- 
saurs,  tortoises,  pterodactyles,  crocodiles,  and  lizards,  the  latter 
including  some  forms  related  to  the  living  tuatara  (Sphenodon 
punctatum)  of  New  Zealand.  The  deinosaurs  were  herbivorous, 
and  among  the  principal  genera  distinguished  by  Marsh  are 
Atlantosaurus,  Brontosaurus,  and  Stegosaurus. 

26 


402  A  TEXT-BOOK  OF  GEOLOGY. 

Associated  with  these  reptilian  remains,  there  have  also  been 
found  many  genera  of  small  marsupial  mammals,  including  the 
genera  Allodon,  Docodon,  Tinodon,  and  many  others. 

In  California  the  thickness  of  the  Jurassic  rocks  is  about  2000 
feet,  part  of  which  is  volcanic  tuff  ;  and  in  Nevada,  5000  or  6000 
feet,  the  upper  4000  feet  of  which  are  slates,  the  remaining  lower 
beds  being  limestones. 

In  this  State  the  Jurassic  rocks  are  sharply  folded  and  frequently 
much  metamorphosed.  The  fauna  of  this  region,  so  far  as  it  has 
been  studied,  shows  a  relationship  to  that  of  the  Central  European 
Biological  Province.  Generally  speaking,  the  Jurassic  System 
plays  a  subordinate  part  in  the  geological  structure  of  North 
America,  the  greater  portion  of  which  was  dry  land  from  the  close 
of  the  Triassic  till  the  Cretaceous. 

Australasia. — Jurassic  rocks  have  been  identified  in  Borneo 
and  New  Guinea,  whence  they  extend  southward  to  Queensland, 
New  South  Wales,  Victoria,  South  Australia,  Western  Australia, 
and  New  Zealand. 

In  the  Australian  Continent  the  rocks  for  the  most  part  consist 
of  sandstones  (greywackes),  shales,  and  conglomerates  of  the 
continental  facies.  They  are  distinguished  by  the  presence  of 
fossil  plants,  reptilian  and  fish  remains. 

A  characteristic  and  widespread  fern  is  Tceniopteris  daintreei  ; 
but  the  succession  of  the  Jurassic  rocks  has  not  yet  been  worked 
out  in  detail. 

In  New  Zealand  the  Jurassic  rocks  consist  of  two  types,  the 
Coastal  and  Alpine.  The  rocks  of  the  coastal  type  contain  thin 
seams  of  bituminous  coal  and  occasional  bands  of  estuarine  sand- 
stones and  shales.  Plant  remains  are  abundant  and  include 
Tceniopteris,  Pecopteris,  and  Sphenopteris.  The  molluscs  are 
mostly  those  found  in  shallow  seas  and  estuaries.  They  include 
Pecten,  Lima,  Avicula,  Ostrea,  Pinna,  Modiola,  Ammonites,  and 
Belemnites. 

The  Jurassic  rocks  of  the  Alpine  type  are  mainly  developed  in 
the  Alpine  Chain,  where  they  attain  a  thickness  exceeding  10,000 
feet.  They  consist  of  a  vast  pile  of  alternating  greywackes  and 
shales  of  the  Flysch  facies,  and,  so  far  as  known,  are  devoid  of  all 
fossils.  They  appear  to  have  been  formed  in  the  delta  of  a  river 
which  may  have  drained  the  southern  portion  of  the  ancient  Gond- 
wana  Land. 

Calcareous  rocks  are  conspicuously  absent  in  the  Jurassic  System, 
as  developed  in  Australia  and  New  Zealand. 

Zonal  Distribution  of  Faunas. — As  the  faunas  become  more 
highly  organised  and  differentiated,  they  are  increasingly  subject 
to  the  influences  of  climatic  conditions  ;  and  the  tendency  of 


MESOZOIC  ERA:    JURASSIC  SYSTEM.  403 

progressive  biological  development  is  to  bring  into  existence  forms 
adapted  to  their  peculiar  environment. 

There  is  not  much  evidence  until  we  reach  the  Jurassic  period, 
that  the  distribution  of  the  marine  inhabitants  was  influenced  by 
climatic  conditions.  But  in  the  faunas  of  the  marine  limestones 
and  marls  of  this  period  there  is  abundant  evidence  that  the 
climatic  or  faunal  zones,  which  are  so  characteristic  of  the  present 
time,  had  already  been  well  established. 

The  detailed  study  of  the  Jurassic  faunas  of  Europe  appears  to 
show  that  they  may  be  divided  into  three  zones  encircling  the  globe 
in  a  direction  parallel  to  the  equator. 

In  geological  times  later  than  the  Jurassic,  the  faunas  in  similar 
climatic  zones  show  a  biological  relationship  in  the  corresponding 
latitudes  in  each  hemisphere. 

Jurassic  Biological  Zones  or  Provinces.— In  Continental  Europe 
where  the  Jurassic  faunas  have  been  studied  more  closely  than 
elsewhere,  Neumayr  has  distinguished  three  Jurassic  marine 
provinces  characterised  by  different  faunas  : — 

(1)  The   Mediterranean  Province,  which   includes   the   deposits 

of  the  Balkan  Peninsula,  Carpathians,  Cevennes,  Italy, 
Spain,  Crimea,  Caucasus,  Asia  Minor,  and  Further  India. 

(2)  The  Middle   European  Province,  which  includes  the  extra- 

Alpine  Jurassic  of  France,  Germany,  the  Jurassic  of 
England,  North- West  Spain,  Portugal,  the  Baltic  Region, 
Japan,  and  California. 

(3)  The  Russian  or  Boreal  Province,  which  includes  the  Jurassic 

rocks  of  Central  and  Northern  Russia,  Nova  Zembla, 
Spitzbergen,  Greenland,  and  Alaska. 

All  three  facies  occupy  broad  belts  or  zones  passing  round  the 
globe  in  the  direction  of  the  parallels  of  latitude.  These  isozoic 
zones  coincide  with  the  climatic  zones : — 

The  Mediterranean  Province  =the  Alpine,  Equatorial,  or  Tropical 
Zone. 

The  Middle  European  Province  =the  Temperate  Zone. 
The  Russian  or  Boreal  Province  =ihe  Arctic  or  Boreal  Zone. 

The  Jurassic  faunas  of  the  Southern  Hemisphere  occur  in  the 
same  isozoic  zones  as  in  the  Northern.  For  example,  the  Jurassic 
faunas  of  South  Australia,  New  Zealand,  Cape  Colony,  Chile, 
Bolivia,  Peru,  and  Argentine,  exhibit  a  striking  resemblance  to 
the  Jurassic  faunas  of  England  and  Swabia. 

The  Equatorial  Zone  is  characterised  by  the  extraordinary 
development  of  Ammonites  of  the  genera  Phylloceras,  Lytoceras, 


404  A  TEXT-BOOK  OF  GEOLOGY. 

and  Simoceras  ;  and  of  the  brachiopods,  Terebratula  diphya  is 
peculiar  to  this  province. 

In  the  Temperate  Zone  Phylloceras  and  Lytoceras  are  not  com- 
mon, while  Harpoceras,  Peltoceras,  Aspidoceras,  and  Oppelia  are 
very  abundant.  Coral  reefs  are  prominent  and  frequently  of  great 
extent  and  thickness. 

Tn  the  Boreal  Zone  the  Ammonite  genera  Harpoceras,  Lytoceras. 
and  Phylloceras  are  entirely  absent,  and  coral  reefs  are  unknown. 
On  the  other  hand,  an  Ammonoid  Cardioccras  and  the  Lamelli- 
branch  Aucella  are  characteristic  and  widespread. 


CHAPTER   XXX. 
CRETACEOUS   SYSTEM. 

THE  Cretaceous  is  the  youngest  of  the  three  great  systems  into 
which  the  Mesozoic  is  divided.  It  received  its  name  in  England 
from  its  most  important  member  the  Chalk,  for  which  the  Latin 
name  is  or  eta. 

The  Cretaceous  System  is  world-wide  in  distribution,  and 
embraces  a  considerable  variety  of  sandy,  clayey,  and  calcareous 
deposits. 

At  the  close  of  the  Jurassic,  the  form  of  the  great  continents 
was  already  clearly  outlined ;  hence  the  Cretaceous  sediments 
were  laid  down  for  the  most  part  in  seas  marginal  to  the  existing 
continents,  or  in  land-locked  estuaries  and  basins  to  which  the  sea 
had  free  access.  As  a  result  of  this  marginal  distribution,  the 
deposits  of  this  period  are  frequently  covered  over  to  a  considerable 
extent  with  the  succeeding  Tertiary  formations,  which  were  also 
laid  down  as  marginal  sheets  mantling  round  the  shores  of  the 
continents  and  larger  islands. 

Rocks. — The  prevailing  rocks  are  sands,  clays,  shales,  and  lime- 
stones, but  the  deposits  frequently  exhibit  considerable  local 
variations  due  to  differences  in  the  conditions  of  deposition.  Even 
the  Chalk,  which  is  so  prominent  and  important  in  England,  North 
France,  Belgium,  Baltic  area,  and  North  America,  is  absentjin 
Central  Germany,  Alps,  Africa,  and  Australia. 

The  lower  members  of  the  system  in  North- West  Europe  are 
frequently  sandy  beds  of  terrestrial  and  estuarine  origin  with 
plant  remains  and  seams  of  coal.  Following  these  come  alternating 
sandy  and  clayey  deposits  of  marine  origin  frequently  containing 
lines  of  septarian  concretions  ;  and  in  their  turn  these  are  succeeded 
by  chalk  and  other  calcareous  beds  which  in  many  regions  close 
the  succession,  but  in  others  are  followed  by  estuarine  and  terrestrial 
deposits  with  coal-seams. 

The  sandy  beds  of  both  hemispheres  are  frequently  dark  green 
in  colour,  due  to  the  presence  of  glauconitic  grains. 

In  both  hemispheres  the  Upper  Cretaceous  seas  advanced  over 
the  low-lying  maritime  lands  of  all  the  continents,  and  this  unpre- 

405 


406         A  TEXT-BOOK  OF  GEOLOGY. 

cedented  transgression  was  so  rapid  and  universal  that  the  Upper 
Cretaceous  sediments  extend  far  beyond  the  limits  of  the  Lower 
Cretaceous,  and  rest  on  the  worn- down  surfaces  of  the  older  forma- 
tions on  which  they  trespass  in  some  regions  for  thousands  of 
square  miles. 

At  the  close  of  the  Cretaceous,  the  long  era  of  quietude  and 
immunity  from  volcanic  disturbance  was  broken  by  the  revival 
of  eruptions  on  a  gigantic  scale  ;  and  since  that  date  volcanic 
activity  of  a  more  or  less  intense  kind  has  been  in  evidence  in 
some  part  of  the  globe  up  to  the  present  day. 

From  the  above  it  would  appear  that  the  Cretaceous  deposits 
were  laid  down  on  a  slowly  sinking  sea-floor  until  the  middle  of 
the  period,  when  a  sudden  invasion  of  the  sea  took  place.  There- 
after, subsidence  continued  in  many  regions  until  the  close  of  the 
Chalk,  when  the  uplift  began  which  eventually  led  to  the  deposition 
of  the  estuarine  and  terrestrial  deposits  of  the  uppermost  beds  of 
the  Cretaceous. 

Distribution. — The  Cretaceous  is  found  in  all  the  great  continents. 
It  is  one  of  the  most  extensively  developed  of  all  the  rock-systems, 
and  in  some  regions  covers  hundreds  of  thousands  of  square  miles 
in  one  continuous  sheet. 

In  Europe  the  Cretaceous  presents  two  distinct  palseontological 
facies,  the  Central  European  and  Mediterranean.  The  Central 
European  is  well  developed  in  England,  North  France,  Belgium, 
Hanover,  Westphalia,  Saxony,  Bohemia,  and  Baltic  area ;  and  the 
Mediterranean  type  on  both  sides  of  that  basin,  in  Portugal, 
Spain,  South  France,  Italy,  Switzerland,  Sicily,  Greece,  Carpathians, 
Morocco,  Algiers,  Tunis,  Egypt,  Syria,  and  Palestine. 

The  Mediterranean  facies  also  stretches  eastwards  into  Asia, 
and  covers  enormous  tracts  in  Asia  Minor,  Persia,  Arabia, 
Afghanistan.  Baluchistan,  Northern  Himalayas,  Tibet,  and  China. 

The  Cretaceous  System  is  also  present  in  Japan,  Australia,  New 
Zealand,  Antarctic  region  near  Graham's  Land,  Patagonia,  Chile, 
Peru,  Bolivia,  United  States,  British  Columbia,  Alaska,  and  North 
Greenland.  In  the  North  American  continent  the  northern  and 
southern  facies  are  as  well  marked  as  in  Europe. 

From  this  biological  relationship  we  know  that  the  Cretaceous 
sea  extended  from  the  Atlantic  eastward  through  the  Mediterranean 
area  to  Asia  Minor,  Persia,  and  India ;  and  during  the  great  trans- 
gression spread  over  the  greater  part  of  the  desert  area  of  North 
Africa.  From  the  North  Atlantic  long  arms  of  the  Cretaceous 
sea  extended  through  Germany  and  Baltic  area  to  Northern 
Russia,  reaching  as  far  as  Spitzbergen. 

In  the  West  Atlantic  a  broad  prolongation  of  the  sea  stretched 
through  the  great  Western  Interior  Basin  of  the  Western  States 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  407 

to  Alaska,  its  waters  laving  the  western  foothills  of  the  Rocky 
Mountains. 

Flora.— The  earliest  Cretaceous  flora  is  closely  related  to  that 
of  the  Jurassic,  and  ferns,  cycads,  and  conifers  are  still  the  dominant 
forms  of  vegetation  ;  but  there  was  a  remarkable  change  impending 
in  the  character  of  the  land  vegetation,  and  in  the  Upper  Creta- 
ceous we  witness  the  world-wide  appearance  of  the  angiosperms, 
both  monocotyledons  and  dicotyledons,  which  represent  the 
highest  forms  of  vegetation  prevailing  at  the  present  time. 

The  advent  of  mammals  and  other  highly  organised  vertebrates 
a  full  geological  period  ahead  of  the  less  delicately  organised 
angiosperms  is  a  biological  puzzle  the  solution  of  which  is  not  very 
obvious. 

Fauna. — Many  of  the  Jurassic  genera  appear  in  the  Cretaceous 
together  with  new  forms,  and  generally  we  may  say  that  the 
fauna  is  stamped  with  a  distinctly  Mesozoic  facies. 

Foraminifera  are  exceedingly  abundant  as  builders  of  chalk 
and  other  limestones,  the  most  common  genera  being  Globigerina 
and  Orbitolina,  the  former  characteristic  of  the  true  chalk  of  North- 
West  Europe,  the  latter  of  the  Alpine  Cretaceous. 

Calcareous  sponges  are  common  in  the  middle  of  the  system, 
and  siliceous  sponges  abound  in  the  Chalk. 

Corals  of  the  reef-building  type  are  rare.  The  few  known  genera 
of  corals  are  such  simple  forms  as  Parasmilia,  Micrabacia.  and 
Trochocyathus,  the  last,  well  known  in  the  Lower  Tertiary. 

Sea-urchins  are  numerous  in  the  Chalk,  the  genera  Micraster, 
Holaster,  Hemiaster,  Echinobrissus,  and  Cidaris  being  common.  A 
few  starfishes  are  known ;  and  crinoids  are  represented  by  the  genus 
Marsupites.  Polyzoans  are  common  in  the  Calcareous  division. 

Brachiopods  are  abundant,  and  include  Terebratula,  Terebratella 
Magas,  and  Rhynchonella — all  living  genera — as  well  as  the  ancient 
Crania. 

Molluscs  are  well  represented.  Among  the  common  Lamelli- 
branchs  are  Inoceramus,  Exogyra,  Ostrea,  Trigonia,  Gervillia, 
and  Spondylus,  as  well  as  the  curious  genus  Hippurites,  which 
forms  massive  beds  of  limestone  in  the  Alpine  facies  of  Southern 
Europe.  Of  these  Inoceramus,  Exogyra,  and  Hippurites  are 
distinctively  Cretaceous,  and  the  last  is  limited  to  this  system. 

The  shell  of  Inoceramus  is  composed  of  aragonite  built  up  of 
fibrous  layers  lying  at  right  angles  to  the  axis  of  the  shell,  which 
is  consequently  very  fragile.  Fragments  of  Inoceramus  are  common 
in  the  Chalk. 

Of  Gasteropods,  the  genera  Pleurotoma,  Aporrhais,  Rostellaria, 
Cerithium,  and  Fusus  are  abundant. 

Cephalopods  swarmed  in  the  Cretaceous  seas,  and  are  chiefly 


408 


A    TEXT-BOOK    OF    GEOLOGY. 


represented  by  Ammonites  and  Belemnites  which  make  their 
last  appearance.  The  Ammonite  genera  are  specially  character- 
ised by  the  free-whorled  (Crioceras),  hooked  (Hamites,  fig.  217),  and 
horn-shaped  and  turreted  ( Turrilites,  fig.  218)  forms,  many  of  which 
possessed  beautifully  ornamented  shells. 

In  the  Upper  Cretaceous  the  place  of  the  true  Belemnites 
is  taken  by  Belemnitella  and  its  sub-genus  Actinocamax.  The 
genus  Nautilus,  the  most  ancient  and  persistent  of  all  Cephalopods, 
is  represented  by  many  species  in  the  Upper  Cretaceous,  some  of 
large  size. 

Fishes  were  common  in  the  Cretaceous  seas  and  rivers,  and  in- 


FIG.  217.— Hamites. 


FIG.  218.— Turrilites. 


eluded  many  Teleosts  (bony  fishes)  which  became  very  plentiful 
towards  the  close  of  the  period. 

The  reptilians  Ichthyosaurus,  Plesiosaurus,  and  Pterosaur  us, 
which  dominated  the  vertebrate  life  of  the  Jurassic,  are  seen  for 
the  last  time  in  the  Cretaceous. 

The  huge  deinosaurs  reach  their  maximum  development  in 
this  period,  and  disappear  at  its  close.  They  are  specially  repre- 
sented by  Iguanodon,  Megalosaurus,  and  Cetiosaurus.  The  gigantic 
pythonomorph  or  sea-serpent  Mosasaurus,  one  of  the  extinct 
monsters  of  the  Cretaceous  seas,  is  estimated  to  have  attained  a 
length  of  75  feet.  The  Cretaceous  rocks  of  the  Western  States 
of  North  America  have  yielded  a  rich  harvest  of  deinosaurs, 
pterosaurs,  crocodilians,  sea-saurians,  turtles,  and  sea-serpents. 


PLATE   XLVIII. 

CHARACTERISTIC  CRETACEOUS  AMMONITES. 

1.  Ammonites    (Schlcenbachia)    rostratus    (Sow.).     Upper    Greensand     and 

Gault,  Devizes  and  Folkestone. 

Group  Cristati.     Fam.  Schloenbachia. 

2.  Ammonites  (Hoplites)  lautus  (Sow.).     Gault  and  Upper  Greensand. 

Group  Tuberculati.     Fam.  Schloznbachia. 

3.  Ammonites  clypeiformis  (Sow.).     Upper  Greensand. 

Group  Clypeiforme.     Fam.? 

4.  Ammonites  (Acanthoceras)  Rottiomagensis.     Chalk  Marl  and  Lower  Chalk. 

Group  Rothomagenses.     Fam,  Stepkanoceratites. 

5.  Ammonites  (Cosmoceras)  Leopoldianus.     Neocomian. 

Group  Flexuosi.     Fam.  Ste.phanoceratites. 

6.  Ammonites  (Hoplites}  interruptus  (Sow.). 

Group  Dentati.     Fam.  Stephanoceratites. 

7.  Ammonites  (Hoplites)  Deshayesii  (Leym.).     Upper  Neocomian. 

Group  Angulicostati.     Fam.? 

8.  Ammonites  catillus  (Sow.).     Upper  Greensand. 

Group  Compressi.     Fam.? 


PLATE   XL VIII. 

C  R  I  ST A  T  I  . 


ROTHOMAGENSES 


To  face  page  408. 
T  U  B  E  R  C  U  LAT  I. 


CLYPEIFORIMI. 


D    E  N  T  AT  I  . 


COMPRESS!- 


ANGULICOSTATI. 


*NE&  ERSKINE.LITH.  EDIN" 

CHARACTERISTIC  CRETACEOUS   AMMONITES. 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM. 


409 


Perhaps  no  less  remarkable  than  the  deinosaurs  are  the  toothed 
birds  of  the  Cretaceous  of  Kansas,  among  the  most  interesting 
of  which  is  Ichthyornis  victor.  The  only  bird  remains  found  in 
the  English  Cretaceous  are  those  of  the  genus  Enaliornis. 

With  the  deinosaurs,  crocodiles,  and  other  reptilians  found  in 
the  Upper  Cretaceous  rocks  of  Dakota  and  Wyoming,  there  have 
been  found  numerous  jaws  and  teeth  of  small  marsupial  mammals 
related  to  the  Jurassic  and  Triassic  forms. 

Subdivisions, — -The  subdivisions  of  the  Cretaceous  System  in 
England  where  the  succession  was  first  accurately  determined, 
with  the  names  of  the  corresponding  subdivisions  in  North  France, 


32  i  23 

FIG.  219. — General  section  across  Wealden. 
1,  Hastings  Sand.  4,  Gault. 


2.  Weald  Clay. 

3,  Lower  Greensand. 


5,  Upper  Greensand  and  Chalk. 


which  are  now  commonly  used  as  stage  or  time-names  for  the 
different  groups  of  beds,  are  as  follow  : — 


Upper 

Cretaceous  " 

5. 

4. 
3. 

England. 
Absent,    . 
f  Upper  Chalk,  . 
Chalk      J  Middle  Chalk,  . 
I^Lower  Chalk 
Upper  Greensand,     . 
Gault,        . 

France. 
Danian. 
Senonian. 
Turonian. 

>Cenomanian 
Albian. 

Lower 
Cretaceous 

p. 
I1- 

Lower  Greensand, 
Wealden/^eald  Clay 
^Hastings  Sand 

Aptian. 
>Neocomian. 

The  Cretaceous  rocks  of  England  occur  in  three  distinct  areas, 
namely,  the  Southern,  Middle,  and  Northern.  In  the  Southern 
District  the  deposits  are  mainly  freshwater  ;  in  the  Middle  District 
where  the  lower  members  are  absent,  marine  ;  and  in  the  Northern 
District,  marine. 

In  the  Southern  District  they  occupy  almost  the  whole  of  the 
central  portion  of  the  Wealden  Dome  lying  between  the  North 
and  South  Downs.  They  also  appear  in  the  Isle  of  Wight  where 
they  are  tilted  at  high  angles  (fig.  219A),  in  the  Isle  of  Purbeck, 
and  near  Weymouth,  but  these  outcrops  are  subordinate  in  extent 


410  A  TEXT-BOOK  OF  GEOLOGY. 

to  that  in  the  Weald,  which  is  the  largest  and  most  important 
development  of  the  Cretaceous  in  England  or  in  the  British  Isles. 

The  Cretaceous  rocks  of  the  Middle  District  are  chiefly  developed 
in  Bedfordshire  and  Cambridgeshire. 

The  Northern  District  extends  from  North  Norfolk  to  Flam- 
borough  Head,  and  in  this  area  the  Cretaceous  System  is  best 
displayed  in  Lincolnshire  and  Yorkshire. 

The  Cretaceous  rocks  fall  into  two  great  natural  divisions :  the 
Lower  Cretaceous,  well  displayed  in  the  Southern  and  Northern 
Districts ;  and  the  Upper  Cretaceous,  found  in  each  of  the  three 
geographical  areas  referred  to  above. 

Lower  Cretaceous. 

SOUTHERN  DISTRICT. 

The  Lower  Cretaceous  of  the  south,  which  is  freshwater,  is  quite 
unlike  the  Lower  Cretaceous  of  the  north,  which  is  marine  and 
palseontologically  shows  a  relationship  to  the  Lower  Cretaceous 
of  the  Baltic  area. 

While  freshwater  basins  existed  in  the  south  of  England,  and  the 
Middle  District  formed  dry  land,  a  sea  existed  in  the  Northern 
District.  But  when  subsidence  took  place,  the  southern  sea 
invaded  the  freshwater  basins,  and  encroached  on  the  Middle 
District.  Thereafter  there  was  a  continuous  sea  from  south  to 
north,  and  the  fauna  of  the  southern  sea  spread  northward  until 
it  reached  the  Northern  District. 

Wealden  (Neocomian). — This  is  a  freshwater  series  which  derives 
its  name  from  the  Weald  of  Sussex,  Surrey,  and  Kent,  where  it  is 
typically  developed.  It  is  overlain  conformably  by  the  Lower 
Greensand. 

The  Wealden  comprises  two  main  groups,  namely  : — 

2.  Weald  Clay. 
1.  Hastings  Sand. 

The  total  thickness  of  the  Wealden  Series  is  over  2000  feet,  of 
which  the  Weald  Clay  comprises  about  1000  feet.  The  conditions 
of  deposition  were  deltaic,  and  the  sediments  were  apparently  laid 
down  during  a  period  of  slow  but  progressive  subsidence. 

The  Wealden  flora  includes  ferns,  cycads,  and  conifers.  Among 
the  ferns  are  Sphenopteris  and  Alethopteris .  The  molluscs  include 
the  freshwater  forms  Unio  valdensis  (Plate  XLIX.,  fig.  1),  Cyrena 
media  (Plate  XLIX.,  fig.  2),  Viviparus  fluviorum,  and  a  few  littoral 
shells,  including  Mytilus,  Exogyra,  and  Ostrea. 

Among  the  fish  we  have   Lepidotus  Mantelli,  a  ganoid  related 


.XI 


KaciJA:- 


: 
• 


PLATE   XLIX. 

PURBECK  AND  WEALDEN  FOSSILS. 

1.  Unio  valdensis  (Mant.).     Hastings  Sand,  etc.     Wealden,  Isle 

Hastings. 

2.  Cyrena  media  (Sow.).     Weald  Clay.     Kent,  Surrey,  Sussex. 

3.  Cypridea    valdensis    (Sow.).     Weald    Clay.     Isle    of    Wight,    Tunbridge 

Wells,  Dorset. 

4.  Vicarya  lujani  (Du  Verneuil).     Punfield  beds  and  Upper  Neocomian. 

Isle  of  Wight  and  Punfield. 

5.  Vertical  section  of  same,  showing  ridges. 

6.  Cypridea  tuberculata  (Sow.).     Upper  Purbeck  beds. 

7.  Cypridea  fasciculata.     Middle  Purbeck  beds. 

8.  Modiola  Fittoni.     Purbeck  beds. 

9.  Archceoniscus  Brodiei  (M.  Edw.).     Purbeck.     Vale  of  Wardour. 
10.  Cidaris  Purbeckensis  (Forbes).     Middle  Purbeck  (Cinder  bed). 


To  face  page  410.] 


[PLATE    XLIX. 


PURBECK    AND    WEALDEN    FOSSILS. 


[PLATE  L. 


CRETACEOUS  FOSSILS. 
(Neocomian.) 


PLATE   L. 

CRETACEOUS   FOSSILS. 
(Neocomian.) 

1.  Exogyra  sinuata  (Sow.).     Upper  Neocomian.     Kent,   Sussex,   Speeton, 

etc. 

2.  Perna  mulleti  (Desh.).     Upper  Neocomian.     Kent,  Isle  of  Wight,  etc. 
2a.  .Do.  Hinge-line  showing  vertical  dentition. 

3.  Pecten  cinctus  (Sow.).     Middle  Neocomian.     Lincolnshire. 

4.  Crioceras  (Ancyloceras)  Duvallii  (Leveille).     Upper  Neocomian.     Speeton, 

Yorkshire. 

5.  Ancyloceras  gigas  (Sow.).     Upper  Neocomian.     Isle  of  Wight,  Sandgate, 

etc. 

6.  Nautilus  plicatus  (Fitton).     Upper  Neocomian.     Kent,  Isle  of  Wight,  etc. 

7.  Ammonites  (Hoplites)  noricus.     Lower  Neocomian.     Speeton. 

8.  Ammonites  Deshayesii  (Leym.).     Upper  Neocomian.     Isle  of  Wight,  etc. 


(uoJii1 
:»  >K  i-, 

OJ         :., 


To  face  page  411.] 


[PLATE    LI. 


CRETACEOUS  FOSSILS. 
( Various. ) 


PLATE  LI. 

CRETACEOUS  FOSSILS. 
(Various.) 

1.  Ananchytes  ovatus  (Leske).     Upper  Chalk.     Kent,  Sussex,  Surrey,  Isle 

of  Wight,  Wilts,  etc. 

2.  Micraster  cor-anguinum  (Leske).     Upper  Chalk.     Kent,  Surrey,  Sussex, 

Norfolk,  Wilts,  etc. 

3.  Bourgueticrinus  ellipticus  (Miller).     Portion  of  stem  and  calyx.     Upper 

Chalk.     Norfolk,  Kent. 

4.  Rhynchonella  octoplicata  (Sow.).     Upper  Chalk.     Norfolk,  Kent,  Sussex, 

Wilts. 

5.  Terebratulina  striata  (Wahl.).     Upper  Chalk. 

6.  Crania   parisiensis   (Defr.).     Upper   Chalk.     Kent,    Norfolk,    Brighton, 

etc. 

7.  Marsupites  ornatus  (Miller).     Upper  Chalk.     Lewes,  Basingstoke,  Bland- 

ford,  Brighton. 

8.  Plicatula  placunea  (Lam.).     Upper  Neocomian. 

9.  Pecten  (Janira)  quinquecostatus  (Sow.).     Lower  Chalk,  Upper  Greensand, 

Gault,  Neocomian. 

10.  Nucula  pectinata  (Sow.).     Gault.     Folkestone,  Cambridge,  etc. 

11.  Exogyra  sinuata  (Sow.).     Neocomian.     Kent,  Sussex, 

12.  Hamites,  sp.     Gault.     Folkestone.  • 

13.  Balemnitella  mucronata  (Schloth.).     Upper  Chalk.    Norfolk,  Kent,  Sussex, 

Cambridge. 

14.  Baculites  Faujasii  (Stow.).     Upper  Chalk.     Norwich,  Sussex,  etc. 

15.  Ptychoceras  adpressum  (Sow.).     Gault.     Folkestone. 


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MESOZOIC    ERA  :     CEETACEOUS    SYSTEM.  411 

to  the  gar-pike  of  the  North  American  rivers.  Reptilians  are 
abundant  and  represented  by  plesiosaurs,  deinosaurs,  and  flying 
pterodactyls.  Among  the  deinosaurs,  the  gigantic  Iguanodon 
was  common. 

The  local  subdivisions  of  the  Wealden  are  as  follow  : — 

f  2.  Weald  Clay  ] 

Wealden   !  Cc.  Tunbridge  Wells  Sand   I     Neo- 

(Deltaic)    j  <  6.  Wadhurst  Clay  f  comian. 

[1.  Hastings  Sand  [a.  Ashdown  Sand  J 

Lower  Greensand. — The  progressive  subsidence  of  the  Neo- 
comian,  which  affected  the  whole  of  North- West  Europe,  enabled 
the  sea  to  encroach  on  the  Wealden  Delta  where  the  marine  sedi- 
ments of  the  Lower  Greensand  were  laid  down  conformably  follow- 
ing the  Weald  Clay. 

The  stages  of  the  Lower  Greensand  are  as  follow  : — 

f  Folkestone  Beds  "] 

T    !  Sandgate  Beds 
Lower  Greensand  •{  jj  ^    ge  -.  f  Mainly  Aptian. 

[  Atherfield  Beds    j 

The  rocks  of  this  series  consist  mainly  of  grey,  yellow,  and  green 
sands  intercalated  with  beds  of  clay,  limestone,  and  ironstone. 
The  green-coloured  sands,  from  which  this  division  derives  its 
name,  owe  their  prevailing  green  hue  to  the  presence  of  glauconitic 
grains. 

Some  of  the  calcareous  bands,  notably  those  in  the  Hythe  stage, 
pass  into  more  or  less  compact  limestones,  such  as  that  locally  called 
Kentish  Rag,  which  is  extensively  used  as  a  building- stone  and  for 
burning  into  lime. 

The  Lower  Greensand  contains  a  large  assemblage  of  molluscs, 
among  which  littoral  shells  are  conspicuous.  The  most  common 
forms  are  Ostrea,  Exogyra,  Perna,  and  Area,  with  which  are  associ- 
ated many  Ammonites  and  Belemnites. 

Among  the  characteristic  species  are  Terebratula  sella,  Exogyra 
sinuata  (Plate  L.,  fig.  1),  Pcrna  mulleti  (Plate  L.,  fig.  2),  Gervillia 
sublanceolata,  and  Ammonites  Deshayesii  (Plate  L.,  fig.  8). 

NORTHERN  DISTRICT. 

The  Lower  Cretaceous  of  Lincolnshire  and  Yorkshire  is  wholly 
marine,  and  shows  a  palseontological  relationship  to  the  Cretaceous 
of  the  Baltic  area  ;  and  many  of  the  species  of  molluscs,  although 
unknown  in  the  South  of  England  or  in  North  France,  are 
common  in  Northern  Russia. 


412  A  TEXT-BOOK  OF  GEOLOGY. 

The  deposits  are  mainly  dark-coloured  clays  and  shales  which 
follow  the  Jurassic  with  no  appearance  of  a  stratigraphical  break. 
Since  they  are  marine  and  contain  a  different  fauna,  these  beds 
cannot  be  correlated  stage  by  stage  with  the  deltaic  series  in  the 
south  of  England. 

The  Speeton  Clay,  which  is  so  well  displayed  in  the  neighbour- 
hood of  Speeton,  north  of  Flamborough  Head  in  Yorkshire,  is  the 
most  important  division  of  the  northern  Lower  Cretaceous,  and 
may  be  regarded  as  typical  of  the  whole  series  of  which  it  forms  the 
major  part.  It  contains  a  prolific  molluscous  fauna  dominated 
by  Belemnites  and  Ammonites  ;  but  in  one  thin  band  the  character- 
istic sea-urchin  E chinos patagus  cordiformis  is  fairly  common. 

Palseontologically  the  series  has  been  divided  by  Lamplugh  into 
four  Belemnite  zones  : — 

4.  Zone  of  Belemnites  minimus  (base  of  Gault). 

3.         ,,  „  hrunsvicensis. 

2.         „  „          jaculum. 

1.         „  ,,  lateralis  (passage-bed). 

0.  Coprolite  Bed. 

The  Coprolite  Bed  is  a  seam  of  phosphatic  nodules  about  four 
inches  thick,  which  appears  to  rest  quite  conformably  on  the 
Upper  Kimeridgian  with  Belemnites  Oweni.  The  coprolitic * 
character  of  the  bed  might,  however,  be  taken  to  indicate  a  short 
cessation  of  deposition  before  the  deposition  of  the  marine  clays 
commenced. 

Upper  Cretaceous. 

A^  a  result  of  the  great  Cenomanian  transgression  of  the  sea 
the  Upper  Cretaceous  was  deposited  over  a  wider  and  more 
uniform  sea  than  the  Lower  Cretaceous  ;  hence  it  extends  far 
beyond  the  limits  of  that  division.  In  some  regions  the  overlap 
is  so  great  that  the  Lower  and  Upper  Cretaceous  might  very  well 
be  regarded  as  two  distinct  systems. 

Lithologically  the  Upper  Cretaceous  is  divided  into  three  distinct 
stages,  namely  an  argillaceous  stage  at  the  base  =  the  Gault;  a 
sandy  stage  in  the  middle  =the  Upper  Greensand  ;  and  a  calcareous 
stage  at  the  top  =the  Chalk. 

In  England,  the  Upper  Cretaceous  is  well  developed  in  the 
Southern,  Middle,  and  Northern  Districts  ;  and  in  each  district 
the  various  divisions  exhibit  a  remarkable  uniformity  of  character, 
except  the  chalk,  which  in  the  Northern  District  is  thinner  than  in 
the  South,  and  not  argillaceous  at  its  base. 

,  and  lithos  =  Si  stone.. 


To  face  page  413.] 


[PLATE    LI  I. 


CRETACEOUS  FOSSILS. 
( Upper  Green-sand  and  Chalk. ) 


PLATE   LII. 

CRETACEOUS  FOSSILS. 

(Upper  Qreensand  and  Chalk.) 

1.  Scaphites  cequalis  (Sow.).     Lower  Chalk,  Chalk  Marl.     Lewes,  Evershot, 

Chardstoek. 

2.  Ammonites    (Acanthoceras)    Rhothomagensis    (Brongn.).     Lower    Chalk. 

Sussex,  Hampshire,  etc. 

3.  Turrilites  costatus  (Lam.).    Lower  Chalk.    Hamsey,  Folkestone,  Compton, 

Norwich. 

4.  Inoceramus  Guvieri  (Sow.).     Upper  and  Lower  Chalk.     Lewes,  Royston, 

Petersfield,  etc. 

5.  Pecten  Beaveri  (Sow.).     Lower  Chalk.     Kent,  Wilts,  Sussex,  Norfolk,  etc. 

6.  Lima  Hoperi  (Sow.).     Upper  Chalk.     Norwich,  Lewes,  Surrey,  Kent,  etc. 

7.  Ostrea  vesicularis  (Lam.).     Upper  Chalk.     Kent,  Sussex,  Norfolk. 

8.  Lima  spinosa  (Sow.).     Lower  and  Upper  Chalk.     Norfolk,  Sussex,  Kent. 

9.  Pecten  (Janira)  quinquecostatus  (Sow.).     Lower  Chalk,  Gault,  and  Neo- 

comian — passim. 

10.  Terebrirostra  lyra  (Sow.).     Chloritic  Sand  and  Upper  Greensand.     War- 

minster. 

11.  Terebratula  biplicata  (Sow.).     Upper  Greensand  (Cambridge  Greensand). 

Cambridge,  Warminster,  etc. 


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MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  413 

("Upper  Chalk — Senonian. 
3.  ChalW  Middle  Chalk— Turonian. 


Upper 
Cretaceous 


("Upper  Chalk — Senonian. 
:<{  Middle  Chalk— Turonian. 
I  Lower  Chalk     Cenomanian. 


2.  Upper  Greensand, 
1.  Gault, 


The  Gault.— The  Gault  is  dominated  by  the  argillaceous 
facies  of  sediments.  Lithologically  it  consists  of  stiff,  dark  blue, 
marine  clay,  in  places  sandy  and  marly,  with  lines  of  pyritic  and 
phosphatic  nodules.  Its  thickness  varies  from  100  to  300  feet,  and 
in  many  places  it  overlaps  the  Lower  Cretaceous. 

The  Gault  contains  many  beautifully  preserved  fossils,  large 
numbers  of  which  may  be  seen  at  low  tide  at  Copt  Point,  on  the 
coast  near  Folkestone,  where  the  Chalk  rests  directly  on  the  Gault. 
Ammonites  are  plentiful,  and  among  other  molluscs  are  Aporrhais, 
Pleurotoma,  Cerithium,  Fusus,  Natica,  Dentalium,  Corbula,  Pinna, 
Cucullcea,  Mytilus,  Ostrea,  Pecten,  Inoceramus,  Cyprina,  and 
Pholas,  the  last  seven  being  commonest  in  the  higher  beds. 

A  small  Belemnite,  Belemnites  minimus,  is  very  abundant  and 
Characteristic,  as  also  are  Terebratula  biplicata  (Plate  LIL,  fig.  11), 
Inoceramus  sulcatus,  I.  concentricus,  Ammonites  interruptus,  and 
A.  rostratus,  all  of  which  are  present  in  the  contemporaneous 
Red  Chalk  of  Yorkshire. 

Upper  Greensand. — This  division  is  dominated  by  sandy  beds, 
but  there  is  no  sharp  line  of  demarcation  between  it  and  the 
Gault.  The  prevailing  colour  is  dark  green,  due  to  the  presence 
of  glauconitic  grains.  In  places  where  the  glauconite  has 
become  oxidised,  the  sands  assume  a  yellow,  yellowish-brown, 
or  red  colour. 

Nearly  half  the  molluscs  of  the  Gault  pass  up  into  the  Upper 
Greensand,  which  is  now  known  to  be  the  local  equivalent  of 
different  horizons  of  the  Chalk  series. 

Palseontologically  the  Upper  Greensand  is  divided  into  two 
well-marked  zones — 

2.  Zone  of  Pecten  as  per. 

1.         „        Ammonites  rostratus. 

The  Lower  Zone  contains,  among  characteristic  species,  Venus 
submersa,  Area  glabra,  Pecten  quinquecostatus  (Plate  LIL,  fig.  9), 
Ammonites  rostratus,  and  Hamites  alternatus',  and  the  Upper 
Zone,  Terebratula  biplicata,  Pecten  asper,  and  Ammonites  varians. 

In  the  Central  District  the  Gault  and  Upper  Greensand  (Albian) 
possess  the  same  physical  characteristics  and  fossils  as  in  the  south 
of  England,  but  going  northward  the  sandy  beds  comprising  the 


414  A  TEXT-BOOK  OF  GEOLOGY. 

Upper  Greensand  are  gradually  replaced  by  clay,  and  in  Bedford- 
shire finally  disappear,  so  that  north  of  this  the  Upper  Greensand 
is  no  longer  recognisable  as  a  separate  member  of  the  Upper 
Cretaceous. 

In  the  Northern  District  the  Albian  (Gault  +  Upper  Greensand), 
now  mainly  represented  by  clay,  thins  out  and  gradually  passes 
into  a  bed  of  red  chalk  which  is  well  seen  in  the  sea-cliffs  of 
Hunstanton  in  Norfolk,  where  it  is  about  three  feet  thick  and  con- 
tains the  characteristic  fossils  of  the  Albian  of  the  south  of  England. 

Going  northwards,  the  Red  Chalk  expands  to  ten  or  twelve  feet, 
and  in  the  neighbourhood  of  Speeton  still  further  thickens  and 
passes  into  beds  of  reddish-coloured  marls  and  clays  with  irregular 
seams  of  red  chalky  marl. 

The  Chalk. — The  Chalk  is  the  most  conspicuous  of  the  Upper 
Mesozoic  formations  of  North- West  Europe.  It  is  a  soft  earthy 
limestone  mainly  composed  of  the  shells  of  foraminifera  among 
which  the  genus  Globigerina  predominates.  At  its  base  it  becomes 
argillaceous,  forming  what  is  called  Chalk  Marl ;  and  in  some 
places  it  contains  grains  of  glauconite. 

Nodules  of  flint  arranged  in  lines  parallel  to  the  original  planes 
of  deposition  are  scattered  throughout  the  Chalk  and  are  particu- 
larly prevalent  in  the  Upper  Chalk,  which  has  for  that  reason  been 
called  White  Chalk  with  flints.  The  Lower  Chalk  has  been  called 
the  White  Chalk  without  flints ;  but  this  basis  of  subdivision  is  not 
satisfactory,  since  the  lower  part  of  the  Chalk  frequently  contains 
flints,  and  the  upper  part  in  some  cases  does  not. 

The  Chalk  Series  is  divided,  on  palseontological  grounds,  into 
three  distinct  stages,  namely  : — 


/Upper  Chalk  — Senonian. 
Middl 


Chalk  Series^  Middle  Chalk— Turonian. 

I^Lower  Chalk — Cenomanian. 

Conditions  of  Deposition. — The  Chalk  is  mainly  composed  of 
foraminiferal  ooze,  a  kind  of  deposit  which  at  the  present  day  is 
always  associated  with  deep  oceanic  waters.  The  geographical 
position  of  the  Chalk  of  North- West  Europe  and  North  America, 
and  the  presence  in  it  of  sandy  beds  as  well  as  a  mixed  molluscous 
fauna,  including  Terebratulina,  Rhynchonella,  Pecten,  Ammonites, 
Belemnitella,  and  other  genera,  besides  numerous  sea-urchins  and 
the  crinoid  Marsupites,  would  seem  to  indicate  that  the  original 
calcareous  sediments  were  laid  down  in  clear  but  comparatively 
shallow  waters  such  as  now  exist  in  the  fiords  of  Norway  and  New 
Zealand. 

The  palseontological  zones  into  which  the  Upper  Cretaceous 
is  divided  are  as  follow  : — 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM. 


415 


Upper  Chalk 


Zone  of  Ostrea  lunata 


Middle  Chalk 


Lower  Chalk 


{ 

•{ 
f 


Upper 

Greensand       ^ 
and  Gault        | 


l 


Belemnitella  mucronata 
Actinocamax  quadrat  us 
Marsupites  testudinarius 
Micraster  cor-anguinum 

„         cor-testudinarium 
Holaster  planus 

Terebratulina  lata 
Rhynchonella  Cuvieri 

Holaster  subglobosus 

Ammonites  varians 

Pecten  asper 

Ammonites  rostratus 
„  lautus 

„  interruptus 

mammillatus 


Senonian. 


Turonian. 


>Cenomaniau, 


Albian. 


Climate. — The  character  of  the  land  vegetation,  reptilians,  and 
marine  mollusca  would  seem  to  indicate  the  prevalence  of  a  semi- 
tropical  to  tropical  climate  and  warm  seas  such  as  may  now  be 
found  on  the  coasts  of  West  Africa  and  Malaysia. 

Scotland. 

Cretaceous  rocks  occur  in  the  Isle  of  Mull  and  on  the  margin 
of  the  neighbouring  Morvern  Peninsula.  In  these  areas  they  owe 
their  preservation  to  the  covering  of  Tertiary  basalts.  The  rocks 
belong  to  the  Upper  Cretaceous,  and  contain  evidence  of  deposition 
on  the  shores  of  an  estuary,  or  landlocked  inlet  of  the  sea. 

Ireland. 

Upper  Cretaceous  rocks  appear  round  the  borders  of  the  Antrim 
plateau,  and,  as  in  West  Scotland,  owe  their  preservation  to  the 
covering  plateau  of  basalts.  They  rest  unconformably  on  Jurassic 
and  older  rocks,  and  bear  witness  to  the  wide-spread  character 
of  the  great  Cenomanian  transgression. 

Cretaceous  of  other  Countries. 

North  France  and  Belgium. — The  Cretaceous  rocks  of  North 
France  and  Belgium  are  lithologically  and  palseontologically 
closely  related  to  the  Cretaceous  of  England  of  which  they  are 
obviously  the  eastern  extension  laid  down  in  a  prolongation  of 
the  same  sea. 


416  A  TEXT-BOOK  OF  GEOLOGY. 

The  subdivisions  recognised  in  this  region  are  :— 

(8.  Danian. 
7.  Senonian. 
6.  Turonian. 
5.  Cenomanian. 

[4.  Albian. 

Lower  Cretaceous  j  |  u^an. 
[1.  Neocomian. 

The  characteristic  fossils  of  the  corresponding  English  sub- 
divisions are  well  represented  in  these  stages. 

Danian. — The  Danian  stage,  so  called  from  its  typical  develop- 
ment in  East  Denmark,  seems  to  bridge  the  hiatus  between  the 
Senonian  and  the  Landenian  (or  lowermost  Eocene)  as  developed 
in  England.  It  comprises  both  marine  and  freshwater  sediments  ; 
and  its  fauna,  while  mainly  Cretaceous,  contains  many  Cainozoic 
types. 

Rocks  of  Danian  age  are  welljdeveloped  in  the  northern  Creta- 
ceous basin  of  Western  Europe,  where  they  consist  chiefly  of  grey 
and  yellowish-coloured  chalk  and  chalky  marls  that  usually  rest 
on  an  eroded  surface  of  the  underlying  Senonian  chalk. 

The  so-called  Pisolitic  Limestone  of  French  geologists  occurs  in 
isolated  patches  in  the  neighbourhood  of  Paris,  and  in  the  depart- 
ment of  Oise  and  Marne,  and  rests  unconformably  in  different  parts 
of  the  Cretaceous  series,  forming  passage-beds  into  the  Tertiary 
formations.  The  lowermost  of  these  deposits  is  a  hard,  coarse- 
grained limestone  containing  the  characteristic  species  Neithea 
quadricostata  and  Nautilus  hebertinus.  The  concretionary  lime- 
stone of  the  upper  division,  representing  the  Montian  sub-stage 
of  the  Cretaceous  system,  has  yielded  among  many  fossil  molluscs, 
Pleurotoma  penultima,  Neithea  quadricostata,  Lima  tacta,  and  the 
very  characteristic  Danian  cephalopod,  Nautilus  danicus. 

At  Mons,  in  South  Belgium,  the  calcareous  beds  of  Danian  age 
underlying  the  town  have  been  proved  by  boring  to  be  300  feet 
thick.  The  Mons  Chalk  is  mainly  composed  of  Cretaceous  fora- 
minifera  and  calcareous  algae,  and  with  these  are  associated  many 
Tertiary  genera,  including  Triton,  Fusus,  as  well  as  a  few  fresh- 
water forms,  such  as  Pupa,  Physa,  and  Bithinia. 

The  Maestricht  Chalk  in  Holland  contains  a  rich  fauna,  which 
includes  Nautilus  danicus,  Baculites  Faujasi,  Belemnitella  mucro- 
nata,  Ostrea  vesicularis,  Cidaris  Faujasi,  Micraster  tercensis,  some 
hippurites,  many  fish  remains,  and  numerous  bones  of  Mosasaurus 
camperi.  the  last  of  the  great  Cretaceous  mosasaurids. 


To  face  page,  417. 


[PLATE    LIII. 


CRETACEOUS  FOSSILS. 
(Chalk.} 


PLATE  LIII. 

CRETACEOUS  FOSSILS. 

1.  Hippurites  organisans.     Chalk  of  France. 

2.  Hippurites  bioculata.  „  „ 

3.  Spkerulites  ventricosa.  „  „ 

4.  Ostrea  carinata  (Frons.).     Chalk,  Marl,  etc. 

5.  Head  (Upper  and  Lower  Jaw)  of  Mosasaurus.     From  the  Upper  Chalk  of 

Maastricht. 


lo 


. 


MESOZOIC  ERA  :    CRETACEOUS  SYSTEM.  417 

The  Faxoe  Chalk,  which  forms  the  lower  division  of  the  Danian 
in  East  Denmark,  is  a  hard  yellow  limestone  full  of  bryozoa,  with 
Nautilus  danicus  and  numerous  echinoderms,  the  latter  including 
the  genera  Holaster,  Tremnocidaris,  and  Dorocidaris.  The  Saltholm, 
or  upper  division,  which  is  a  chalk  with  flints,  has  been  proved  by 
boring  to  occupy  a  wide  tract  around  Copenhagen  under  the  glacial 
drift.  It  contains  an  abundant  fauna  in  which  Nautilus  danicus, 
Baculites  Faujasi,  Belemnitella  mucronata,  Ostrea  vesicularis,  and 
Terebratula  carnea  are  conspicuous.  Similar  strata  and  fossils 
occur  in  the  south  of  Sweden. 

The  Danian  is  also  strongly  developed  in  the  south  of  France, 
where  it  is  represented  by  marly,  chloritic,  and  compact  lime- 
stones over  600  feet  thick,  with  an  abundant  fauna,  which  in- 
cludes such  distinctive  forms  as  Nautilus  danicus  and  Micraster 
tercensis. 

The  presence  of  Hippurites  and  large  echinoderms  in  the  Danian 
of  Denmark  and  Sweden  indicates  the  prevalence  of  a  compara- 
tively warm  climate  in  the  Baltic  zone  just  prior  to  the  advent 
of  the  Eocene. 

The  Danian  stage  appears  to  have  no  representative  in  England, 
unless  the  uppermost  Cretaceous  beds  which  appear  on  the  Norfolk 
coast,  at  Trimmingham,  near  Cromer,  are  the  equivalents  of  the 
lowest  Danian  of  the  continental  area. 

A  very  complete  succession  of  Cretaceous  strata  occurs  in  Persia, 
India,  Japan,  and  United  States,  including  representatives  of  the 
Senonian  and  lower  Danian. 

The  highest  division  of  the  Upper  Cretaceous  on  the  east  coast 
of  Southern  India,  from  Pondicherri  to  Trichinopoli,  contains  the 
well-known  Danian  fossil  Nautilus  danicus,  which  has  also  been 
identified  in  the  Upper  Cretaceous  of  Persia. 

The  great  freshwater  Laramie  formation,  which  forms  the  chief 
Lignitic  series  of  North  Utah  and  Wyoming,  reaches  from  the  Sen- 
onian to  the  Danian,  and  is  separated  by  a  strong  unconformity 
from  the  lowermost  Eocene.  It  is  believed  by  some  writers  to  form 
a  passage-bed  leading  up  to  the  Tertiary  formations. 

No  strata  of  Danian  age  have  been  recognised  in  Australia  or 
New  Zealand.  Wherever  Cretaceous  and  Eocene  formations  are 
present  in  these  regions,  the  palseontological  break  is  always 
sharply  defined,  even  in  places  where  the  stratigraphical  discord- 
ance is  absurdly  insignificant. 

It  should  be  noted  that  Ammonites,  Belemnites,  and  Turrilites 
disappear  before  the  Danian  stage  is  reached. 

Germany. — The  Cretaceous  rocks  of  Germany  and  the  Baltic 
area  were  laid  down  in  prolongations  of  the  same  sea  as  the 
English  Cretaceous  of  the  Northern  District,  and  consequently 

27 


418  A  TEXT-BOOK  OF  GEOLOGY. 

present  the  same  palseontological  succession,  and  to  some  extent 
the  same  lithological  features. 

In  Germany,  the  Cretaceous  System  is  well  developed  in  Bohemia, 
Saxony,  Hanover,  and  Westphalia.  The  soft  chalk  of  North  France 
when  traced  eastward  into  Westphalia  passes  into  sands,  soft 
sandstones,  and  marly  beds,  which  expand  to  an  enormous  thickness 
in  the  gorge  of  the  Elbe. 

The  Upper  Cretaceous  terrestrial  beds  of  Aix-la-Chapelle  contain 
an  abundant  flora,  comprising  many  monocotyledons  and  dicoty- 
ledons, some  of  which  show  a  curious  resemblance  to  forms  found 
in  the  Upper  Cretaceous  beds  of  Northern  Greenland. 

Russia. — Cretaceous  rocks  cover  an  extensive  tract  in  the  valleys 
of  the  Dneister,  Don,  and  Volga,  and  generally  bear  a  relationship 
to  the  Cretaceous  of  North- West  Europe. 

Mediterranean  Basin. — There  is  a  great  development  of  the 
southern  facies  of  the  Cretaceous  in  the  regions  abutting  on  the 
Mediterranean  Basin,  notably  in  Portugal,  Spain,  South  France, 
Sicily,  Italy,  Switzerland,  the  Carpathians,  Greece,  and  Asia  Minor. 
Also  in  Morocco,  Algiers,  Tunis,  and  Egypt  they  cover  a  vast  area 
which  extends  almost  to  the  southern  limits  of  the  Sahara  Desert. 

Palseontologically,  the  Mediterranean  facies  is  characterised 
by  the  extraordinary  prevalence  of  the  peculiar  Lamellibranch 
Hippurites,  which  is  a  cone-shaped  shell  provided  with  a  lid.  The 
Hippurites  lived  in  banks  in  shallow  water,  and  grew  in  such 
numbers  as  to  compose  thick  beds  of  limestone.  Ammonites 
attain  a  large  size  and  belong  to  the  sub-genera  with  free-whorled, 
hooked,  and  highly  ornamented  shells.  Among  these,  Buchiceras 
is  widely  spread  and  characteristic. 

A  remarkable  and  interesting  feature  of  the  Cretaceous  as 
developed  in  the  Alps  is  a  vast  pile  of  sandstones  and  shales 
commonly  known  to  Continental  geologists  as  the  Flysch  or  Vienna 
Sandstone.  This  rock  formation  extends  from  south-west  Switzer- 
land through  the  northern  Alps  to  Vienna.  It  is  conspicuously 
unfossiliferous,  with  perhaps  the  exception  of  some  fucoid-like 
markings  which  afford  no  evidence  of  its  age.  The  lower  portions 
of  this  great  accumulation  of  fluviatile  deposits  are  known  to  be 
Cretaceous  from  the  presence  of  fragments  of  Inoceramus  and 
intercalated  beds  of  limestone  which  contains  Neocomian  fossils. 
The  upper  portion  may  be  Eocene  or  even  later  date,  but  this  is 
not  certain. 

The  lithological  character  of  this  mass  of  unfossiliferous  rocks  is 
so  distinctive  that  the  name  Flysch  is  now  recognised  as  a  descriptive 
term  for  all  such  accumulations  of  similar  unfossiliferous  strata, 
regardless  of  their  age. 

India. — The  Cretaceous  System  is  represented  in  India  by  a 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  419 

great  assemblage  of  marine  and  fluviatile  deposits  occurring  both 
in  the  Peninsular  area  and  the  Himalayan.  The  rocks  are  largely 
limestones  and  shales,  but  sandstones  and  shales  of  the  Flysch 
facies  are  extensively  developed  in  both  regions. 

The  Hippurite  limestones  which  are  characteristic  of  the  Medi- 
terranean facies  of  Southern  Europe  stretch  into  Asia  Minor  and 
Persia,  whence  they  pass  into  Baluchistan,  Northern  Himalayas, 
Tibet,  Upper  Burma,  and  China. 

All  the  stages  of  the  Cretaceous  have  been  recognised  in  the 
coastal  region  by  their  faunas,  which  show  a  remarkable  relationship 
to  those  of  North- West  Europe. 

(Danian,  with  Nautilus  danicas. 
Senonian. 
T 
Cenomanian.    Not  known  in  Himalayan  region. 
Albian.     Absent  in  Himalayan  region. 

Lower  Cretaceous  /  Aptian. . 

\Neocomian. 

The  effects  of  the  Cenomanian  transgression  are  particularly 
evident  in  the  Peninsular  region,  where  the  Upper  Cretaceous 
covers  large  tracts  that  in  many  places  extend  inland  far  beyond 
the  limits  of  the  Lower  Cretaceous. 

A  notable  feature  of  the  Upper  Cretaceous  of  India  is  the  evidence 
of  volcanic  outbursts  on  a  titanic  scale  and,  so  far  as  is  known, 
unparalleled  in  the  history  of  the  globe.  Towards  the  close  of 
this  period  a  succession  of  floods  of  lavas  overwhelmed  the  greater 
portion  of  the  Peninsular  area,  in  places  attaining  a  depth  of 
10,000  feet.  The  lavas  are  mainly  augite  basalts  and  dolerites 
that  constitute  what  is  commonly  known  as  the  Deccan  Trap. 

Concurrent  with  these  outbursts,  violent  volcanic  eruptions 
also  took  place  in  the  Himalayan  area,  the  ejected  material  con- 
sisting mainly  of  tuffs  supposed  to  be  submarine,  rhyolitic,  andesitic, 
and  basaltic  lavas,  all  of  which  are  intruded  by  later  gabbros  and 
chrome-bearing  serpentines. 

North  America.— The  Cretaceous  rocks  of  North  America  fall 
into  two  great  formations  that  differ  greatly  in  lithological  character, 
fauna,  and  geographical  distribution,  this  last  arising  from  the 
Cenomanian  transgression,  the  effects  of  which  are  perhaps  more 
marked  in  North  America  than  in  any  other  continent. 

In  North  America  there  is  the  same  faunal  contrast  between 
the  north  and  south  facies  as  in  Europe.  In  Mexico,  Texas,  and 
California,  the  southern  or  equatorial  facies  is  characterised  by  the 
presence  of  Hippurites,  Nerincea,  and  the  Ammonite  Buchiceras, 
all  found  in  Southern  Europe,  as  also  in  Syria,  Persia,  and  India. 


420         A  TEXT-BOOK  OF  GEOLOGY. 

The  northern  facies  with  white  chalk  is  typically  developed  in 
Colorado. 

The  Lower  Cretaceous,  or  Comanchean  System,  as  it  is  sometimes 
called  by  American  geologists,  extends  as  a  narrow  strip  along  the 
old  Atlantic  border  from  New  Jersey  southward  to  South  Carolina, 
and  through  Virginia,  Georgia,  Alabama,  and  Tennessee.  From 
the  Mississippi  Basin  it  sweeps  round  the  Mexican  Gulf,  whence  it 
passes  northward  to  Texas  and  southward  to  Mexico.  It  is  also 
typically  developed  on  the  Pacific  side  of  the  continent,  notably  in 
the  Sacramento  Valley  and  coastal  ranges  of  California,  Oregon, 
and  Washington. 

The  Upper  Cretaceous  follows  the  Lower  Cretaceous  round  the 
old  Atlantic  border  and  Mexican  fringe,  whence  it  spreads  out 
over  Texas.  Here  it  overlaps  the  Lower  Cretaceous  and  extends 
northward  as  a  broad  belt  through  the  Western  Interior  Basin  to 
British  Columbia  and  Alaska. 

In  this  region  the  Cenomanian  transgression  amounted  to  over 
two  thousand  miles,  and  curiously  enough  it  followed  a  line  of 
depression  running  parallel  with  the  axis  or  fulcrum  along  which 
the  tilting  of  the  continent  took  place  in  the  Middle  Mesozoic. 

The  Lower  Cretaceous  Series  consists  mainly  of  sandy,  clayey, 
and  calcareous  deposits  of  marine  origin  ;  and  the  Upper  Cretaceous 
mainly  of  estuarine,  lacustrine,  and  terrestrial  sediments. 

Lower  Cretaceous.  —  On  the  Atlantic  border,  this  great  group  of 
beds  is  known  as  the  Potomac  Series,  and  in  the  Mexican  Gulf 
region  as  the  Tuscaloosa  Series.  These  two  series  are  in  part,  or 
perhaps  mainly,  contemporaneous. 

The  Potomas  Series  is  chiefly  composed  of  estuarine  and  terres- 
trial deposits,  and  the  Tuscaloosa  Series  of  marine  sediments, 
among  which  chalk  and  compact  limestones  are  well  represented. 

In  Mexico  the  Lower  Cretaceous  attains  a  vast  thickness,  which 
is  variously  estimated  at  from  10,000  to  20,000  feet  ;  and  in  Cali- 
fornia the  Shastan  Series  (  =the  Comanchean)  has  an  estimated 
maximum  thickness  of  26,000  feet. 

Upper  Cretaceous.  —  The  subdivisions  of  this  series  in  the  Interior 
Basin,  where  we  have  its  greatest  development,  are  as  follow  :  — 

4.  Laramie.  —  Mainly  brackish    waters,  lacus- 
trine and  terrestrial,  with  seams  of  lignite. 
3.  Montana.  —  Lower  division,  marine  ;  upper, 


Upper  Cretaceous     9    p 

2.  Colorado.  —  Lower     portion  mostly  shales  ; 

upper  portion,  chalk. 

1.  Dakota.  —  Mainly  continental  and  estuarine, 
with  coal-seams. 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  421 

The  Upper  Cretaceous  rocks  at  one  time  stretched  in  a  continuous 
sheet  from  the  Gulf  of  Mexico  to  Alaska,  and  were  laid  down  in 
an  inland  basin  on  the  shores  of  which  coal  vegetation  grew 
luxuriantly.  The  sea  had  free  access  to  the  basin  until  the  middle 
of  the  Montana  stage,  when  a  general  uplift  introduced  brackish- 
water  conditions,  and  eventually  cut  off  all  communication  with  the 
sea.  It  was  in  this  great  land-locked  basin  that  the  famous  Laramie 
lignitic  formation  was  laid  down.  This  inland  basin  was  2000 
miles  long  and  500  miles  wide. 

The  Laramie  Series  follows  the  Montana  Series  conformably, 
and  is  overlain  unconformably  by  the  Eocene.  It  is  the  principal 
coal-bearing  formation  of  the  Western  States,  altogether  covering 
an  area  of  about  100,000  square  miles.  The  coal  belongs  to  the 
lignitic  variety,  except  in  some  parts  of  Colorado,  where  it  has  been 
altered  to  anthracite  by  local  igneous  intrusions. 

At  the  close  of  the  Cretaceous  there  was  a  sudden  and  violent 
revival  of  volcanic  activity  in  many  parts  of  North  America  ;  and 
the  outbursts  were  particularly  intense  in  the  Crazy  Mountain 
area  of  Montana. 

Fauna  and  Flora. — The  Cretaceous  Systems  of  North  America 
contain  prolific  and  varied  faunas  and  floras,  among  which  the 
northern  and  southern  facies  of  Europe  are  typically  developed. 

The  reptilian  fauna  is  specially  notable  for  the  number  and 
variety  of  its  deinosaurs,  pterodactyls,  crocodiles,  turtles,  and 
plesiosaurs. 

The  marine  molluscous  fauna  is  mainly  dominated  by  Ammonites 
and  Belemnites,  many  of  which,  as  in  Europe  and  Asia,  possess  a 
zonal  importance.  All  the  characteristic  genera  of  Lamellibranchs, 
Gasteropods,  and  Sea-urchins  that  distinguish  the  European 
Cretaceous  are  well  represented  in  North  America. 

The  land  flora  of  the  Laramie  Lignitic  Series  contains  a  large 
assemblage  of  forest  trees,  including  representatives  of  the  oak, 
willow,  beech,  plane,  poplar,  maple,  hickory,  fig,  and  sassafras, 
with  many  ferns,  cycads,  and  conifers. 

South  America.  —  Cretaceous  rocks  are  widely  distributed 
throughout  Brazil,  Peru,  Chile,  and  Patagonia. 

In  Brazil  the  Upper  Cretaceous  division  extends  far  beyond 
the  domain  of  the  Lower  Cretaceous,  and  consists  mainly  of  marine 
sediments.  Rocks  of  Senonian  age  rise  into  the  summit  of  the 
Eastern  Andes,  and  in  some  parts  of  the  chain  appear  at  a  height 
of  over  14,000  feet  above  the  sea. 

The  presence  of  extensive  sheets  of  intercalated  lavas  and  tuffs 
shows  that  the  close  of  the  Cretaceous  in  this  continent  was  a  time 
of  intense  volcanic  activity. 

North  Africa. — The  Lower  Cretaceous  rocks  of  North  Africa  are 


422  A  TEXT-BOOK  OF  GEOLOGY. 

mostly  confined  to  the  region  fringing  the  south-west  borders  of 
the  Mediterranean  basin,  including  the  Atlas  Mountains.  But  as 
a  result  of  the  Cenomanian  transgression,  the  Upper  Cretaceous 
Sea  overspread  all  the  low-lying  areas  of  North  Africa,  thereby 
covering  the  greater  portion  of  what  are  now  the  Saharan,  Libyan, 
and  Nubian  deserts.  Altogether  the  sea  invaded  an  area  amounting 
to  many  hundred  thousand  square  miles. 

The  best-known  and  perhaps  most  widespread  member  of  the 
Upper  Cretaceous  in  North-East  Africa  is  the  Nubian  Sandstone, 
a  reddish-brown  or  grey  sandstone  which  contains  only  silicified 
wood  and  in  many  places  bears  a  curious  resemblance  to  the 
Desert  Sandstone  of  Queensland.  In  the  Libyan  Desert  the  Nubian 
Sandstone  is  overlain  by  white  chalk  which,  among  other  fossils, 
contains  the  sea-urchin  Ananchytes  ovata,  a  characteristic  form  of 
the  Senonian  of  North- West  Germany. 

From  Egypt  the  Nubian  Sandstone  passes  into  Syria  and 
Arabia,  where  Upper  Cretaceous  rocks  with  Hippurites  are  very 
largely  developed,  and  include  the  famous  fish-bearing  Senonian 
rocks  in  Lebanon. 

South  Africa. — The  Cretaceous  rocks  of  South  Africa  are  divided 
into  two  groups,  which  occupy  separate  geographical  areas.  Both 
are  shallow-water  marine  deposits,  and  each  group  begins  with  a 
coarse  basal  conglomerate. 

The  two  groups  are  the  Uitenhage  Series  and  the  Pondoland 
Series.  The  latter  occupies  two  narrow  strips  along  the  coast  ; 
while  the  former  is  mainly  displayed  in  a  disturbed  and  folded  zone 
lying  between  the  Karoo  and  the  coast. 

The  Uitenhage  Series  consists  mainly  of  sandstones,  clays, 
shales,  and  limestones,  with  conglomerates  at  the  base.  It  is 
divided  into  three  stages  : — 

f3.  Sunday's  Kiver  Beds") 

Lower  Cretaceous  J  2.  Wood  Beds  ^Mainly  Neocomian. 

\l.  Enon  Beds  j 

The  clays  of  the  Enon  Beds  have  yielded  the  remains  of  deino- 
saurs  ;  and  the  Wood  Beds  contain  an  interesting  series  of  fossil 
plants  comprising  many  ferns,  cycads,  and  conifers.  Among  the 
ferns  are  such  well-known  genera  as  Taniopteris,  Sphenopteris, 
and  Cladophlebis.  Some  of  the  beds  are  crowded  with  the  broad 
fronds  of  the  cycad  Zamites,  of  which  four  species  have  been 
identified,  including  Z.  recta  and  Z.  africana.  The  conifers  are 
represented  by  Araucarites,  Taxites,  Conites,  and  others. 

Intercalated  with  the  Wood  Beds,  there  are  two  or  more  bands 
of  marine  deposits  in  which  the  molluscs  Ostrea,  Psammobia, 
Pecten,  and  Turbo  have  been  found.  In  the  fossil  wood  found  in 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  423 

this  formation  there  have  also  been  discovered  many  examples 
of  the  small  boring  mollusc  Actceonina  atherstonei. 

The  Sunday's  River  Beds  are  shallow-water  or  estuarine,  and 
contain  a  rich  molluscous  fauna  which  consists  mainly  of  Lamel- 
libranchs,  Gasteropods,  and  Cephalopods.  The  Lamellibranchs 
include  such  littoral  shells  as  Astarte,  Avicula,  Cardita,  Cucullcea, 
Exogyra,  Gervillia,  Lima,  Pecten,  Perna,  Pinna,  Mytilus,  Modiola, 
and  Trigonia ;  the  Gasteropods,  Natica,  Patella,  Trochus,  and 
Turbo  ;  and  the  Cephalopods,  Baculites,  Crioceras,  Hamites,  and 
Belemnites. 

The  Pondoland  Series  occurs  in  two  strips,  the  Umzamba  and  the 
Embotyi.  The  Umzamba,  which  is  the  more  important,  lies  on 
the  coast  near  the  Natal  boundary.  It  consists  mainly  of  alternat- 
ing bands  of  shelly  limestone  and  hard  marly  clays  of  marine  origin, 
with  a  large  number  of  fossil  molluscs,  including  Cephalopods,  which 
show  a  singular  relationship  to  those  of  the  Upper  Senonian  of 
Northern  India,  Japan,  Vancouver  Island,  and  Chile. 

Australasia. — Cretaceous  rocks  occupy  an  enormous  area  in  the 
Northern  Territory,  Queensland,  and  Western  New  South  Wales. 
They  are  divided  into  two  series,  namely  : — 

Upper  Cretaceous  (2) — Desert  Sandstone. 
Lower  Cretaceous  (1) — Rolling  Downs  Series. 

The  Rolling  Downs  Series  consists  mainly  of  marine  clays  which 
follow  the  Jurassic  rocks  quite  conformably.  It  is  mainly  developed 
in  Queensland,  where  it  occupies  or  underlies  an  area  of  500,000 
square  miles.  The  clays  of  this  widespread  formation  constitute 
an  impervious  covering,  and  thereby  imprison  the  fresh  water 
contained  in  the  underlying  sandstones.  The  water  rises  freely 
to  the  surface  when  tapped  by  bore-holes.  The  economic  import- 
ance of  the  Rolling  Downs  formation  to  the  dry  interior  of 
Queensland  is  almost  incalculable. 

The  Desert  Sandstone  consists  of  rusty-brown,  gritty  sandstones 
which  are  frequently  current-bedded.  It  mostly  occurs  as  isolated 
patches  and  outliers  which  form  hills,  ranges,  and  plateaux  scattered 
throughout  Queensland  and  Western  New  South  Wales.  Its 
present  distribution  shows  conclusively  that  it  was  at  one  time  a 
continuous  sheet  which  occupied  an  area  exceeding  500,000  square 
miles.  This  great  formation  is  quite  undisturbed,  and  rests 
unconformably  on  the  Lower  Cretaceous  and  on  older  rocks.  We 
now  have  evidence  that  the  perplexing  Cenomanian  transgression 
of  the  sea  was  world-wide  and  probably  contemporaneous  in  both 
hemispheres. 

The  Desert  Sandstone  was  laid  down  partly  in  a  shallow  sea 
bordering  a  great  continent,  and  partly  on  the  neighbouring  low- 


424  A  TEXT-BOOK  OF  GEOLOGY. 

lying  desert  lands.  Near  the  bottom  of  the  series  on  the  present 
coast-line  it  contains  intercalated  marine  beds  from  which  a  few 
molluscs  have  been  obtained,  including  Rhynchonella  croydonensis, 
Leda  elongata?  Avicula  alata,  and  casts  of  Belemnites.  The  same  beds 
contain  the  sea-urchin  Micraster  sweeti.  In  the  Desert  Sandstone 
in  Western  New  South  Wales  there  have  been  found  the  remains  of 
the  reptilian  Cimoliosaurus  ;  and  in  Central  Queensland,  broken 
plants  and  silicified  trees  in  abundance.  At  Cooktown  it  contains 
coal-seams  and  silicified  trees. 

The  surface  of  the  Desert  Sandstone  is  frequently  covered  with 
a  thin  enamel  or  glaze  of  silica  deposited  by  water.  The  siliceous 
cement  stones  of  South  New  Zealand,  and  the  surfaces  of  the 
mushroom-shaped  deposits  of  siliceous  sinter  in  the  Hauraki 
Peninsula  in  North  New  Zealand,  are  also  covered  with  the  same 
glaze,  which  is  only  formed  on  weathered  surfaces. 

On  the  Darling  Kiver  the  Desert  Sandstone  contains  deposits 
of  opal. 

In  New  Zealand  the  Cretaceous  Waipara  Series  consists  of  basal 
conglomerates  and  sandy  beds  with  seams  of  coal  and  shales  which 
contain  the  leaves  of  dicotyledonous  plants.  The  Coal-Measures 
are  followed  by  marly  or  shaly  clays  with  lines  of  calcareous  concre- 
tions that  frequently  contain  saurian  remains.  Then  follow 
glauconitic  greensands  which  are  conformably  overlain  by  chalky 
and  hard  limestones  which  close  the  succession.  Passing  northward 
from  Waipara,  the  chalky  limestone  (Amuri  Limestone)  expands 
and  replaces  the  hard  limestone  (Weka  Pass  Stone),  and  is  called 
the  Grey  Marl. 

The  reptilian  remains  found  in  the  marly  clays  below  the  green- 
sands  include  representatives  of  Plesiosaurus,  Mauiosaurus, 
Polycotylus,  and  others.  In  the  same  beds  there  are  found  the 
teeth  of  Lamna  and  Hybodus,  and  the  molluscs  Trigonia,  Con- 
chothyra  parasitica,  which  is  a  characteristic  Cretaceous  form, 
Rostellaria  and  Belemnites. 

The  Waipara  Series  is  marginal  to  the  present  coasts,  but  in  a 
few  places  it  creeps  into  land-locked  mountain  basins  to  which  the 
sea  had  free  access. 

Landscape  and  Physical  Features. — The  Cretaceous  System,  as 
seen  in  different  lands,  presents  a  great  diversity  of  surface  forms. 
The  effects  of  denudation  are  found  to  vary  with  the  character  and 
succession  of  the  rocks,  the  amount  of  tilting  and  faulting  they 
have  suffered,  the  height  above  the  sea,  the  amount  of  rainfall, 
and  general  climatic  conditions.  Even  the  same  rock-formation 
may  assume  different  landscape  forms  in  different  regions. 

The  Chalk,  in  England,  owing  to  its  superior  resisting  power 
to  the  effects  of  subaerial  denudation,  wherever  it  occurs,  forms 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  425 

striking  features  in  the  landscape.  Thus  the  ranges  of  hills  known 
as  the  North  and  South  Downs,  the  Salisbury  Plain,  Chilterns, 
Lincolnshire  Wolds,  and  Yorkshire  Wolds,  all  owe  their  origin  to  the 
wearing  away  of  the  softer  clays  and  sands  and  the  survival  of 
the  chalk  notwithstanding  the  wasting  influence  of  the  weather. 
In  the  same  way  the  white  cliffs  of  Dover,  in  South-East  England, 
and  of  Flamborough  Head  on  the  north-east  coast,  owe  their 
striking  appearance  to  the  resistance  the  chalk  has  offered  to  the 
assaults  of  the  sea.  Moreover,  in  the  Isle  of  Wight,  where  the 
Cretaceous  rocks  are  tilted  at  high  angles,  the  chalk  forms  the 
central  ridge  which  traverses  the  island,  the  surrounding  softer 
rocks  having  been  worn  away  by  denudation. 

In  arid  regions  the  effects  of  subaerial  denudation  are  always 
strikingly  uniform,  and  there  is  seldom  seen  the  diversity  of  surface 
features  which  characterises  temperate  climates  with  an  abundant 
rainfall.  In  temperate  regions  the  rock-formations  become  dis- 
sected into  a  complex  of  ridges  and  valleys,  but  in  arid  regions 
the  general  effect  of  the  varying  temperature  and  wind  is  to  reduce 
the  whole  landscape  to  a  monotonous  level  surface. 

The  Nubian  Sandstone  of  North  Africa  forms  long  lines  of  even 
escarpment  in  the  Libyan  Desert  ;  and  the  Desert  Sandstone  of 
North-East  Australia  frequently  assumes  the  form  of  isolated  flat- 
topped  ridges  bounded  by  steep  walls,  like  the  mesas l  of  Colorado  ; 
or  pyramidal  hills  barred  with  the  horizontal  parallel  lines  of 
stratification,  like  the  buttes  of  Wyoming. 

Economic  Minerals. — The  Cretaceous  System  is  notable  for  its 
valuable  deposits  of  lignitic  coal  as  found  in  the  Laramie  formation 
of  the  Western  Interior  States  of  North  America  ;  and  in  the 
Waipara  Series  of  New  Zealand.  The  chalk  and  other  limestones 
of  this  system  are  also  of  great  economic  importance,  in  England, 
France,  and  elsewhere,  as  a  source  of  lime  for  the  manufacture  of 
cement,  and  for  agricultural  purposes. 

SUMMARY. 

(1)  The  Mesozoic  era  is  divided  into  three  great  systems,  the 
Triassic,     Jurassic,    and     Cretaceous.     Generally    speaking,     the 
Triassic   is  the  connecting-link  with  the  Palaeozoic  era,  and  the 
Cretaceous  with  the  Cainozoic. 

(2)  The  Triassic  as  developed  in  England  is  continental,  and 
shows    a    continuation    of   the  conditions  that  prevailed  in  the 
Permian ;    but    in    Continental    Europe   there    are    two    distinct 
facies   of   deposits,  each  occupying  different   geographical  areas, 
and  each  characterised  by  a   distinctive  fauna.     The  two  facies 

1  Sp.  Mesa==&  table. 


426  A  TEXT-BOOK  OF  GEOLOGY. 

are  the  Continental  and  the  Marine,  the  former  typically  developed 
in  Central  Germany  and  hence  called  the  German  facies  ;  and  the 
latter  in  the  Alps,  and  hence  known  as  the  Alpine  facies. 

The  deposits  of  the  continental  facies  were  laid  down  in  inland 
basins  or  Mediterranean  seas  that  after  a  time  were  cut  off  from 
all  access  to  the  sea.  The  extensive  and  valuable  beds  of  rock- 
salt  and  gypsum  associated  with  these  deposits  show  that  the 
climatic  conditions  of  Central  Europe  and  the  corresponding 
latitudes  in  North  America  were  not  unlike  those  now  prevailing 
in  the  arid  regions  of  North  Africa  and  Central  Australia. 

The  Triassic  deposits  of  the  Alpine  facies  are  marine,  but  the 
fauna  is  that  of  shallow  water. 

The  Triassic  System  is  specially  distinguished  by  the  appearance 
in  it  of  the  earliest  known  mammals  which  belong  to  a  primitive 
type  apparently  related  to  the  existing  marsupials  of  Australia. 

(3)  The  deposits  and  fossils  of  the  Jurassic  System  show  that  the 
continents  of  that  period  were  clothed  with  a  rank  vegetation,  while 
the  estuaries  and  seas  swarmed  with  molluscs,  fishes,  and  huge 
reptiles.     Moreover,  the  forests  teemed  with  insects,  and  the  sea- 
shore was  frequented  by  the  peculiar  toothed  Archceopteryx,  the 
earliest  known  bird. 

The  faunas  of  the  Jurassic  may  be  divided  into  geographical 
zones  which  encircle  the  globe  in  a  direction  parallel  to  the  equator, 
and  correspond  to  the  biological  zones  that  now  exist  in  each 
hemisphere.  This  is  the  first  evidence  of  the  existence  of  climatic 
zones  on  the  globe. 

Ammonites  were  very  numerous  in  the  Jurassic  seas  ;  and  the 
different  species  were  so  widely  distributed  throughout  the  globe 
and  so  limited  in  vertical  range,  that  they  are  now  useful  in  sub- 
dividing the  various  stages  of  the  system  into  palaeontological 
zones. 

Cycads  were  so  abundant  and  prominent  among  the  land  vegeta- 
tion, and  reptiles  so  numerous  on  the  land,  in  the  air,  in  the  deltas 
and  seas,  that  the  Jurassic  has  been  sometimes  called  the  Age  of 
Cycads,  and  sometimes  the  Age  of  Reptiles. 

(4)  The   Cretaceous    System   everywhere   falls   into   two   great 
divisions,  the  Lower  Cretaceous  and  the  Upper  Cretaceous.     These 
two  divisions  are  frequently  associated  in  the  same  regions  ;    but 
in  all  the  continents,  in  both  hemispheres,  the  Upper  Cretaceous 
passes  on  to  older  rocks  and  stretches  far  beyond  the  limits  of  the 
Lower  Cretaceous.     This  remarkable   distribution  of  the  Upper 
Cretaceous  rocks  was  due  to  a  relatively  sudden  invasion  or  trans- 
gression of  the  sea  all  over  the  globe,  whereby  all  the  low-lying 
lands  and  valleys  fringing  the   continents  were  overwhelmed  by 
the  sea.     The  cause  of  this  great  inundation  is  unknown,  but  it 


MESOZOIC    ERA  :     CRETACEOUS    SYSTEM.  427 

may  have  been  connected  with  the  collapse  of  the  Gondwana- 
Land  continent,  which  we  know  existed  in  the  Indian  Ocean  area 
up  till  the  close  of  the  Jurassic  and  well  into  the  Cretaceous 
period. 

The  marine  life  of  the  Cretaceous  was  even  more  abundant  than 
that  of  the  Jurassic  ;  and  while  its  general  facies  is  distinctively 
Mesozoic,  it  is  characterised  by  the  appearance  of  many  genera 
of  marine  molluscs  which  still  live  in  our  seas. 

The  Cretaceous  flora  is  specially  notable  for  the  advent  of 
angiosperms  or  flowering  plants,  including  monocotyledons  and 
dicotyledons,  among  which  were  representatives  of  most  of  the 
forest  trees  of  the  present  day.  Ferns,  cycads,  and  conifers  now 
grew  side  by  side  with  the  oak,  beech,  plane,  willow,  and  other 
familiar  trees  ;  hence  the  general  aspect  of  the  forests  must  have 
resembled  that  of  the  existing  forests  of  the  warm  temperate 
zones  of  the  present  day. 

The  Jurassic  reptiles  were  still  present  in  all  parts  of  the  globe, 
and  particularly  numerous  in  North  America  ;  but  they  disappeared 
before  the  close  of  the  Cretaceous  period,  as  also  did  the  voracious 
Ammonites  and  Belemnites.  The  Nautilus,  the  most  ancient 
of  the  Cephalopods,  survived  the  Cretaceous  and  still  lives  in  warm 
tropical  seas.  Its  persistence  is  possibly  due  to  its  habitat  lying 
in  the  open  deep  seas,  where  it  would  be  less  affected  by  continental 
changes  than  the  shallow-water  Ammonites  and  Belemnites. 

Two  distinct  types  of  marine  fauna  are  present  in  Europe  and 
America,  the  northern  and  southern,  the  former  or  Central  European 
characterised  by  soft  foraminiferal  chalk,  and  the  latter  or  Equa- 
torial by  hard  limestones  frequently  composed  of  the  shells  of 
the  curious  Hippurites,  which  also  spread  eastward  as  far  as 
Northern  India  and  Tibet,  and  westward  to  Jamaica,  Texas,  and 
California,  but  appear  to  be  unknown  in  the  Southern  Hemisphere. 


CHAPTER   XXXI. 
CAINOZOIC   OR  TERTIARY  ERA. 

Eocene  and  Oligocene  Systems. 

THE  Cainozoic  is  the  youngest  of  the  four  grand  divisions  into 
which  geological  time  is  divided,  and  embraces  the  period  from 
the  end  of  the  Cretaceous  to  the  present  time. 

The  palseontological  break  between  the  Lower  Tertiary  and  the 
Chalk  is  the  most  striking  and  universal  in  the  geological  history 
of  the  globe.  But  the  stratigraphical  break  is  not,  as  might 
reasonably  be  expected,  correspondingly  great.  On  the  contrary, 
it  is  seldom  conspicuous,  and  in  many  places  is  scarcely  visible, 
which  renders  the  sudden  change  in  the  organic  life  of  the  Earth 
all  the  more  remarkable  and  puzzling. 

Great  changes  took  place  in  the  relative  distribution  of  land 
and  sea  during  the  interval  bridging  the  Chalk  and  Eocene  ;  and 
there  is  abundant  evidence  that  they  were  mainly  due  to  a  world- 
wide recession  of  the  sea.  But  these  changes  were  inconsiderable 
compared  with  those  caused  by  the  great  Cenomanian  transgression, 
which,  as  we  know,  was  followed  by  no  conspicuous  acceleration 
in  organic  development.  Nevertheless,  we  are  probably  not  far 
from  the  truth  when  we  assume  that  the  remarkably  sudden  dis- 
appearance of  old  forms,  and  the  advent  of  many  new  inhabitants 
of  the  globe  in  the  interval  between  the  Chalk  and  Lower  Tertiary 
period,  were  mainly  due  to  physical  and  climatic  changes  ;  and 
although  the  stratigraphical  break  is  apparently  small,  the  time 
occupied  in  these  changes  must  have  covered  a  vast  period  of  time. 

Fauna  and  Flora. — The  life  of  the  Tertiary  era  is  distinguished 
from  the  Cretaceous  by  the  disappearance  of  many  well-established 
Mesozoic  genera  and  the  sudden  appearance  of  numerous  highly 
organised  forms  of  which  we  can  find  no  trace  of  probable  ancestors. 
Ammonites,  Belemnites,  Baculites,  Hamites,  Inocerami,  Hippurites, 
and  the  remarkable  reptilian  Plesiosaurs,  Ichthyosaurs,  Ptero- 
dactyls, and  monstrous  Deinosaurs  disappear  as  completely  as  if 
they  had  never  existed,  and  their  place  is  immediately  filled  by 
a  congeries  of  highly  developed  placental  mammals. 

428 


CAINOZOIC    ERA  :     EOCENE    SYSTEM.  429 

Many  genera  survived  from  the  Mesozoic,  but  the  organic  hiatus 
is  so  complete  that  no  single  species  higher  in  the  scale  than 
the  primitive  Forarninifera  passed  from  the  Cretaceous  to  the 
Tertiary. 

Foraminifera  are  numerous  throughout  the  whole  of  the  Tertiary 
era  and  particularly  abundant  in  the  Middle  and  Upper  Eocene  ; 
and  the  reef-building  corals  comparatively  rare  in  the  Chalk  again 
become  prominent  in  the  Equatorial  zones. 

Brachiopods  show  a  marked  decline,  except  perhaps  in  the 
Australian  waters ;  but  marine  Lamellibranchs  and  Gasteropods 
are  more  numerous  than  ever.  Ammonites  and  Belemnites  have 
disappeared,  but  Nautilus  and  gigantic  Aturia  are  still  common. 
Crustaceans  are  now  represented  by  numerous  short-tailed 
decapods. 

Among  the  vertebrates  we  have  a  great  array  of  fishes,  as  well 
as  many  sea-snakes,  crocodiles,  and  birds.  Placental  mammals, 
including  ancestral  forms  of  most  of  the  living  ungulates  (hoofed- 
herbivores),  appear  in  the  Lower  Tertiary  for  the  first  time,  and 
become  prominent  almost  at  once  ;  and  associated  with  them  we 
have  representatives  of  the  non-placental  marsupials  which  are 
still  the  dominant  endemic  mammals  of  the  Australian  continent. 

Before  the  close  of  the  Tertiary  era  there  appeared  the  anthro- 
poid apes,  and  finally  man. 

The  flora  is  now  dominated  by  the  flowering  Angiosperms,  which 
are  represented  by  a  vast  assemblage  of  monocotyledonous  and 
dicotyledonous  forms,  which  include  the  cactus,  numerous  palms, 
laurel,  myrtle,  magnolia,  etc.,  comprising  a  luxuriant  evergreen 
vegetation. 

Rocks. — The  sedimentary  rocks  of  the  Cainozoic  era  in  the 
Northern  facies  are  mostly  incoherent  sands,  clays,  and  pebbly  beds 
with  subordinate  layers  of  marls  and  hard,  shelly  limestones; 
but  in  the  Southern  or  Equatorial  facies  of  the  Lower  Tertiary 
hard  limestones,  sandstones,  and  shales  predominate. 

Marine  and  estuarine  beds  are  largely  represented,  but  deltaic, 
fluviatile,  lacustrine,  and  desert  deposits  play  an  important  rdle 
in  all  the  Tertiary  formations.  Towards  the  close  of  the  era, 
glacial  accumulations  are  conspicuous  in  many  temperate  latitudes. 

Generally  speaking,  the  marine  Tertiary  deposits  are  marginal 
to  the  continental  areas,  and  usually  still  lie  horizontal,  except 
where  they  have  been  involved  in  the  structural  folds  of  the  great 
mountain-chains,  or  locally  disturbed  by  volcanic  outbursts. 

Where  the  Tertiary  systems  are  fully  represented,  they  form  a 
great  succession  of  conformable  strata  ;  but  where  the  succession 
is  incomplete,  there  may  be  physical  breaks  of  considerable  magni- 
tude. In  England,  for  example,  where  the  Miocene  is  entirely 


430  A  TEXT-BOOK  OF  GEOLOGY. 

absent,  the  Pliocene  rests  unconformably  on  the  Eocene  and  older 
rocks. 

The  volcanic  activity  which  revived  at  the  close  of  the  Cretaceous 
after  nearly  an  era  of  quiescence  continued  with  periods  of  rest 
throughout  the  whole  of  the  Cainozoic  era.  There  is  much  evidence 
in  favour  of  the  belief  that  all  great  crustal  movements  have 
been  preceded  or  accompanied  by  violent  displays  of  volcanic 
activity. 

Distribution. — The  Tertiary  systems  are  found  in  all  parts  of 
the  globe  ;  and  in  many  regions  there  is  a  close  geographical 
relationship  between  the  Lower  Tertiary  formations  and  the 
Cretaceous.  It  would  appear  that  many  Cretaceous  areas  of 
deposition  after  a  lapse  of  time  became  areas  of  deposition  in 
the  Tertiary  era  ;  and  in  regions  where  the  Cretaceous  rocks 
suffered  little  deformation,  the  Tertiary  strata,  frequently  rest  on 
them  with  no  visible  appearance  of  stratigraphical  discordance. 

Tertiary  sedimentary  rocks  are  found  involved  in  the  folds  of 
all  the  great  mountain-chains  of  the  globe,  and  from  this  we  know 
that  the  Cainozoic  has  been  the  greatest  mountain-building  era 
of  which  we  have  any  certain  knowledge. 

Kocks  of  Lower  Tertiary  age  take  part  in  the  structure  of  the 
Alps,  Apennines,  Carpathians,  Caucasus,  Atlas,  Himalayas,  Andes, 
Rocky  Mountains,  Sierras,  and  many  other  great  chains,  all  of  which 
are  therefore  comparatively  young.  When  we  pause  to  remember 
that  the  site  of  a  gigantic  mountain  complex  such  as  the  Himalayan 
was  a  sea-floor  so  recently  as  the  Middle  Tertiary,  we  begin  to  catch 
a  faint  conception  of  the  comparative  rapidity  of  great  earth- 
movements  and  of  the  titanic  forces  of  which  they  are  the  visible 
expression. 

The  uplift  of  the  Tertiary  rocks  in  some  regions  is  enormous. 
In  the  Alps  the  Lower  Tertiary  Nummulitic  Limestone  occurs  at 
a  height  of  11,000  feet  above  the  sea,  and  in  the  Himalayas  17,000 
feet. 

Distribution  of  Land  and  Water. — The  beginning  of  the  Tertiary 
era  still  found  the  Tethys  or  Central  Sea  in  existence,  and  on  its 
floor  and  borders  were  laid  down  a  great  succession  of  Lower 
Tertiary  deposits,  including  the  Nummulitic  Limestone.  The 
Central  Sea,  as  already  described,  extended  from  the  Atlantic 
eastwards  through  the  Mediterranean  Basin,  covered  the  whole  of 
Southern  Europe  and  North  Africa  and  stretched  over  Asia  Minor, 
Arabia,  Persia,  Baluchistan,  Himalayan  area  to  Further  India  ; 
and  although  shrunken  in  size  since  the  Cretaceous  period,  it  still 
formed  a  great  inland  sea  that  girdled  half  the  globe. 

In  the  Middle  Tertiary  the  eastern  half  of  the  Central  Sea 
became  occupied  by  the  Himalayas,  Caucasus,  and  the  mountains 


OAINOZQIC    EKA  :     EOCENE    SYSTEM. 

of  Persia,  Arabia,  and  Asia  Minor  ;  and  its  northern  limits  were 
curtailed  by  the  rise  of  the  Carpathians,  Apennines,  Alps,  and 
Pyrenees. 

The  crustal  corrugation  and  folding  of  the  Himalayas,  Alps,  and 
other  great  chains  continued  until  the  close  of  the  Miocene,  when 
the  Central -Sea  became  broken  up  into  disconnected  inland  seas 
and  salt-water  lakes. 

In  Pliocene  and  later  times,  due  to  the  continued  recession  of 
the  ocean,  the  Central  Sea  diminished  in  size  until  in  our  own  time 
the  Mediterranean  Sea  is  all  that  now  remains  to  mark  its  former 
existence. 

The  present  distribution  of  animals  and  plants  tends  to  show 
that  there  was  a  Tertiary  land  connection  between  Europe  and 
North  America  through  the  Faroe  Islands  and  Iceland,  and 
between  Alaska  and  North-East  Asia  across  the  present  Behring 
Straits.  About  the  same  time  land-bridges  probably  joined 
South  Africa,  South  America,  New  Zealand,  and  Australia  with 
the  Antarctic  continent. 

It  is  a  singular  fact  that  the  Tertiary  mollusca  of  Chile  presents 
a  greater  resemblance  to  the  living  and  fossil  mollusca  of  the  Medi- 
terranean Basin  than  to  the  mollusca  now  living  on  the  coast  of 
Chile.  The  inference  to  be  drawn  from  this  is  that  the  isolation 
of  the  Chilian  region  did  not  take  place  till  some  time  between  the 
mid-Tertiary  and  the  beginning  of  the  Pleistocene. 

Climate. — The  Tertiary  climate  of  Europe  and  North  America 
was  at  first  warm,  and  then  tropical ;  but  gradually  the  temperature 
became  cooler,  and  at  last  Arctic  cold  prevailed.  This  last  phase 
took  place  in  quite  late  Tertiary  times,  and  has  since  been  succeeded 
by  a  cold  temperate  climate. 

Similar  variations  of  climate  also  took  place  in  the  Southern 
Hemisphere. 

The  changes  of  climate  are  indicated  by  the  character  of  the  land 
animals  and  plants,  and  to  some  extent  by  the  marine  faunas. 
The  period  of  refrigeration  witnessed  a  great  advance  of  the  polar 
ice- sheets,  and  the  accumulation  of  gigantic  glaciers  on  the  higher 
mountain- chains. 

Subdivision. — In  the  Cainozoic  era,  climate  exercises  a  more 
potent  influence  than  ever  in  the  distribution  of  animals  and  plants ; 
and  in  consequence  the  methods  of  subdivision  and  correlation 
of  rock-formations  by  some  characteristic  fossils  so  successfully 
applied  to  the  Mesozoic  and  Palaeozoic  systems  can  no  longer  be 
employed.  In  these  circumstances  it  became  necessary  to  devise 
some  new  method  of  subdivision  in  order  that  the  formations 
in  one  region  should  be  equivalent  in  time  to  those  in  another 
region. 


432  A  TEXT-BOOK  OF  GEOLOGY. 

The  method  of  classification  first  suggested  in  1830  by  the  French 
geologist  Deshayes,  and  subsequently  adopted  by  Lyell,  for  the 
chronological  subdivision  of  the  Cainozoic  rock-formations  is 
based  on  the  proportion  of  living  to  extinct  forms  contained  in 
the  complete  fauna.  The  principle  underlying  this  method  is 
that  the  older  a  formation  is,  the  fewer  living  species  will  it  contain  ; 
and  the  younger  it  is,  the  greater  the  number. 

The  percentage  method  of  classification  has  proved  accurate, 
and  is  commonly  adopted. 

When  groups  of  beds  in  two  distant  regions  are  classified  as 
Eocene,  it  does  not  necessarily  follow  that  they  are  contempor- 
aneous, for  it  is  evident  that  through  various  physical  and  climatic 
conditions  a  larger  proportion  of  species  may  contrive  to  sur- 
vive in  one  region  than  in  another.  Moreover,  the  rate  of 
evolution  is  not  the  same  in  the  different  orders  of  the  animal 
kingdom. 

The  age  of  formations,  as  determined  by  the  percentage  of  living 
species,  is  comparative  rather  than  actual ;  hence  the  correlation 
of  distant,  disconnected  groups  of  beds  by  this  method  is  never 
satisfactory  without  supporting  evidence. 

The  main  divisions  of  the  Cainozoic,  or  Neozoic  1  era  as  it  is 
sometimes  called,  are  as  follow  : — 

/  6.  Recent. 

Upper  Cainozoic  or  )  5.  Pleistocene2  =  mostly  recent  species. 
Neogene  \  4.  Pliocene3        -majority  recent  species. 

(3.  Miocene4        =  minority  recent  species. 
Lower  Cainozoic  orj^2.  Oligocene5      —few  recent  species. 
Palaeogene          \l.  Eocene6          =  dawn  of  recent  species. 

The  Foraminifera  are  such  persistent  organic  types  that,  stand- 
ing by  themselves,  they  are  of  little  value  for  the  division  of  the 
Cainozoic  era  into  time  periods  ;  while  the  higher  land  vertebrates 
show  such  a  rapid  biological  development  and  limited  distribution, 
combined  with  a  constitution  so  acutely  sensitive  to  climatic  and 
geographical  changes,  that  they  are  as  untrustworthy  for  purposes 
of  subdivision.  The  more  stable  and  wide-spread  marine  mollusca 
form  the  best  available  basis  of  classification. 

The  percentages  of  living,  i.e.  recent,  species  of  the  molluscous 
fauna  used  as  a  basis  of  classification  are  as  follow  : — 


1  Gr.  weos  =  new,  and  zoe=life. 

2  Gr.  pleiston  =  the  most,  and  kainos  (cene)=  recent. 

3  Gr.  pleion=moie,  and  Tea inos = recent. 

4  Gr.  meion=leas,  and  kainos=  recent. 

5  Gr.  oligos=tew,  and  ka inos = recent. 

6  Gr.  eo-s  =  the  dawn,  and  kainos  =  recent. 


CAINOZOIC    ERA  :     EOCENE    SYSTEM.  433 

Recent          =100  per  cent. 
Pleistocene  =  90 — 100  per  cent. 
Pliocene        =  40 —  90  per  cent. 
Miocene        =  10 —  40  per  cent. 
Eocene          =     0 —  10  per  cent. 

The  main  divisions  of  the  Cainozoic  are  sometimes  recognised 
as  separate  systems,  but  they  represent  periods  of  time  so  much 
shorter  than  the  systems  of  the  Palaeozoic  and  Mesozoic  eras  that 
they  are,  by  some  writers,  grouped  into  two  great  divisions  or 
systems  called  the  Palceogene  and  Neogene.  Since  the  systems 
are  admittedly  of  unequal  value  as  measurers  of  time,  it  is  unim- 
portant whether  we  regard  the  Eocene,  Oligocene,  etc.,  as  systems 
or  merely  as  series.  Obviously  the  relative  importance  of  a  suc- 
cession of  strata  cannot  be  measured  by  the  thickness  of  the  deposits 
which  it  comprises,  but  by  the  organic  changes  which  took  place 
during  the  time  occupied  in  the  deposition  of  the  sediments. 

Although  the  Cainozoic  may  cover  a  relatively  short  period  of 
time,  it  is  certain  that  it  has  witnessed  a  more  striking  and 
momentous  development  of  organic  life  and  physical  changes  of 
greater  magnitude  than  the  Mesozoic.  Hence,  for  our  present  pur- 
pose, we  will  regard  the  Cainozoic  as  an  era  and  the  main  divisions 
as  systems. 

EOCENE   SYSTEM. 

At  the  close  of  the  Cretaceous  there  was  a  recession  of  the  sea 
all  over  the  globe.  Hence  the  Eocene  deposits  occupy  a  smaller 
area  than  the  Cretaceous ;  and  shallow  water,  estuarine  and  even 
terrestrial  sediments  follow  the  Chalk. 

The  recession  of  the  sea  caused  a  widespread  migration  of  the 
Cretaceous  life,  and  none  survived  until  the  Eocene.  All  the 
forms  slowly  disappeared  or  became  modified  by  the  development 
of  structural  features  better  adapted  to  the  new  conditions  and 
environment. 

Distribution. — In  Europe  the  Eocene  is  mainly  distributed  in 
two  geographical  regions  :  the  Anglo- Gallic,  which  embraces 
South  England,  North  France,  and  Belgium ;  and  the  Alpine 
or  Mediterranean,  which  in  the  main  follows  the  former  limits  of 
the  Central  Sea  and  embraces  the  whole  of  Southern  Europe  and 
North  Africa  and  a  wide  zone  extending  eastward  to  Further  India. 

Rocks. — In  the  Anglo-Gallic  region,  which  at  one  time  spread 
over  a  large  portion  of  North-West  Europe,  the  rocks  of  the  Eocene 
System  are  mainly  loose  incoherent  sands,  clays,  and  pebbly  beds  with 
occasional  bands  of  hard  shelly  limestone,  and  in  the  Alpine  region, 
massive  beds  of  hard  limestone,  compact  sandstones,  and  shales. 
The  limestones  of  this  region  are  largely  composed  of  Nummulites. 

28 


434  A  TEXT-BOOK  OF  GEOLOGY. 

The  Nummulites  are  the  most  complex  and  most  highly  organised 
of  all  the  Foraminifera.  A  few  appeared  in  the  Jurassic  and 
Cretaceous,  but  their  maximum  development  took  place  in  the 
Middle  Eocene.  They  are  equatorial  in  habitat,  and  lived  in  the 
Great  Centre  Sea  (Tethys)  in  extraordinary  profusion.  They  are 
unknown  in  the  Tertiary  rocks  of  Australia,  New  Zealand,  South 
Africa,  and  America. 

In  the  Swiss  and  Maritime  Alps,  in  the  Apennines,  Carpathians, 
and  eastwards  to  the  Himalayas,  the  Upper  Eocene  is  also  repre- 
sented by  an  extraordinary  thickness  of  grey  sandstones  and  shales 
of  the  Flysch  facies.  These  rocks  occur  in  close  association  with 
massive  beds  of  the  nummulitic  limestone,  but  are  themselves 
practically  devoid  of  all  organic  remains.  The  same  facies  of 
rocks  is  also  largely  developed  in  California. 

The  volcanic  activity  which  revived  at  the  close  of  the  Cretaceous 
continued  into  the  Eocene.  The  outbursts  were  local  and  inter- 
mittent. Eocene  volcanic  rocks  occur  in  North  Ireland,  West 
Scotland,  Faroe  Islands,  Iceland,  Greenland,  and  in  the  States  of 
California,  Oregon,  Washington,  Montana,  Wyoming,  and  Colorado. 

British  Isles. 

The  Eocene  deposits  of  England  occupy  two  triangular  areas 
in  the  south-east  end  of  the  island,  namely,  the  London  Basin  in 
the  Thames  Valley,  and  the  Hampshire  Basin,  with  its  base  lying 
along  the  coast  of  the  mainland  opposite  the  Isle  of  Wight.  Out- 
liers occur  on  the  Isle  of  Wight,  Salisbury  Plain,  Chilterns,  and 
elsewhere,  and  indicate  a  former  extension  of  the  Eocene  strata 
over  the  whole  of  South-East  England. 

The  Eocene  beds  everywhere  rest  on  the  Chalk,  usually  without 
any  visible  stratigraphical  unconformity. 

The  beds  of  the  London  and  Hampshire  basins  exhibit  a  close 
relationship  both  in  fauna  and  lithological  sequences,  and  it  is 
certain  that,  although  now  separated  by  a  ridge  of  chalk,  they 
were  all  laid  down  on  the  floor  of  the  same  continuous  sea. 

Local  Subdivisions. — The  subdivisions  usually  recognised  in  the 
two  basins  are  as  follow  : — 

London  Basin.  Hampshire  Basin. 

CQ.  Upper  Bagshot  Sands.  Barton  Beds. 

Upper  Eocene «j  5.  Middle  Bagshot  Sands.  Bracklesham  Beds. 

|^4.  Lower  Bagshot  Sands.  Lower  Bagshot  Beds. 

/  3.  London  Clay.  London  Clay. 

Lower  Eocene  1  2"  Woolwich  and  Riding 

j  Beds.  Beading  Beds. 

(  1.  Thanet  Sands.  (Absent.) 


CAINOZOIC    ERA  :     EOCEKE    SYSTEM. 


435 


The  Thanet  Sands  consist  of  light-coloured  sands  which  are  clayey 
at  the  base,  and  contain  glauconitic  grains.  Where  they  rest  on 
the  Chalk,  they  contain  a  basal  layer  of  unworn,  green-coated 
flints,  which  was  apparently  formed  after  the  sands  were  deposited. 
The  flints  are  the  insoluble  residuum  left  behind  after  the  upper 
layers  of  Chalk  in  which  they  were  imbedded  had  been  removed 
by  the  action  of  percolating  water.  Examples  of  underground 
chemical  erosion  are  not  infrequent  where  limestones  are  followed 
by  porous  sands  or  sandstones  containing  moving  water. 

The  fossils  of  the  Thanet  Sands  are  marine  and  mostly  Lamelli- 
branchs  and  Gasteropods.  Among  the  more  abundant  forms  are 
C orb ula  regulbiensis  and  Aporrhais  Soiverbyi  (Plate  LV.,  fig.  3). 

The  Woolwich  and  Reading  Beds  vary  considerably  in  character. 
In  East  Kent  they  consist  of  marine  sands,  and  in  West  Kent  and 


N 


FIG.  219A. — Section  across  the  Isle  of  Wight.     (After  H.  W.  Bristow.] 
a.  Chalk. — Cretaceous. 
6.  Reading  Beds. 

c.  London  Clay. 

d.  Lower  Bagshot  Beds. 

e.  Bracklesham  Beds. 
/.   Barton  Clay. 

g.  Barton  Sand. 


Eocene. 


h.  Headon  Beds. 
*.  Osborne  Beds. 
k.  Bembridge  and 

Hamstead  Beds. 
m.  Gravels. — Recent. 


Oligocene. 


Surrey  of  estuarine  sands  and  grey  clays,  which  may  be  taken  as 
an  indication  that  the  land  lay  to  the  westward. 

In  the  Hampshire  Basin,  only  the  Reading  Sands  are  present. 
The  Oldhaven  and  Blackheath  Beds,  which  consist  of  fluviatile 
pebbly  sands  and  pebbles,  are  local  subdivisions  overlying  the 
Woolwich  Series. 

The  London  Clay  is  perhaps  the  most  important  division  of  the 
Eocene  in  England.  It  is  usually  a  stiff  marine  clay  of  a  bluish- 
grey  colour,  except  at  the  surface  where  it  weathers  to  a  brown  hue. 
It  contains  layers  of  calcareous  concretions  arid  nodules  of  pyrites. 
In  some  places  crystals  of  selenite  are  common.  At  London, 
which  lies  about  the  centre  of  the  basin,  the  thickness  of  the  clay 
is  400  or  500  feet.  In  the  Hampshire  Basin  the  London  Clay  is 
more  sandy  than  in  the  eastern  basin. 

Fossils  are  abundant  and  mostly  marine  molluscs,  crustaceans, 
fishes,  and  land  plants.  Among  the  molluscs  are  Aporrhais 
Sowerbyi  and  Aturia  ziczac  ;  and  the  genera  Pleurotoma,  Fusus, 
Murex,  and  Natica  are  represented  by  numerous  species. 


436  A   TEXT-BOOK   OF   GEOLOGY. 

The  fishes  include  many  forms  of  rays  (Myliobates),  and  the 
ubiquitous  sharks  (Lam-na,  Otodus,  etc.).  Among  the  numerous 
reptilians  are  turtles,  tortoises,  crocodiles,  and  a  sea-snake.  The 
remains  of  several  birds  have  also  been  found.  The  mammals 
include  ancestral  forms  of  the  tapir,  bat,  and  opossum. 

The  plants  include  fan-palms,  feather-palms,  cactus,  fig,  elm, 
poplar,  beech,  planes,  maple,  and  many  other  angiosperms. 

The  land  animals  and  vegetation  would  indicate  the  prevalence 
in  the  Middle  Eocene  of  a  warm,  temperate,  or  semi-tropical  climate 
in  the  south  of  England  resembling  that  of  New  Zealand  at  the 
present  time. 

The  Bagshot  Beds  are  divided  into  three  subdivisions,  Lower, 
Middle,  and  Upper.  They  occupy  a  smaller  area  than  the  London 
Clay,  which  is  probably  a  result  of  denudation.  The  Upper  and 
Lower  divisions  consist  mainly  of  sandy  beds,  and  the  Middle 
division,  of  clays.  In  the  London  Basin  they  do  not  contain 
many  fossils,  but  the  corresponding  beds  in  the  Hampshire  Basin — 
the  Bracklesham  Beds  on  the  coast  of  Sussex,  and  Barton  Beds  in 
the  Isle  of  Wight — contain  a  rich  molluscous  fauna  which  includes 
Cardita  sulcata  (Plate  LV.,  fig.  12),  Crassatella  sulcata  (Plate  LV., 
fig.  11),  Valuta  ambigua  (Plate  LV.,  fig.  7),  V.  athleta  (Plate  LV., 
fig.  8),  and  Pleurotoma  dentata. 

The  well-known  Grey  Wethers  of  the  south  of  England  are  tabular 
masses  of  siliceous  cement-stone,  probably  derived  from  portions 
of  the  Bagshot  sands  solidified  by  the  infiltration  of  siliceous  waters. 

Bovey  Tracey  Beds. — These  beds  consist  of  a  series  of  gravels, 
sands,  and  clays  with  seams  of  lignite  lying  in  an  old  lake-basin  in 
the  valley  of  the  Teign,  between  Newton  Abbey  and  Bovey  Tracey, 
in  Devonshire.  They  rest  on  a  highly  eroded  surface  of  the 
Devonian  and  Carboniferous  rocks. 

This  series  of  lacustrine  sediments  varies  from  200  to  300  feet 
thick ;  and  is  interesting  as  representing  the  continental  facies  of 
the  Aquitanian  in  South  England. 

The  fossil  plants  are  numerous  and  frequently  well  preserved. 
They  show  that  the  adjacent  lands  at  the  close  of  the  Oligocene 
were  clothed  with  luxuriant  subtropical  evergreen  forests.  Among 
the  species  are  ferns,  including  Osmunda  and  numerous  angiosperms, 
represented  by  Sequoia,  spindle-trees,  cinnamon,  oak,  fig,  laurel, 
willow,  and  vines. 

Contemporaneous  Volcanic  Activity.— At  the  time  the  streams 
and  rivers  were  discharging  their  load  of  detritus  into  the  Anglo- 
Gallic  sea  and  its  estuaries,  there  was  a  great  display  of  volcanic 
activity  in  the  north  of  Ireland  and  west  coast  of  Scotland.  Succes- 
sive sheets  of  lava  were  poured  over  the  land  and  formed  wide 
basaltic  plateaux. 


as 

. 

. 

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. 

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PLATE   LV. 

EOCENE  FOSSILS. 

1.  Cyrena  cuneiformis  (Sow.).     Woolwich  and  Reading  beds  (Plastic  Clay). 

Charlton,  Upnor,  etc. 

2.  Melania  (Melanatria)  inquinata  (Defr.).     Plastic  Clay.     Woolwich,  New 

Cross,  Plumstead. 

3.  Rostellaria  Sowerbyi  (Aporrhais)  (Mant.).     London  Clay  Series.     Herne 

Bay,  Highgate,  Bognor,  Watford. 

4.  Valuta  nodosa  (Sow.).     Middle  Eocene.     Barton,  etc. 

5.  Phorus   extensus   (Sow.).     London   Clay.     Highgate,   Sheppey,   Bognor, 

etc. 

6.  Pyrula  reticulata  ?     Middle  Eocene.     Bracklesham. 

7.  Voluta  ambigua  (Brand).     Middle  Eocene.     Barton. 

8.  Voluta  athleta  (Sow.).     Middle  Eocene.     Barton,  Bracklesham. 

9.  Terebellum   convolutum    or   sopita    (Brand.).     Middle    Eocene.     Barton, 

Bracklesham. 

10.  Terebellum  fusiforme  (Lam.).     Barton  and  Bracklesham. 

11.  Crassatdla  sulcata  (Sow.).     Middle  Eocene.     Barton. 

12.  Cardita  sulcata  (Brander).     Middle  Eocene.     Barton,  Hordwell. 


To  face  page  436.] 


[PLATE  LV. 


EOCENE  FOSSILS. 


CAINOZOJC    ERA  :     EOCENE    SYSTEM.  437 

The  Antrim  Plateau,  which  is  but  the  remnant  of  a  greater 
plateau,  extends  from  Belfast  Lough  to  Lough  Foyle,  and  from 
the  south  of  Lough  Neagh  to  the  Giant's  Causeway,  and  covers 
an  area  of  about  two  thousand  square  miles.  The  great  cliffs  which 
form  the  striking  coastal  scenery  of  the  famous  Giant's  Causeway 
have  been  carved  out  of  the  edge  of  a  thick  sheet  of  basalt,  which 
exhibits  a  symmetrical  columnar  structure  of  great  beauty  (fig.  125). 

From  Antrim  the  basalts  extend  to  the  Islands  of  Staffa,  Mull, 
Skye,  and  the  other  islands  of  the  Inner  Hebrides.  At  Staffa 
they  form  the  celebrated  Fingal's  Cave. 

These  basalts  were  poured  out  by  successive  eruptions,  some  of 
which  were  separated  by  considerable  intervals  of  quiescence. 
In  these  periods  of  rest  subaerial  denudation  became  active,  and 
the  neighbouring  hollows  and  lagoons  were  soon  filled  with  sands 
and  gravels.  The  surface  of  the  lava-flows  also  became  weathered 
and  disintegrated,  and  formed  soils  on  which  a  rank  semi-tropical 
vegetation  grew  long  enough  to  form  peaty  deposits  that  have  since 
become  changed  into  beds  of  lignite.  Later  eruptions  overwhelmed 
the  forests  and  covered  up  the  newly  formed  sediments  and  peat- 
bogs. 

The  plant  remains  enclosed  in  the  intercalated  soils  and  detrital 
material  comprise  various  palms,  cactus,  oak,  laurel,  and  other 
evergreen  trees  found  in  the  London  Clay. 

It  is  claimed  by  some  writers  that  the  volcanic  region  of  North 
Ireland  and  West  Scotland  belongs  to  the  petrographical  province 
which  includes  the  Faroe  Islands,  Iceland,  and  the  eastern  portion 
of  Greenland. 

Eocene  of  Other  Countries. 

France. — The  Eocene  System  is  very  fully  developed  in  the 
Paris  Tertiary  Basin,  which  is  a  continuation  of  the  English  basins 
across  the  Channel.  The  deposits  are  mainly  marine,  but  beds  of 
estuarine  and  freshwater  deposits  are  also  present.  Molluscs  are 
exceedingly  abundant ;  and  the  genera  Fusus,  Pleurotoma,  and 
Cerithium  are  represented  by  the  greatest  number  of  species. 

French  geologists  recognise  three  divisions,  namely,  the  Lower, 
Middle,  and  Upper. 

The  Lower  Eocene  consists  mainly  of  estuarine  and  terrestrial 
marls,  sands,  and  plastic  clay  with  seams  of  brown  coal. 

The  Middle  Eocene  is  represented  by  a  band  of  impure  shelly 
limestone,  30  feet  thick.  This  is  the  principal  building-stone  in 
Paris,  and  hence  has  become  the  best-known  member  of  the  Eocene 
in  this  basin.  The  lower  beds  of  the  limestone  contain  Nummulites 
in  great  abundance  ;  also  many  sea-urchins  and  a  great  assemblage 
of  Lamellibranchs  and  Gasteropods.  Among  the  last  is  the 


438 


A   TEXT-BOOK    OF   GEOLOGY. 


gigantic  Cerithium  giganteum  (Plate  LVL,  fig.  1),  which  sometimes 
reaches  a  length  of  nearly  three  feet. 

The  Upper  Eocene  is  a  marine  sand  about  48  feet  thick, 
crowded  with  molluscs.  It  is  intercalated  at  St  Ouen  with  a  band 
of  freshwater  limestone  which  contains  a  great  many  examples 
of  Limncea  longiscata. 

Belgium. — The  Eocene  rocks  of  Belgium  show  a  similar  develop- 
ment to  those  of  the  London  and  Paris  Basins,  to  which  they  are 
closely  related.  The  lowest  beds  at  Mons  contain  Cidaris  Tom- 
becki  and  other  Cretaceous  sea-urchins. 

Southern  Europe. — The  Eocene  deposits  laid  down  on  the  floor 
of  the  great  Central  Sea  extend  from  Portugal  eastward  through 
the  Alps,  Apennines,  Carpathians,  and  Caucasus  to  Asia  Minor, 
whence  they  pass  still  further  east  to  the  Himalayas.  On  the 
south  side  of  the  Mediterranean  they  stretch  from  the  Atlas  Moun- 


FIG.  220. — Fragment  of  Nummulitic  Limestone. 

tains  and  Morocco  eastwards  to  Egypt,  Libyan  Desert,  Syria, 
Palestine,  Arabia,  and  Persia,  where  they  merge  into  the  broad 
zone  passing  through  Central  Asia. 

The  deposits  laid  down  in  this  great  inland  sea  belong  to  the 
Alpine  or  Equatorial  facies,  arid  differ  vastly  from  those  of  the 
Northern  facies  as  developed  in  the  Anglo-Gallic  region.  In  the 
first  place,  the  rocks  are  not  loose  and  incoherent,  but  mainlv 
masses  of  compact  limestone  or  hard  sandstones  and  gritty  shales 
of  the  Flysch  type.  Palseontologically  they  are  characterised  by 
the  abundance  of  Nummulites,  which  are  large  disc-shaped 
Fora  minif  era  provided  with  a  complicated  chambered  shell. 

The  Nummulitic  Limestone  of  Upper  Eocene  age  consists  of  a 
series  of  beds  of  massive  limestone,  in  places  3000  feet  thick.  It- 
can  be  traced  from  the  Pyrenees  eastwards  through  the  Alps, 
Carpathians,  and  Balkan  States  to  Asia  Minor.  On  the  south 
side  of  the  Mediterranean,  it  stretches  far  south  into  the  Sahara, 
Libyan  Desert,  and  Egypt.  From  Arabia  and  Asia  Minor,  as 
already  described,  it  passes  eastward  to  the  Himalayas  and 


To  face  page  439.] 


[PLATE  LVI. 


EOCENE  FOSSILS. 


PLATE   LVI. 
EOCENE  FOSSILS. 

1.  Cerithium   giganteum    (Lam.).     Middle    Eocene.      Bracklesham,    Grignon, 

etc.,  France. 

2.  Rostellarin  (Hippocrenes)  macroptera  (Lam.)  =  R.  ampla  (Brander).     Middle 

Eocene.      Barton,  Bracklesham.      Lower  Eocene  (L.  Clay).      Kingston 
and  Whetstone. 

3.  TypJiis  pungens  (Brander).     Middle  Eocene.     Barton,  Alum  Bay  (Isle  of 

Wight). 

4.  Cardita  (  V enericardia)  planicosta  (Lam.).      Middle  Eocene.      Bracklesham. 

5.  Nummulites  Icevigatus  (Brongn.).     Middle  Eocene.     Bracklesham,  Isle  of 

Wight. 


'•'••'• 


M&TO' 


CAINOZOIC    ERA  :     EOCENE    SYSTEM.  439 

Further  India,  and  thence  through  Java,  Sumatra,  Borneo,  and 
Philippines.  In  Egypt  it  was  the  chief  source  of  the  building- 
stone  used  in  the  construction  of  the  Pyramids,  near  Cairo. 

India. — The  Eocene  of  India  belongs,  as  we  have  seen,  to  the 
Southern  or  Equatorial  facies,  and  was  laid  down  in  the  eastern 
end  of  the  Central  Sea.  It  is  principally  characterised  by  the 
great  development  of  the  nummulitic  limestones. 

The  Eocene  of  India  is  divided  into  three  stages  or  series  : — 

Upper  Eocene  (3) — Khirthar  Series. 
Middle  Eocene  (2) — Laki  „ 

Lower  Eocene  (1) — Ranikot       „ 

The  Ranikot  Series  consists  mainly  of  basal  fluviatile  sandstones 
followed  by  marine  beds.  It  is  restricted  to  a  small  area  in 
Scind. 

The  Laki  Series  is  in  some  places  sandy,  in  others  calcareous. 
The  well-known  Laki  Limestone  abounds  in  Foraminifera,  of  which 
the  genera  Nummulites  and  Alveolina  are  prominent.  This  series 
is  of  great  economic  importance  for  its  valuable  coal-seams,  which 
are  well  developed  in  the  Punjab,  Assam,  and  Baluchistan. 

The  Khirthar  Series  consists  chiefly  of  limestones  which,  on  the 
Scind-Baluchistan  border,  attain  a  thickness  of  3000  feet,  and 
contain  some  zones  remarkably  rich  in  Nummulites,  among 
which  Nummulites  Icevigatus  (Plate  LVL,  fig.  5),  N.  perforatus,  and 
N.  complanata  are  the  most  common. 

North  America. — Rocks  of  Eocene  age  cover  large  tracts  in  the 
United  States,  and  in  the  main  follow  the  outcrops  of  the 
Cretaceous,  although  they  occupy  a  much  smaller  area  as  a  result 
of  the  recession  of  the  sea  which  followed  the  close  of  the  Cretaceous. 

They  comprise  three  distinct  types  of  deposits,  each  occupying 
a  separate  geographical  region.  The  marine  type  is  distributed 
as  a  fringe  along  the  Atlantic  border  and  Gulf  of  Mexico  ;  the 
brackish-water  occurs  mainly  in  Washington  and  Oregon  ;  and 
the  freshwater  or  lacustrine  occupies  old  lake-basins  among  the 
mountains  of  the  Western  States. 

The  marine  Eocene  beds  are  typically  developed  in  the  Gulf 
region,  and  are  perhaps  best  displayed  in  the  State  of  Alabama, 
where  three  divisions  are  recognised,  each  richly  fossiliferous. 

The  Eocene  beds  of  Texas  are  mainly  estuarine  and  partly 
terrestrial,  the  latter  facies  interstratified  with  layers  of  salt  and 
gypsum-bearing  sediments  and  seams  of  lignite.  From  Texas 
these  beds  extend  northwards  to  Arkansas. 

The  Puget  Sound  coal-bearing  series  of  Washington  is  estuarine 
and  terrestrial.  It  attains  a  thickness  variously  estimated  at 
from  10,000  to  20,000  feet, 


440  A  TEXT-BOOK  OF  GEOLOGY. 

Numerous  patches  of  coal-bearing  strata  of  Eocene  age,  usually 
much  disturbed,  are  scattered  around  the  coastal  fringe  and 
maritime  valleys  of  Alaska. 

The  marine  fauna  of  the  North  American  Eocene  System  is 
remarkably  rich  in  molluscs,  among  which  Lamellibranchs  and 
Gasteropods  largely  predominate.  But  the  most  notable  feature 
of  this  period  is  the  sudden  appearance  of  numerous  placental 
mammals,  many  of  which  possess  structural  features  that  seem 
to  connect  them  with  the  placentals  of  the  present  day. 

Among  the  primitive  types  of  land  placentals  have  been  found 
what  are  believed  to  be  ancestral  forms  of  the  rhinoceros,  deer, 
horse,  tapir,  cat,  dog,  otter,  badger,  etc. 

The  Eocene  seas  were  also  peopled  with  the  earliest  marine 
mammals,  the  cetaceans  being  represented  by  whales  (Zeuglodons), 
dolphins,  and  porpoises  ;  the  sirenians  by  the  dugong  ;  and  the 
pinnipeds  by  seals  and  sea-lions. 

The  Eocene  vegetation,  as  in  Europe,  indicates  a  temperate 
climate  in  the  early  part  of  the  period,  followed  by  semi-tropical 
conditions  in  the  Middle  and  Upper  Eocene. 

It  is  noteworthy  that  many  of -the  leading  types  of  plant  life 
in  the  Eocene  of  North  America  and  Europe  are  allied  to  types  that 
still  survive  in  India  and  Australia. 

Australasia. — Marine  deposits  of  Eocene  age  are  unknown 
throughout  Eastern  Australia,  but  are  well  developed  at  Mount 
Gambier  and  Murray  Flats  in  South  Australia,  where  they  contain 
a  rich  fauna  which  includes  numerous  corals,  sea-urchins,  brachio- 
pods,  Lamellibranchs,  and  Gasteropods,  as  well  as  the  remains  of 
fishes  and  cetaceans. 

Eocene  deposits  are  well  developed  in  Tasmania  and  New 
Zealand.  In  the  latter  they  contain  seams  of  bituminous  coal 
of  great  economic  value. 

OLIGOCENE   SYSTEM. 

The  Oligocene  System  was  formerly  regarded  as  the  uppermost 
portion  of  the  Eocene,  with  which  it  is  always  intimately  connected 
where  the  full  Lower  Tertiary  succession  is  present.  In  this  case 
the  condominion  is  perfect.  The  separation  has  no  palaeonto- 
logical  or  lithological  basis,  and  was  mainly  made  in  deference  to 
geographical  considerations. 

In  South  England,  where  the  Eocene  is  so  well  developed,  the 
Oligocene  is  poorly  developed  and  the  Miocene  is  altogether  absent  ; 
but  in  Germany,  where  the  Eocene  is  absent,  the  Oligocene  is  an 
important  formation.  From  thisTwe^flearnTthat  considerable 
geographical^changes  took  place  in  North- West  Europe  at  the  close 


CAINOZOIC   ERA  :     OLIGOCENE    SYSTEM.  441 

of  the  Eocene.  South-East  England  and  North  France,  which  were 
then  covered  by  the  sea,  became  dry  land,  and  Germany,  which  was 
dry  land,  became  inundated  by  the  sea.  The  uplift  in  England 
was  balanced  by  subsidence  in  Germany. 

It  was  principally  owing  to  these  changes  of  land  and  sea  that 
it  was  considered  convenient  to  separate  the  Oligocene  from  the 
Eocene  in  England  and  Continental  Europe.  The  Oligocene  is 
not  a  natural  division  ;  and  in  regions  outside  Europe  where  the 
marine  Tertiary  succession  is  complete,  the  formations  arrange 
themselves  into  the  three  natural  divisions,  Eocene,  Miocene,  and 
Pliocene. 

Distribution. — Except  in  Germany  where  the  Lowest  Tertiary 
beds  are  absent,  the  Oligocene  occupies  the  same  areas  as  the 
Eocene.  In  Europe  it  occurs  in  two  distinct  geographical  provinces, 
namely,  the  Northern  and  the  Southern. 

The  Northern  Province  embraces  the  greater  portion  of  the 
Anglo- Gallic  basin,  and  the  whole  of  Germany  except  the  Alpine 
portion. 

The  Southern  Province  covers  the  region  now  occupied  by  the 
Alps,  Apennines,  and  Carpathians. 

In  the  Northern  Province  the  sediments,  as  in  the  Eocene,  are 
mainly  represented  by  loose  sands,  clays,  and  pebbly  beds  with 
thin-bedded  limestones  ;  and  in  the  Southern  by  sandstones  and 
shales  of  the  Flysch  type,  together  with  massive  beds  of  soft  pebbly 
sandstones  and  coarse  conglomerates,  which  constitute  the  series 
of  deposits  called  Molasse  by  Swiss  geologists.  The  lower  portion 
of  the  Molasse,  called  Older  Molasse,  is  referred  to  the  Oligocene,  and 
the  upper  portion  to  the  Miocene. 

From  the  Carpathians  eastwards  through  the  Balkans.  Asia 
Minor,  Arabia,  Persia.  Baluchistan,  the  Himalayas,  and  Burma, 
the  Oligocene  is  co-extensive  with  the  Eocene. 

In  Baluchistan  and  Scind,  the  strata  are  mainly  unfossiliferous 
shales  of  the  Flysch  facies,  and  massive  beds  of  nummulitic  and 
coralline  limestones. 

In  North  America  the  Oligocene  has  not  been  separated  from  the 
Eocene. 

British  Isles. 

The  Oligocene  strata  play  an  unimportant  part  in  Britain,  and 
are  confined  to  a  small  area  in  the  Isle  of  Wight  and  Hampshire 
Basin,  where  they  rest  conformably  on  the  Eocene.  They  are  not 
represented  in  the  London  Basin. 

The  deposits  consist  mainly  of  sand,  clays,  marls,  and  thin- 
bedded  limestones,  and  they  are  partly  marine,  partly  estuarine, 
and  partly  freshwater.  They  were  obviously  laid  down  in  the 


442  A  TEXT-BOOK  OF  GEOLOGY. 

Hampshire  Eocene  delta  at  a  time  when  distinct  but  inconsiderable 
oscillations  of  the  land  were  in  progress. 

The  subdivisions  or  stages  of  the  Oligocene  of  South  England  are 
as  follow  : — 

4.  Hamstead  Beds. 


Oligocene^   ~  rr"     "*g  ^ 
I  2.  Osborne  Beds. 

1.  Headon  Beds. 

Marine  Beds  are  intercalated  with  the  Headon,  Bembridge,  and 
Hamstead  Beds.  The  remaining  beds  are  mainly  deltaic  and 
freshwater.  In  the  Headon  Beds  there  occur  seams  of  lignite. 

Fossils  are  found  in  all  the  different  divisions,  and  are  abundant 
on  some  horizons. 

The  land  snails  include  Helix  and  Amphidromus  ;  and  among 
the  common  forms  of  pond  and  river  molluscs  are  the  Lamelli- 
branchs  Unio  and  Cyrena  ;  and  of  the  Gasteropods  Viviparus, 
Limncea,  and  Planorbis. 

The  fossil  vertebrates  include  the  remains  of  rays  (Myliobates), 
sea-snakes,  crocodiles,  alligators,  turtles,  and  a  whale  (Balcenoptera). 

The  Bembridge  Beds  have  yielded  the  bones  of  many  mammals, 
including  those  of  the  pachyderms  Palceotheriiim,1  Anoplotherium,2 
Hyopotamus?  and  Chceropotamus. 

The  fossil  plants  are  mostly  subtropical  evergreens,  such  as  fan- 
palms,  feather-palms,  conifers,  oaks,  laurels,  and  vines. 

The  nucules  of  Chara  (Plate  LVIL,  fig.  11),  a  freshwater  alga, 
are  plentiful  in  the  Bembridge  Limestone  in  the  Isle  of  Wight. 

The  marine  and  brackish- water  beds  contain  Ostrea,  Cyrena, 
Cytherea,  Cerithium,  Melania,  and  many  other  molluscs.  Of  the 
Middle  Headon  division  Cytherea  incrassata  is  the  characteristic 
fossil,  and  Ostrea  velata  forms  thick  beds.  In  the  Osborne  Beds 
Melania  excavata  is  a  common  form. 

Continental  Europe. 

The  Oligocene  is  fully  developed  in  the  Paris  Basin,  where  only 
the  middle  division  is  marine  ;  in  Belgium,  where  the  strata  consists 
of  alternations  of  marine  and  freshwater  deposits  ;  and  in  North 
Germany,  where  the  beds  are  essentially  marine. 

Among  the  fossils,  land,  freshwater,  estuarine,  and  marine 
molluscs  are  plentiful,  and  also  the  remains  of  plants,  fishes,  and 
mammals. 

The    Lower   Oligocene  of  the  Paris   Basin  is  characterised  by 

1  Gr.  palaios= ancient,  and  therion  =  a,n  animal. 

2  Gr.  anaplos  =  unarmed,  and  therion  =  a,n  animal. 

3  Gr.  hus  —  a.  log,  and  potamits  =  Si  river. 


To  face  page  443.] 


[PLATE    LVII. 


I 


OLIGOCENE  FOSSILS. 


PLATE   LVII. 

OLIGOCENE  FOSSILS. 

1.  Paludina  orbicularis.     Bembridge  Limestone.     Isle  of  Wight. 

2.  Helix  occlusa  (Edw.).     Sconce  and  Headon. 

3.  Rissoa  Chastellii  (Nyst.).     Hempstead,  Isle  of  Wight. 

4.  Paludina    lenta    (Brander).     White  Cliff,    Hempstead.     Middle  Eocene. 

Hordwell. 

5.  Cerithium  elegans  (Desh.).     Hempstead  Cliff,  Isle  of  Wight. 

6.  Cerithium  plicatum  (Lam.).     Hempstead. 

7.  Bulimus    ellipticus    (Sow.).      Bembridge    Limestone.     Isle    of    Wight. 

Half  natural  size. 

8    Planorbis  discus  (Edw.).     Bembridge  Limestone.     Isle  of  Wight. 
9.  Cyrena  semistriata  (Desh.).     Hempstead  Cliff,  Isle  of  Wight. 

10.  Corbula  pisum  (Sow.).     Hempstead  Cliff,  Isle  of  Wight. 

11.  Chara  tuberculata  (Seed- Vessel).     Bembridge  Limestone.     Isle  of  Wight. 


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CAINOZOIC    ERA  :     OLIGOCENE    SYSTEM.  443 

mammalian  remains,  which  include  Palceotherium,  Anoplotherium, 
etc. ;  first  described  by  Cuvier. 

The  Middle  Oligocene  of  the  Paris  Basin  and  Ehine  Valley  con- 
tains a  rich  fauna,  which  includes  Cerithium  plicatum  (Plate  LYIL, 
fig.  6),  Buccinum  cassidaria,  Cyrena  semistriata,  and  Leda 
Deskayesiana.  The  last  is  also  characteristic  of  the  Septaria  Clay, 
which  is  a  well-marked  member  of  the  German  Middle  Oligocene. 

The  Upper  Oligocene  of  the  Paris  Basin  is  freshwater,  and  con- 
tains numerous  species  of  Helix,  Viviparus,  Planorbis,  and 
Limncea.  The  marine  equivalent  of  these  beds  in  the  Mainz 
Basin  of  the  Khine  Valley  contains  Cerithium  plicatum,  Perna 
Soldani,  and  other  molluscs. 

The  Flysch  sandstones  and  shales  of  the  Southern  European 
Province  are  practically  unfossiliferous,  but  contain  at  Glarus  a 
bed  of  slaty  shale  crowded  with  well-preserved  fish  remains. 

The  Molasse  Series  of  Switzerland  rises  into  high  picturesque 
mountains,  as  in  the  Rigi  and  Rossland.  It  consists  essentially  of 
fluviatile  drifts  discharged  into  a  freshwater  lake-basin.  In  many 
places  its  sediments  have  preserved  numerous  remains  of  the  vegeta- 
tion which  clothed  the  slopes  of  the  surrounding  ranges.  Curiously 
enough,  the  plants  include  palms  related  to  an  American  type,  the 
Californian  conifer  Sequoia,  as  well  as  the  alder,  fig,  cinnamon,  oak, 
and  many  other  evergreen  forest  trees. 

The  geographical  names  usually  applied  to  the  Oligocene  of 
France,  Belgium,  Switzerland,  North  Italy,  and  regions  where 
that  system  has  been  recognised  are  as  follow  : — 

Upper  Oligocene — 3.  Aquitanian,  so  named  from  Aquitania. 
Middle  Oligocene — 2.  Stampian,  „  fitampes. 

Lower  Oligocene — 1.  Tongrian,  ,,  Tongres  in 

Limbourg. 

Perhaps  the  most  interesting  Oligocene  deposits  in  Europe  are 
the  amber-bearing  beds  near  Konigsberg  in  Pomerania.  The  lower 
beds  consist  of  glauconitic  sands  with  a  rich  Lower  Oligocene  fauna, 
which  includes  molluscs,  sea-urchins,  numerous  crustaceans,  and 
the  teeth  of  sharks  ;  and  the  upper  beds  of  lignite-bearing  sands. 

The  amber  for  which  these  beds  have  long  been  famous  is  the 
fossil  resin  of  various  pines,  especially  of  Pinus  succinifera.  It  is 
found  in  what  is  locally  called  the  Blue  Earth,  not  far  from  the 
base  of  the  glauconitic  sands. 

The  scientific  importance  of  the  amber  depends  on  the  remark- 
able number  of  insects,  spiders,  etc.,  and  plant  remains  enclosed 
in  it,  usually  in  a  perfect  state  of  preservation.  Altogether  over 
2000  species  of  insects  ;  and  of  plants,  over  100  Dicotyledons  alone 
have  been  identified ;  also  palms  and  other  Monocotyledons. 


444  A  TEXT-BOOK  OF  GEOLOGY. 

India. 

The  Oligocene  is  represented  in  Baluchistan  by  the  Kojak  Shales 
of  the  Flysch  facies,  and  in  Scind  and  Baluchistan  by  massive  beds 
of  fossiliferous  marine  strata,  which  are  divided  into  two  series,  the 
Nari  and  Gaj  : — 

Lower  Miocene —  3.  Mekran. 

Oligocene 

The  Nari  Series  includes  massive  beds  of  Nummulitic  Limestone, 
which  are  particularly  rich  in  large  Foraminifera,  some  of  which 
attain  a  diameter  of  several  inches. 

The  Gaj  Series  consists  of  shales  and  coral  limestones. 

Origin  of  the  Flysch  Facies  of  Deposits. 

Deposits  of  the  Flysch  type  are  conspicuous  in  the  structure  of 
the  Alps,  Carpathians,  Caucasus,  and  mountain  systems  of  Persia, 
Baluchistan,  and  Northern  India. 

They  consist  of  alternating  grey  sandstones  and  gritty  shales,  or 
simply  of  shales  alone,  and  are  everywhere  conspicuously  unfossili- 
ferous.  Bands  of  fossiliferous  calcareous  strata  are  in  a  few  places 
intersected  with  them,  and  in  the  adjacent  areas  they  are  frequently 
associated  with  massive  beds  of  marine  limestones. 

The  Flysch  Series  comprises  a  pile  of  strata  which  ranges  in  age 
from  the  Upper  Cretaceous  to  the  Oligocene.  They  are  obviously 
composed  of  deltaic  detritus  that  accumulated  on  the  borders  of 
the  Central  Sea  at  the  mouths  of  the  rivers  draining  the  great 
northern  continent.  The  detritus  was  not  spread  out  as  a  con- 
tinuous sheet  along  the  shores  of  the  Central  Sea,  but  was  piled  up 
to  a  great  thickness  in  the  deltaic  areas.  At  its  outward  fringes 
the  material  was  in  places  intercalated  with  thin  sheets  of  marine 
sediments. 

In  the  clearer  and  deeper  waters  of  the  adjacent  parts  of  the 
Central  Sea  there  also  accumulated,  contemporaneously  with  the 
deltaic  sediments,  the  thick  deposits  of  calcareous  sediments,  which 
now  form  the  massive  beds  of  Nummulitic  Limestone  so  con- 
spicuous in  the  Alps,  Carpathians,  Baluchistan,  and  Himalayas. 

The  character  of  the  sediments  composing  the  Flysch  Series,  and 
the  absence  of  organic  remains,  would  tend  to  show  that  there  was 
a  considerable  rainfall  all  over  the  northern  continent,  attended 
with  rapid  denudation  of  the  land,  and  a  correspondingly  rapid 
accumulation  of  fluviatile  detritus — so  rapid  as  to  preclude  the 
existence  of  living  organisms  within  the  area  of  deposition. 


CAINOZOIC    ERA  :     OLIGOCENE    SYSTEM.  445 

The  total  thickness  of  the  Flysch  sandstones  and  shales  has  been 
variously  estimated  at  from  10,000  to  20,000  feet ;  but  the  thick- 
ness of  sediments  of  this  character  cannot  be  determined  by 
measurements  taken  across  the  apparent  bedding  planes. 

When  detrital  material  is  shot  into  comparatively  deep  water, 
as  the  reclamation  of  the  fringe  of  the  basin  proceeds,  it  is  pushed 
further  and  further  into  the  basin,  each  successive  layer  assuming 
its  proper  angle  of  rest.  With  the  constant  recurrence  of  floods, 
sands  and  muds  overspread  and  succeed  one  another  in  a  pile  of 
alternating  sheets. 

When  fluviatile  deposition  of  this  kind  begins  on  a  sea-littoral 
where  marine  deposition  has  previously  been  in  progress,  the  first 
layers  of  fluviatile  detritus  discharged  into  the  sea  are  laid  down 
parallel  with  the  bedding-planes  of  the  marine  deposits  ;  but  when- 
ever the  filling  in  has  been  carried  forward  so  far  that  there  is  a 
sudden  drop  into  deep  water,  the  bedding-plane  of  the  detritus 
becomes  parallel  with  the  angle  of  rest  assumed  by  the  material 
as  it  falls  over  the  end  of  the  advancing  delta. 

The  sediments  laid  down  in  the  shallow  waters  of  the  delta  are 
horizontal  or  possess  a  gentle  slope  seaward,  but  those  discharged 
in  the  deeper  waters  at  the  outer  edge  of  the  delta  assume  an  angle 
of  30°  or  more.  It  should,  however,  be  noted  that  it  is  only  where 
the  accumulation  of  fluviatile  detritus  is  relatively  rapid  that  this 
end-tipping,  which  is  merely  an  exaggerated  form  of  false-bedding, 
is  found.  The  process  can  be  advantageously  studied  in  the  inland 
lake-basins  of  New  Zealand,  North  America,  and  elsewhere. 

The  Central  Sea  or  Tethys. 

This  great  inland  sea  is  the  most  striking  and  important  geograph- 
ical feature  of  the  Mesozoic  and  Cainozoic,  and  the  sediments  laid 
down  on  its  floor  and  borders  constitute  nearly  a  complete  geological 
history  of  the  Old  World  in  these  eras. 

This  great  sea  was  first  outlined  during  the  orogenic  movements 
of  the  Carboniferous  period  as  a  deep  corrugation  or  basin,  running 
east  and  west  in  the  latitudes  of  Southern  Europe,  and  lying  between 
the  northern  and  southern  continents,  into  which  the  Old  World  at 
this  time  became  divided. 

On  the  floor  of  the  seas  occupying  this  great  corrugation  was  laid 
down  the  Permo-Jurassic  succession  of  conformable  strata,  so 
largely  represented  in  the  structure  of  the  Alps  and  Himalayas. 
But  it  was  not  until  the  Jurassic  that  the  Central  Sea  formed  a 
continuous  sea  and  completely  severed  the  northern  Russo- 
Siberian  continent  from  the  southern  or  Gondwana-Land  con- 
tinent. 


446  A  TEXT-BOOK  OF  GEOLOGY. 

Before  the  close  of  the  Cretaceous  the  Central  Sea  extended  from 
the  Atlantic  eastward  through  Southern  Europe  to  Further  India 
and  Burma.  It  girdled  half  the  globe  with  its  length  of  9000 
miles,  and  its  width  varied  from  1000  to  2500  miles. 

Gondwana  Land  on  the  southern  shores  collapsed  sometime 
about  the  Middle  Cretaceous,  perhaps  contemporaneously  with 
the  great  Cenomanian  Transgression,  and  Central  and  Eastern 
Africa,  a  portion  of  Peninsular  India,  Malaysia,  Australia,  and  New 
Zealand  are  all  that  now  remain  to  mark  its  former  extent.  Of  the 
surface  forms  of  this  great  continent  which  covered  so  large  a 
portion  of  the  present  Indian  Ocean,  or  of  the  outlines  of  the  Central 
Sea  which  washed  its  shores,  we  have  little  remaining  evidence. 

On  its  north  side  the  Central  Sea  was  deeply  indented  with  bays 
and  great  estuaries,  into  which  the  rivers  draining  the  more  per- 
manent northern  continent  discharged  their  loads  of  detritus. 

These  northern  rivers,  throughout  the  Cretaceous,  Eocene,  and 
Oligocene  periods,  reclaimed  large  deltaic  areas  on  the  fringe  of  the 
sea.  In  the  clearer  waters  beyond  the  reach  of  the  detritus,  marine 
sediments  were  laid  down,  largely  composed  of  marine  organisms, 
among  which  Nummulites  predominated.  From  their  nature  these 
marine  deposits  accumulated  more  slowly  than  the  deltaic  or  Flysch 
detrital  sediments,  and  hence  did  not  attain  the  same  great 
thickness. 

In  this  manner  the  Flysch  and  marine  facies  of  deposits  were 
formed  contemporaneously  in  the  same  continuous  sea,  the  deltaic 
as  detached  but  extensive  deposits  on  the  northern  coasts,  the 
marine  as  continuous  sheets  that  wrapped  round  the  deltaic  and 
extended  seaward  into  the  clear  waters. 

That  the  waters  of  the  Central  Sea  were  clear  and  warm,  except 
in  the  deltaic  areas,  up  till  the  close  of  the  Oligocene  period  is  shown 
by  the  abundance  of  reef-building  corals,  sea-urchins,  and  the  large 
size  of  the  molluscs  and  Foraminifera. 

The  Baltic  Sea.— At  the  close  of  the  Carboniferous  there  came 
into  existence  a  northern  but  smaller  sea,  running  nearly  parallel 
with  the  Central  Sea.  This  sea  followed  the  Baltic  depression,  and 
extended  eastward  to  the  Urals  ;  and  although  varying  in  width 
and  extent  at  different  periods,  it  continued  an  area  of  deposition 
up  to  the  close  of  the  Miocene.  The  Baltic  is  a  remnant  of  that 
ancient  sea. 

Up  till  the  Trias  it  was  separated  from  the  Central  Sea  by  a 
narrow  ridge,  but  in  the  Rhsetic  this  separation  ceased,  and  was 
not  renewed  until  the  Cretaceous  period.  From  that  time  onward 
the  separation  continued  to  be  more  or  less  complete,  and  the 
dividing  chain  was  broader  than  at  any  former  time. 


To  face  page  446.] 


r 


[PLATE    LVIII. 


$9  !§  ^ 


FORAMINIFERA.       WAITEMATA    BEDS,    AUCKLAND,    NEW    ZEALAND        LOWER 

TERTIARY.     (After  Karrer.) 


Fig.  l 

*>    2 

,,    3 

,    4 


Dentalina  cequcdis^  Karr.  Fig.  12. 

Vaginulina  recta,  Karr.  ,,  13. 

Lingulina  costata,  d'Orb.  ,,  14. 

Marginulina  neglecta,  Karr.  , ,  15. 

Cristellaria  mammilligera,  Karr.  ,.  16. 

Robulina  regina,  Karr.  ,,  17. 

Textilaria  Hayi,  Karr.  ,,  18. 

Textilaria  con vexa,  Karr.  „  19. 

Textilaria  minima,  Karr.  ..  20. 

Orbitulites  incertus,  Karr.  .,  21. 
Clavulina  elegans,  Karr. 


Rotalia  Novo-Zelandica,  Karr. 
Rotalia  perforata,  Karr. 
Rosalina  Makayi,  Karr. 
Polystomella  Fichtelliana,  d'Orb. 
Polystomella  tennissima.  Karr. 
Nonionina  simplex,  Karr. 
Amphistegina  Campbetti,  Karr. 
Amphistegina  Aucklandica,  Karr. 
Amphistegina  ornatissima,  Karr. 
Orbitoides  Ornkeiensis,  Karr. 


CHAPTER   XXXII. 
MIOCENE  AND  PLIOCENE. 

Miocene. 

THE  Miocene  Period  witnessed  great  changes  in  the  distribution 
of  land  and  water  in  Europe  and  Asia,  including  the  uplift  of  the 
Pyrenees,  Alps,  Carpathians,  and  Himalayas. 

At  the  close  of  the  Miocene  the  main  physiographic  features  of 
the  Old  World,  as  we  now  know  them,  were  clearly  defined.  The 
British  Isles  and  North  France,  where  Miocene  deposits  are  absent, 
formed  dry  land ;  and  Germany,  which  was  mostly  covered  with 
the  sea  in  the  Oligocene  Period,  now  became  continental. 

In  Northern  Europe  there  was  widespread  uplift,  and  in  Southern 
Europe  the  gigantic  erogenic  movements  which  folded  the  Alps 
and  Carpathians  were  now  in  progress.  The  Eocene  and  Oligocene 
strata,  both  marine  and  deltaic,  laid  down  on  the  floor  and  fringe 
of  the  Central  Sea,  became  involved  in  the  huge  crustal  folds  of 
these  chains.  Moreover,  the  corrugations,  dislocations,  and  over- 
thrusts  which  accompanied  the  crustal  movements  cut  off  portions 
of  the  sea,  that  eventually  became  freshwater  lakes. 

The  North  Sea  now  came  into  existence,  and  marine  deposits 
were  laid  down  on  its  floor  in  North  Germany,  in  the  States  of 
Schleswig-Holstein  and  Friesland. 

Wide  arms  of  the  Atlantic  filled  the  basins  of  the  Loire  and 
Garonne,  and  a  long  prolongation  of  the  Central  Sea  spread  north- 
ward to  the  Mainz  Basin  in  the  Rhine  Firth,  which  at  this  time 
drained  southward,  and  thence  passed  eastwards  through  Upper 
Bavaria  to  the  Vienna  Basin.  Another  arm  of  the  Miocene  sea 
stretched  round  the  Carpathians  into  Moravia,  while  a  greater 
prolongation  spread  over  South  Russia,  following  a  depression,  of 
which  the  Black  Sea  and  Caspian  are  the  remaining  portions. 

The  Alps  and  Carpathians  were  thus  completely  surrounded  by 
the  sea,  and  stood  up  as  rugged  chains  near  the  northern  coasts  of 
the  Central  Sea. 

On  its  south  side  the  Central  Sea  still  overspread  large  tracts 

447 


448  A    TEXT-BOOK    OF    GEOLOGY. 

of  North- West  Africa,  and  overflowed  the  maritime  lowlands  of 
Spain  and  Portugal. 

The  crustal  movements  which  formed  the  Alps  and  Carpathians, 
at  the  same  time  raised  the  floor  of  the  Central  Sea  in  the  areas  now 
occupied  by  Egypt,  Syria,  Arabia,  Asia  Minor,  and  Persia,  and 
uplifted  the  mountains  of  Baluchistan  and  the  Himalayas. 

This  general  Miocene  uplift  broke  up  the  Central  Sea,  and 
detached  the  Mediterranean  portion  from  the  Indian  Ocean. 

The  only  record  of  these  wide-spread,  mountain-building,  crustal 
movements  to  be  seen  in  England  is  the  rnonoclinal  fold  of  the 
Eocene  and  Oligocene  strata  on  the  north  side  of  the  Isle  of  Wight, 
to  which  reference  has  already  been  made  (fig.  73). 

In  North  America  the  Miocene  strata  on  the  Atlantic  border  and 
gulf  region  lie  undisturbed,  but  in  the  Pacific  States  they  are  acutely 
folded,  and  in  places  uplifted  to  a  great  height  in  the  Rocky  Moun- 
tain chain.  On  the  Atlantic  side  of  the  continent,  there  was  a 
slight  emergence  of  the  land,  as  shown  by  the  narrower  limits  of  the 
Miocene  as  contrasted  with  the  underlying  Eocene. 

Fauna  and  Flora. — The  Miocene  flora  of  Europe  shows  a  closer 
relationship  to  the  existing  evergreen  floras  of  India,  Australia, 
and  New  Zealand  than  to  the  existing  European  deciduous  flora. 
Among  the  characteristic  genera  of  the  Lower  Miocene  are  numerous 
palms,  magnolias,  myrtles,  laurels,  vines,  etc.,  which  indicate  the 
prevalence  of  a  warm,  subtropical  climate.  The  absence  of  palms, 
and  the  presence  of  such  hardy  trees  as  the  oak,  elm,  beech,  etc., 
in  the  Upper  Miocene,  show  the  advent  of  cooler  conditions  in  the 
later  half  of  this  period. 

The  character  of  the  molluscous  fauna  confirms  the  evidence  of 
the  flora.  Among  the  common  genera  we  have  Murex,  Ancillaria, 
Cassis,  Mitra,  Terebra,  Area,  Mactra,  Panopcea,  Pectunculus, 
Tapes,  Tellina,  Dosinia,  and  other  forms  that  abound  in  warm 
seas. 

In  Spitzbergen,  Iceland,  Greenland,  and  North  Alaska,  the  fossil 
plants  are  also  subtropical,  but  in  Japan,  Kamtschatka,  Saghalien, 
and  Eastern  Siberia  the  flora  indicates  a  somewhat  cooler  tempera- 
ture than  the  present. 

The  distinguishing  feature  of  the  Miocene  fauna  is  the  appearance 
of  the  gigantic  Proboscidians,  Deinotherium  and  Mastodon,  the  last 
related  to  the  true  elephant,  which  did  not  appear  until  later. 
Among  other  Miocene  mammals  are  many  species  of  rhinoceros, 
hippopotamus,  and  deer  ;  also  whales  and  dolphins. 

The  Miocene  mammalian  fauna  is  prolific  and  varied,  and  marked 
by  a  conspicuous  development  of  the  ungulates  and  carnivores. 
The  rodents  are  not  so  prominent  as  in  the  Eocene  ;  and  the 
insectivores  and  lemuroids  show  a  notable  decline. 


CAINOZOIC    ERA  I     MIOCENE    SYSTEM.  449 

The  marine  mollusca  show  an  increasing  relationship  to  existing 
forms.  In  Europe  and  North  America  frequently  a  third  of  the 
fossil  species  are  living  forms. 

The  zonal  distribution  of  the  marine  faunas  is  conspicuous 
both  in  North  America  and  Europe.  Generally  the  Northern  or 
Maryland  fauna  is  related  to  the  North  European,  and  the  Gulf 
fauna  to  the  Mediterranean. 

Continental  Europe. 

France. — Deposits  of  Miocene  age  are  unknown  in  Britain,  but 
they  are  typically  developed  in  the  district  of  Touraine,  traversed 
by  the  rivers  Loire,  Indre,  and  Cher.  Here  they  occur  in  isolated 
and  widely-separated  patches  of  sediments  that  are  mostly  marine. 
They  form  a  sheet  which  seldom  exceeds  a  thickness  of  50  feet,  and 
is  locally  known  asfaluns,  from  its  fertilising  qualities  as  a  dressing 
for  the  land. 

This  deposit  contains  numerous  corals,  and  over  300  species  of 
molluscs,  of  which  about  25  per  cent,  are  identical  with  living 
forms. 

The  mammals  include  Rhinoceros,  Hippopotamus,  Cheer  opotamus, 
and  Mastodon. 

Vienna  Basin. — This  great  basin  is  bounded  by  the  Eastern  Alps, 
the  plateau  of  Bohemia  and  Moravia,  and  the  Western  Carpathians. 
The  group  of  beds  contained  in  it  are  divided  into  three  series  : — 

3.  Pontian  Series. 
2.  Sarrnatian  Series. 
1.  Mediterranean  Series. 

This  basin  began  as  a  salt-water  sea  and  gradually  became  fresh- 
water. 

The  deposits  of  the  Mediterranean  Series  vary  considerably  in 
different  places.  Generally  they  consist  of  limestones,  clays,  marls, 
and  sandstones.  The  Leithakalk  Limestone  consists  mainly  of 
reef-building  corals,  bryozoans,  and  Foraminifera.  It  also  contains 
numerous  sea-urchins,  pectens,  shark's  teeth,  and  mammalian 
remains. 

The  sandstones  are  mostly  freshwater,  and  with  them  are 
associated  the  brown  coals  of  the  Vienna  Basin. 

Marine  molluscs  are  particularly  abundant  in  this  series,  and  of 
1000  species  many  still  survive  in  the  Mediterranean  and  west 
coast  of  Africa. 

The  flora,  like  the  fauna,  possesses  a  subtropical  aspect. 

The  Sarmatian  is  mainly  composed  of  fresh-  and  brackish- water 
sediments,  with  some  intercalating  marine  beds,  from  which  it 

29 


450  A  TEXT-BOOK  OF  GEOLOGY. 

would  appear  that  the  general  uplift  of  this  stage  was  accompanied 
by  minor  oscillations. 

Corals,  bryozoans,  Foraminifera,  and  sea-urchins,  so  abundant 
in  the  underlying  stage,  are  rare  in  the  Sarmatian,  the  muddy 
estuarine  conditions  being  eminently  unfavourable  for  their 
growth. 

Palms  are  absent  among  the  vegetation  of  this  time,  and  the 
Indian  forms  predominate  over  the  American  types. 

The  Ponlian  Series  consists  of  brackish- water  marly  clays,  with 
Unio,  Congeria,  Cardium,  Melanopsis,  etc.  It  is  the  division  of 
the  Miocene  on  which  Vienna  is  built. 

Switzerland. — The  Swiss  Molasse  occupies  the  whole  area  between 
the  Alps  and  the  Jura.  Near  the  former  it  consists  of  coarse  shore- 
conglomerates,  which,  with  increasing  distance  seaward,  pass  into 
sandy  and  clayey  sediments.  The  progressive  uplift  of  the  Alps 
continued  into  the  Pliocene,  and  the  conglomerates  now  lie  at  a 
height  of  6000  feet,  where  they  exhibit  little  or  no  departure  from 
the  original  horizontal  position  in  which  they  were  deposited. 

The  Lower  Molasse,  frequently  called  the  Grey  Molasse,  contains 
numerous  plant  remains,  mainly  subtropical,  and  a  marine  bed 
with  Ostrea,  Venus,  Murex,  and  Cerithium.  It  is  succeeded  by 
the  true  or  St  Gallen  Molasse,  which  contains  a  rich  molluscous 
fauna  comprising  some  400  species,  of  which  about  one-third  are 
still  living. 

The  St  Gallen  stage  is  followed  conformably  by  the  Upper  Fresh- 
water Molasse,  with  Melania,  Unio,  etc.,  and  seams  of  brown  coal. 

In  the  Tortonian  stage  the  land  surrounding  the  lake  was  clothed 
with  a  luxuriant  vegetation,  and  peopled  with  a  great  assemblage 
of  land  animals,  which  included  the  tapir,  mastodon,  rhinoceros, 
deer,  apes,  opossums,  three-toed  horse,  squirrels,  hares,  beavers, 
and  the  huge  Deinotherium  which  frequented  the  jungle  lands 
fringing  the  lake.  The  waters  of  the  lake  teemed  with  fishes,  and 
the  shallow  pools  and  mud-banks  were  frequented  by  crocodiles. 

The  three  main  subdivisions  of  the  Molasse  placed  in  consecutive 
order  are  as  follow  : — 

3.  Upper  Freshwater  Molasse — Tortonian. 
2.  St  Gallen  Molasse  (Marine) — Helvetian. 
1.  Grey  Molasse  (Lower  Freshwater  Stage) — Mayencian. 

India. — As  previously  described  in  last  chapter,  the  Gaj  Series 
of  uppermost  Oligocene  age  is  conformably  followed  by  the  MeJcran 
Series  of  Older  Miocene  date,  which  is  well  developed  along  the 
Mekran  coast,  in  the  islands  of  the  Persian  Gulf,  Irawadi  Valley 
in  Burma,  and  Andaman  Islands. 


To  face  page  450.J 


PLATE  LIX. 


FOSSIL  FISH  FROM  THE  ESMERALDA  FORMATION,  MIOCENE. 
(Leuciscus  Turner  i,  U.S.  Geol.  Survey.) 


To  face  page  451.] 


[PLATE  LX. 


s  t 

MTOCENE  GASTEROPODS. 


PLATE  LX. 

MIOCENE   GASTEROPODS. 

a,  Turritella  variabilis  (Conrad). 

b,  Scala  say  ana  (Dall). 

c,  Nassa  marylandica  (Martin). 

d,  Terebra  unilineata  (Conrad). 

e,  Solarium  trilineatum  (Conrad). 
/,     Cancellaria  alternata  (Conrad). 
g,    Surcula  biscatinaria  (Conrad). 

h,  Calliostoma  philanthropies  (Conrad). 

i,  Actceon  schilohensis  (Whitfield). 

j,  Oliva  Utter ata  (Lamarck). 

Jc,  Retusa  (Cylichnina)  conulus  (Deshayes). 

I,  Conus  diluvianus  (Green). 

m,  Polynices  (  Neverita)  duplicatus  (Say). 

n,  Fissuridea  alticosta  (Conrad). 

o,  Fissuridea  griscomi  (Conrad). 

p,  Xenophora  conchyliophora  (Born.). 

q,  Crepidula  fornicata  (Linne). 

r,  Fulgur  spiniger  (Conrad  var.). 

s,  Ecphora  quadricostata  (Say). 

t,  Siphonalia  marylandica  (Martin). 

u,  Ilyanassa  (?)  (Paranassa)  porcina  (Say). 

SCAPHOPOD. 
v,  Dentalium  attenuatum  (Say). 

(After  Maryland  Geological  Survey.) 


To  face  page.  451. 


[PLATE  LXI. 


MIOCENE  LAMELLIBRANCHS. 


PLATE  LXI. 

MIOCENE   LAMELLIBRANCHS. 

a  and  b,    Area  (Scapharca)  staminea  (Say). 

c  and  d,   Corbula  idonea  (Conrad). 

e,    Crassatellites  marylandicus  (Conrad). 

/,    Phacoides  (Pseudomiltha)  forem,ani  (Conrad). 

g,    Tellina  (  Angulus)  producta  (Conrad). 

h,    Leda  conccntrica  (Say). 

*,    Modiolus  dalli  (Glenn). 

j,     Astarte  thomassii  (Conrad). 

k,    Ensis  directus  (Conrad). 

I,    Spisula  (  Hemimactra)  marylnndica  (Dall). 

m,   Isocardia  markoei  (Conrad). 

n,    Cardium  ( Cerastoderma)  leptopleurum  (Conrad). 

o,    Pe.cten  ( Chlamys)  madisonius  (Say). 

p,     Venus  ducatelli  (Conrad). 

q,    Ostrea  carolinensis  (Conrad). 

(After  Maryland  Geological  Survey.) 


- 


CAINOZOIC    ERA  :     MIOCENE    SYSTEM.  451 

The  strata  consist  of  clays,  sandstones,  and  conglomerates  of  the 
Flysch  facies,  intercalated  with  a  few  calcareous  bands. 

The  characteristic  Foraminifera  are  Nummulites  and  Amphis- 
tegina.  The  uppermost  beds  contain  many  large  pectens. 

Towards  the  close  of  the  Miocene,  the  Flysch  strata  were  deeply 
involved  in  the  great  folds  which  marked  the  final  uplift  of  the 
Himalayas.  The  crustal  movement  of  this  date  broke  up  the 
great  Central  Sea  into  disconnected  inland  seas,  and  severed  the 
Mediterranean  from  the  Indian  Ocean. 

The  Lower  Miocene  rocks  of  India  do  not  possess  much  economic 
importance  apart  from  the  associated  Pegu  System,  which  contains 
valuable  oil-bearing  strata  in  Burma  and  Assam,  as  well  as 
deposits  of  salt  in  the  Salt  Range  in  the  Punjab. 

North  America. — The  Miocene  strata  of  North  America  are 
typically  developed  on  the  Atlantic  coastal  fringe  and  around 
the  Gulf  of  Mexico.  They  dip  gently  seaward,  and  in  some 
regions  are  largely  obscured  by  later  accumulations. 

In  many  places  along  the  Atlantic  fringe,  the  Miocene  beds  appear 
to  rest  unconformably  on  the  Eocene  (Oligocene),  but  the  strati- 
graphical  break  is  slight. 

Along  the  Atlantic  coast,  they  are  grouped  under  the  name 
Chesapeake  Series.  For  the  most  part  they  consist  of  loose  sands, 
clays,  and  shelly  marls  that  enclose  a  rich  fauna. 

In  the  Gulf  region  the  Florida  Limestone  has  been  largely 
replaced  by  rock-phosphate  deposits  of  great  extent. 

The  Miocene  of  the  Pacific  Coast,  sometimes  called  the  Monterey 
Series,  is  restricted  to  a  narrow  fringe  skirting  the  coast- 
line ;  but  strata  of  this  age  also  invade  the  Central  Valley  of 
California. 

Generally  the  Monterey  Series  of  California  rests  unconformably 
on  the  Eocene.  It  consists  mainly  of  shales,  and  sandstones  with 
a  notable  quantity  of  volcanic  ash. 

A  striking  feature  is  the  extraordinary  thickness  of  the  shales, 
4000  feet,  which  are  largely  composed  of  siliceous  diatoms.  The 
thickness  of  the  whole  series  has  been  estimated  at  from  5000 
to  7000  feet. 

The  Miocene  System  is  of  great  economic  importance  as  one  of 
the  oil-bearing  formations  of  California.  The  older  gold-bearing 
gravels  of  that  State  are  supposed  to  be  Miocene,  and  great  accumu- 
lations of  lacustrine  deposits  occur  in  the  old  lake-basins  east  of 
the  Sierras,  and  also  in  Nevada  and  Montana. 

Volcanic  activity  was  conspicuous  throughout  the  whole  of  the 
Miocene,  and  towards  the  close  of  the  period  culminated  in  gigantic 
effusions  of  basaltic  lavas.  Evidences  of  volcanic  eruptions  are 
abundant  in  all  the  Pacific  States,  in  Columbia,  and  in  the  Yellow- 


452  A  TEXT-BOOK  OF  GEOLOGY. 

stone  National  Park,  where  forests  were  overwhelmed  by  ashes, 
and  in  favourable  situations  the  tree  trunks  were  silicified. 

Altogether  during  the  Middle  Tertiary,  from  200,000  to  300,000 
square  miles  of  the  western  part  of  the  United  States  was  covered 
with  sheets  of  basaltic  lava. 

In  the  Western  States  the  Miocene  was  a  period  of  great  crustal 
disturbance  ;  and  to  this  date  is  assigned  the  erogenic  movements 
which  uplifted  the  Cordilleran  chains,  and  deformed,  partly  by 
folding  and  partly  by  faulting,  the  rocks  of  the  Northern  Sierras 
and  Great  Basin.  In  the  latter  region  the  crustal  movements 
were  accompanied  or  succeeded  by  violent  volcanic  outbursts. 

Greenland. — In  North  Greenland,  now  covered  with  polar  ice, 
the  plant  beds  exposed  on  the  coast  contain  a  prolific  evergreen 
subtropical  flora.  Of  150  species,  about  half  are  forest  trees,  such 
as  the  evergreen  oak,  beech,  planes,  poplar,  maple,  walnut,  lime, 
magnolia,  and  many  conifers,  including  the  giant  Sequoia.  A  third 
of  the  species  are  found  in  the  Miocene  basins  of  Southern  Europe. 

In  the  Miocene  of  North  Greenland,  within  8°  15'  of  the  pole, 
there  occurs  a  seam  of  bituminous  coal  25  feet  thick,  embedded  in 
sandstones  and  shales  which  contain  numerous  species  of  conifers, 
mostly  pines,  spruces,  firs,  and  cypresses,  and  also  the  Arctic 
poplar,  birch,  hazel,  and  elm.  Water-lilies  grew  on  the  shallow 
ponds  which  were  fringed  with  reeds  and  sedges. 

The  Miocene  plant  beds  of  Spitzbergen,  within  12°  of  the  pole, 
contain  an  evergreen  vegetation,  from  which  it  would  appear  that 
the  whole  of  the  polar  region  on  this  side  of  the  Northern  Hemi- 
sphere was  covered  in  that  period  with  rank  subtropical  forests 
and  jungle. 

The  existence  of  coal-seams  in  the  Antarctic  continent  points 
to  similar  changes  of  temperature  in  the  South  Polar  regions. 

The  climatic  changes  which  have  taken  place  in  past  geological 
times  present  one  of  the  most  difficult  problems  that  confront 
the  geologist. 

Australia. — Marine  deposits  of  Miocene  or  younger  date  are 
unknown  in  Queensland  and  New  South  Wales,  which,  since  the 
Eocene,  have  remained  dry  land.  In  the  south-east  and  southern 
parts  of  the  continent  Miocene  beds  are  extensively  developed. 
In  the  State  of  Victoria  they  cover  large  tracts,  more  particularly 
in  Gippsland,  where  the  basal  beds  of  the  system  contain  thick 
seams  of  brown  coal,  one  of  which  at  Latrobe  shows  a  thickness  of 
90  feet. 

Miocene  beds  are  sparingly  displayed  in  Tasmania,  but  at  one 
time  they  probably  covered  a  considerable  area.  At  Table  Cape 
the  deposits  contain  a  rich  molluscous  fauna  as  well  as  the  remains 
of  the  primitive  marsupial  Wynyardia. 


OAINOZOiC   ERA  :     PLIOCENE    SYSTEM.  453 

Marine  strata  of  probably  Miocene  age  occur  as  a  fringe  on  the 
Great  Australian  Bight. 

Except  in  Victoria  and  a  corner  of  Tasmania,  practically  the 
whole  of  Australia  was  dry  land  throughout  the  Miocene  and 
younger  Cainozoic.  The  conditions  of  deposition  were  mainly 
continental ;  hence  the  deposits  of  this  period  are  mostly  sands, 
gravels,  and  clays  deposited  in  inland  basins,  or  washed  into 
hollows  by  torrential  rains.  There  was  widespread  volcanic 
activity  in  the  Middle  Cainozoic,  particularly  in  the  eastern  side  of 
the  continent.  Floods  of  basaltic  lavas  spread  over  the  country 
and  streamed  down  the  valleys,  where  they  covered  up  the  gold- 
bearing  gravels  and  other  detrital  material. 

The  buried  gravels  form  the  famous  deep-leads  of  the  State 
of  Victoria. 

New  Zealand. — Miocene  strata  cover  large  tracts  in  both  the 
main  islands.  The  lower  beds  are  terrestrial  sands  and  conglomer- 
ates that  contain  valuable  seams  of  brown  coals.  They  are 
followed  by  estuarine  clayey  and  sandy  beds.  The  uppermost 
beds  are  marine  limestones  mainly  composed  of  comminuted 
corals,  bryozoans,  and  Foraminifera. 

Generally  the  Miocene  strata  contour  around  the  coasts  and 
ramify  into  the  old  firths  and  valley-basins.  They  are  mostly 
undisturbed  or  dip  gently  towards  the  sea.  In  West  Nelson 
they  have  been  raised  by  a  series  of  powerful  parallel  faults  in 
step-like  blocks  to  a  height  of  3500  feet  above  the  sea.  In  East 
Nelson  they  are  overthrust  and  involved  in  the  overturned  folds 
of  the  Triassic  rocks. 

The  marine  beds  everywhere  contain  a  rich  assemblage  of 
molluscs,  among  which  brachiopods  are  numerous,  also  corals, 
bryozoans,  Foraminifera,  and  sea-urchins.  The  remains  of  a 
zeuglodon  whale  are  common. 

On  the  East  Coast  of  Otago  the  upper  members  of  the  Oamaru 
Series  are  intercalated  with  tuffs  and  basalt  flows. 

Antarctic  Region. — Tertiary  strata  occurs  at  Seymour  Island, 
near  Graham's  Land,  containing  a  rich  molluscous  fauna  considered 
by  Wilckens  to  be  Upper  Oligocene  or  Lower  Miocene. 

Pliocene. 

At  the  close  of  the  Miocene  there  was  a  general  retreat  of  the 
sea  throughout  the  whole  globe,  and  in  the  Pliocene  the  continents 
began  to  assume  the  definite  forms  they  now  possess.  It  is  only 
in  those  areas  where  the  Pliocene  sea-floor  has  been  uplifted  and 
has  escaped  complete  destruction  by  denudation  that  marine 
deposits  of  that  period  are  exhibited  for  our  examination.  And 


454  A  TEXT-BOOK  OF  GEOLOGY. 

since  upward  earth-movements  of  considerable  magnitude  have 
not  been  general  or  even  widespread  in  the  latest  Cainozoic,  the 
amount  of  dry  land  now  occupied  by  Pliocene  marine  strata  is 
relatively  small. 

The  bulk  of  the  Pliocene  beds  to  which  we  have  access  are 
lacustrine  and  fluviatile  deposits  of  the  Continental  facies  that  have 
accumulated  in  inland  basins  and  river- valleys.  In  many  places 
these  terrestrial  deposits,  which  are  mostly  loose,  unconsolidated 
drifts,  owe  their  preservation  to  a  protecting  cover  of  basalt, 
rhyolite,  or  other  igneous  rock. 

Of  the  existing  dry  land  forming  Northern  Europe,  only  small 
areas  in  East  England,  Belgium,  and  North  France  were  covered 
with  the  Pliocene  sea.  Germany,  which  was  mainly  dry  land  in 
the  Miocene,  became  wholly  dry  land  in  the  Pliocene  ;  hence 
marine  beds  of  Pliocene  age  are  not  represented  among  the  rock- 
formation  of  that  region. 

In  Southern  Europe,  around  the  Mediterranean  Basin,  the  sea 
encroached  on  the  maritime  borders  of  Spain,  Algeria,  and  Greece, 
and  covered  large  tracts  in  Sicily,  Central  and  South  Italy. 

In  North  America  there  was  the  same  general  recession  of  the 
sea  as  in  Europe  ;  and  since  the  sea-floor  has  not  been  uplifted  to 
any  extent  since  the  Pliocene,  marine  strata  of  that  date  are  but 
poorly  represented  on  this  continent,  and  occur  only  as  a  narrow 
interrupted  fringe  bordering  the  Atlantic  and  Gulf  coasts.  On 
the  other  hand,  detrital  deposits  of  the  Continental  facies  are 
well  developed  in  the  old  lake-basins,  and  along  the  Atlantic 
and  Gulf  maritime  borders. 

A  general  recession  of  the  sea  took  place  in  India,  Australia, 
South  Africa,  and  South  America,  and  it  is  only  where  uplift  has 
taken  place  that  Pliocene  strata  are  exposed  at  the  surface. 

The  Pliocene  was  a  period  of  notable  volcanic  activity  in  Western 
North  America,  India,  Australia,  and  New  Zealand. 

Fauna  and  Flora. — Most  of  the  molluscs  and  other  invertebrates 
of  the  Pliocene  fauna  are  recent  species  ;  but  of  the  vertebrates, 
almost  all  the  species  and  even  many  of  the  genera  are  extinct. 

Where  the  Cainozoic  succession  is  complete,  the  Pliocene  follows 
the  Miocene  conformably,  and  is  conformably  followed  by  the 
Pleistocene.  The  limits  of  these  systems  are  quite  artificial,  and 
the  faunas  show  a  progressive  organic  development  when  passing 
upwards  from  one  system  to  the  other. 

Perhaps  the  most  notable  feature  of  the  Pliocene  land  fauna, 
is  the  presence  of  the  large  extinct  Proboscidians  Deinotherium,1 
and  Mastodon.2     Other  mammals  are  very  abundant,  and  include 

1  Gr.  deinos= terrible,  and  therton=a,n  animal. 
3  Gr.  mast os=  nipple,  and  odous  =  tooth. 


CAINOZOIC   ERA  :     PLIOCENE    SYSTEM.  455 

many  species  of  rhinoceros,  deer,  antelopes,  giraffes,  ox,  cat,  bear, 
fox,  porcupine,  beaver,  and  various  apes.  The  true  elephant, 
Elephas  meridionalis,  appears  about  the  close  of  the  period. 

The  Equidae  are  represented  by  the  existing  horse,  Equus, 
and  by  the  horse-like  Hippotherium  gracile  with  three  toes  on  each 
foot,  only  the  central  toe  reaching  the  ground. 

The  marine  fauna  of  Southern  Europe  closely  approaches  that 
of  the  living  Mediterranean  fauna,  and  in  North- West  Europe 
that  of  the  boreal  seas.  At  the  same  time  many  Arctic  species 
appear  in  the  Pliocene  deposits  of  both  Italy  and  England. 

Foraminifera,  corals,  and  bryozoans  are  exceedingly  abundant, 
and  the  molluscous  fauna  is  very  prolific,  particularly  in  Lamelli- 
branchs  and  Gasteropods,  of  which  about  90  per  cent,  are  still 
living. 

British  Isles. — The  only  important  Pliocene  deposits  in  England 
occur  as  a  coastal  strip  in  East  Anglia,  where  they  are  well  exposed 
in  the  sea-cliffs  and  beaches  from  Walton  in  Essex  to  Weybourn, 
north-west  of  Cromer  in  Norfolk.  A  few  small  patches  survive  on 
the  Downs  of  East  Kent,  and  a  small  deposit  occurs  at  St  Erth 
in  Cornwall. 

Seven  subdivisions  of  the  Pliocene  are  recognised  in  England, 
but  there  is  no  general  agreement  as  to  how  the^deposits  of  this 
period  should  be  divided  : — 


Newer  Pliocenes 

!7.  Cromer  Forest  Be 
6.  Weymouth  Crag. 
5.  Chillesford  Clays. 
4.  Norwich  Crag. 
3.  Eed  Crag. 

Pliocene 


{J 


The  different  subdivisions  are  seldom  found  superimposed  on 
one  another  ;  but  proceeding  northwards  along  the  coast  from 
Walton  we  pass  successively  from  older  to  younger  beds.  From 
this  it  would  appear  that  the  deposits  were  laid  down  on  the  littoral 
of  a  sea,  slowly  retreating  northwards. 

The  fauna  of  the  Older  Pliocene  is  essentially  that  of  a  warm  sea, 
and  of  the  Newer  Pliocene  of  boreal  waters. 

The  molluscous  fauna  of  the  older  Pliocene  comprises  a  large 
proportion  of  southern  forms.  But  the  uplift  which  caused  the 
northward  retreat  of  the  sea  cut  off  communication  with  the 
southern  sea  about  the  Tied  Crag  Stage.  The  entrapped  southern 
forms  were  unable  to  survive  in  the  colder  waters  of  the  North 
Sea  ;  and  at  the  close  of  the  Ked  Crag  Stage  half  of  them  had 


456  A  TEXT-BOOK  OF  GEOLOGY. 

disappeared,  while  an  increasing  number  of  northern  forms  took 
their  place. 

The  influx  of  northern  forms  and  corresponding  disappearance 
of  the  southern  were  accelerated  by  the  gradual  approach  of  the 
Arctic  cold,  which  culminated  in  the  Pleistocene. 

As  the  polar  ice  crept  southward,  more  and  more  of  the  southern 
forms  succumbed  to  the  increasing  cold,  and  at  the  close  of  the 
Norwich  Crag,  there  was  not  a  survivor  left.  Thereafter,  only 
northern  forms  inhabited  the  North  Sea. 

The  land  flora  of  the  Newer  Pliocene  was  less  affected  by  the 
increasing  cold  than  the  more  delicately  organised  marine  fauna, 
and  at  the  close  of  the  period  still  possessed  the  aspect  of  a 
temperate  climate. 

The  Lenham  Beds  occur  as  a  number  of  small  patches  on  the 
North  Downs  between  Maidstone  and  Folkestone  at  heights 
ranging  from  500  to  600  feet  above  the  sea.  They  contain  some 
species  found  in  the  Miocene,  among  them  being  Pleurotoma 
Jounanetti,  Terebra  acuminata,  and  Area  diluvii. 

The  Coralline  Crag  is  only  known  in  South- East  Suffolk,  where 
it  is  well  exposed  in  the  neighbourhood  of  Aldeburgh  and  Orford. 
It  consists  mainly  of  fragments  of  polyzoans,  molluscs,  sea-urchins, 
and  fish  teeth,  and  may  be  regarded  as  a  raised  shell-bank. 
At  its  base,  at  Sutton,  there  is  a  phosphatic  nodule  bed  with  fossils 
derived  from  the  London  Clay  and  Jurassic  rocks.  This  bed  also 
contains  an  assortment  of  different  rocks,  such  as  flints,  granite, 
quartzite,  quartz,  sandstones,  etc.  The  rounded  blocks  of  brown 
sandstone  are  locally  called  box-stones. 

The  Red  Crag  consists  of  sands  frequently  current-bedded, 
and  shells  usually  broken.  The  sands  and  shells  are  stained  a 
reddish-brown  colour  with  peroxide  of  iron,  hence  the  name  Red 
Crag.  Among  the  southern  forms  in  this  bed  are  Cerithium 
trilineatum,  Nassa  limata,  and  Turritella  inerassata  (Plate  LXIT. 
fig.  3),  and  among  the  northern,  Natica  occlusa,  Pleurotoma 
pyramidalis,  Cardium  groenlandicum,  Astarte  Basterotii  (Plate  LXII. 
fig.  8),  and  A.  borealis  (Plate  LXV.  fig.  3). 

The  Norwich  Crag  is  a  shelly  sand  and  gravel  deposit  of  fluvio- 
marine  origin.  A  considerable  proportion  of  the  shells  are  fresh- 
water. This  stage  is  specially  noted  for  the  large  number  of 
mammalian  bones  which  occur  in  a  pebbly  bed  at  its  base.  The 
mammals  include  representatives  of  the  horse,  mastodon,  and 
elephant. 

Among  the  characteristic  boreal  molluscs  are  Astarte  borealis 
and  Nucula  Cobboldice  (Plate  LXV.  fig.  4). 

The  Chillesford  Beds  in  different  places  rest  on  the  Norwich, 
Bed,  and  Coralline  Crags.  They  consist  mainly  of  finely  laminated 


baa  emll 

rtH 


0  be'A 

- 
.noli. 

' 


ni% 

'^^^  nS*5^w'1'- 

'     -  ' 


PLATE   LXI1. 

PLIOCENE  FOSSILS. 

(Crag.) 

1.  Purpura  tetragona  (Murex  alveolatus)  (Sow.)-     Red  Crag.     Walton  on  the 

Naze,  Sutton. 

2.  Valuta  Lamberti  (Sow.).     Coralline  and  Red  Crag.     Felixstowe,  Walton, 

Ramsholt. 

3.  Turritella  incrassata  (Sow.).     Coralline  and  Red  Crag.     Sutton,  Walton. 

Gedgrave,  and  Recent. 

4.  Natica,  sp.     Red  Crag.     Walton. 

5.  Aporrhais  pes-pelicani  (Linn.).     Coralline  and  Red  Crag,  Post-Pliocene 

and    Recent.      Norfolk,    Clyde,    Sutton,    Ramsholt,    Newbourn,    and 
Gedgrave. 

6.  Buccinum    undatum    (Linn.).     Coralline    Crag.     Ramsholt.     Red    Crag. 

Sutton,  Walton.     Mam -Crag.     Yorkshire  and  Recent. 

7.  Trivia  (Cyprcea]  Europcea  (Mont.).     Red  and  Coralline  Crag.      Sutton 

and  Recent. 

8.  Astarte  Basterotii(La,jonk).    Red  Crag.    Sutton  and  Felixstowe.    Coralline 

Ramsholt  and  Sudburn. 

9.  Trophon  antiquum  (Fusus)  (Miill.).    Red  Crag.     Walton.    Coralline  Crag. 

Bramerton  and  Recent. 

10.  Temnechinus  excavatus  (Wood).     Coralline  Crag.     Ramsholt,  Suffolk. 

11.  Lingula  Dumortieri  (Nyst.).     Coralline  Crag.     Sutton.     (Antwerp  Crag.) 

12.  Pyrula  reticulata  (Lam.).     Coralline  Crag.     Ramsholt. 


To  face  page  456.] 


[PLATE    LXII. 


PLIOCENE  FOSSILS.     (Crag.) 


OAINOZOIC    ERA  :     PLIOCENE    SYSTEM.  457 

micaceous  clays  and  sands.  Among  the  numerous  species  of 
molluscs  are  Astarte  compressa,  Cardium  grcenlandicum,  Lucina 
borealis,  and  Cyprina  islandica. 

The  Weybourn  Crag  is  a  marine  shelly  deposit  which  contains 
most  of  the  molluscs  of  the  Chillesford  Clays,  and  also  the  Lamelli- 
branch  Tellina  balthica,  which  is  unknown  in  older  beds. 

The  Cromer  Forest  Bed  Series  is  well  developed  in  the  Cromer 
Cliffs  on  the  north  coast  of  Norfolk.  It  comprises  four  distinct 
groups  of  beds  : — 

n  f4.  Leda  (Yoldia)  myalis  Bed  (marine). 

v      ?£  A  J  3-  UPPer  freshwater  Bed. 
Forest  Bed  4  ^  y^  Bed  (estuarine). 

I  1.  Lower  Freshwater  Bed. 

The  plant  beds  contain  the  remains  of  many  forest  trees,  almost 
all  of  which  are  still  living  in  Norfolk.  All  the  marine  molluscs 
are  found  in  the  Weybourn  Crag.  They  include  the  extinct 
species  Tellina  obliqua  and  Nucula  Cobboldice. 

The  vertebrates  are  exceedingly  abundant  and  comprise 
numerous  fishes  (perch,  cod,  pike,  sturgeon,  etc.),  reptiles,  and 
birds  (eagle,  owl,  cormorant,  wild  goose,  wild  duck,  etc.).  The 
marine  mammals  include  whales,  seals,  and  walrus.  The  ungulates 
are  represented  by  the  elephant,  rhinoceros,  hippopotamus,  horse, 
bison,  and  wild  boar  ;  and  the  carnivores,  by  the  hyaena,  wolf, 
dog,  fox,  otter,  and  marten. 

Excluding  the  bat,  the  total  species  of  land  mammals  is  forty- 
five,  as  compared  with  twenty-nine  living  species.  And  of  thirty 
large  land  mammals  only  three  are  now  living  in  Britain,  and  only 
six  survive  elsewhere. 

The  Leda  myalis  Bed  is  a  current-bedded,  sandy,  marine  loam 
containing  a  few  marine  shells  of  boreal  aspect,  among  which 
Leda  myalis,  Tellina  balthica,  and  Astarte  borealis  are  common. 

The  Arctic  Freshwater  Bed  which  follows  the  Leda  myalis 
Bed  conformably  is  sometimes  referred  to  the  Pliocene  and  some- 
times to  the  Pleistocene.  It  contains  the  Arctic  willow,  Salix 
polaris,  and  the  Arctic  birch,  Betula  nana,  which  indicate  a  mean 
temperature  20°  Fahr.  lower  than  the  present  mean  temperature, 
sufficiently  cold  to  allow  glaciers  to  form  on  the  mountains,  and 
allow  the  sea  to  be  frozen  over  in  the  winter  months. 

Belgium. — The  Pliocene  deposits  across  the  Channel  are  a 
continuation  of  those  in  the  east  coast  of  England.  The  Older 
Pliocene  deposits  consist  of  shelly  sands,  and  rest  unconformably 
on  the  Miocene  and  older  rocks.  The  fossils  are  mostly  those 
which  characterise  the  Coralline  Crag  and  older  Pliocene  of 
England. 


458  A  TEXT-BOOK  OF  GEOLOGY. 

France. — In  South  France  the  Pliocene  deposits  have  been 
raised  to  a  height  of  1150  feet  above  the  sea.  They  consist  chiefly 
of  shelly  sands  and  marly  clays  with  bands  of  conglomerate. 
The  three  main  divisions  are  as  follow  : — 

3.  Arnusian  Stage — Freshwater,  with  volcanic  tuffs. 
2.  Astian          „        Fluviatile,  lacustrine,  and  marine. 
1.  Plaisancian  ,,        Marine. 

Italy. — The  Pliocene  of  Italy  and  Sicily  are  more  fully  developed 
than  elsewhere  in  Europe.  They  occupy  large  tracts  in  Central 
and  Southern  Italy,  forming  low  hills  on  both  flanks  of  the 
Apennines ;  hence  the  name  Sub-Apennine  Series  frequently 
applied  to  the  Italian  Pliocene. 

In  Sicily  they  cover  about  half  the  area  of  the  island,  and  rise 
to  a  height  of  4000  feet  above  the  sea.  The  volcanic  activity  which 
piled  up  Etna  to  its  present  gigantic  size  began  in  the  Pliocene, 
the  first  eruptions  being  submarine.  Since  then  the  island  and 
surrounding  sea-floor  have  been  steadily  rising.  The  marine 
benches  that  contour  round  the  north  and  east  sides  of  the  island 
are  conclusive  evidence  of  uplift  in  quite  recent  times. 

The  fauna  is  rich  in  molluscs  and  mammals,  most  of  them  related 
to  forms  found  in  the  Pliocene  of  England. 

Vienna  Basin. — The  Pliocene  beds  of  this  basin  follow  the 
Miocene  quite  conformably.  They  are  usually  called  the  Con- 
gerian  Stage  from  the  abundance  of  the  Lamellibranch  Congeria 
subglobosa.  The  deposits  consist  of  clays  followed  by  fluviatile 
drifts. 

The  clays  are  estuarine  and  contain  a  rich  assemblage  of  molluscs 
and  mammalian  remains.  The  overlying  fluviatile  drifts  also 
contain  mammalian  remains. 

The  Congerian  Stage  was  apparently  deposited  in  a  gulf  that 
became  detached  from  the  sea,  forming  an  inland  basin  like  the 
present  Caspian  depression  ;  and,  like  the  Caspian  Sea,  it  gradually 
freshened  by  the  inflow  of  streams  and  rivers. 

In  the  arms  of  the  sea  cut  off  from  the  great  Central  Sea, 
there  accumulated  remarkable  deposits  of  rock-salt,  gypsum,  and 
anhydride,  the  most  famous  of  which  is  that  at  Wieliczka  in  Polish 
Austria,  on  the  northern  flank  of  the  Carpathians,  near  Cracow. 

India. — The  uplift  of  the  Himalayas  and  the  mountains  of 
Baluchistan  and  Burma  culminated  in  the  late  Miocene,  conse- 
quently there  are  no  marine  Pliocene  deposits  in  these  regions. 
But  along  the  foot  of  the  Himalayas  there  remained  a  chain  of 
salt-water  basins  and  shallow  lagoons,  in  which  a  thick  series  of 
deposits  was  laid  down  during  the  Pliocene  Period.  These  beds 
consist  principally  of  fluviatile  sands,  clays,  and  conglomerates, 


CAINOZOIC   ERA  :     PLIOCENE    SYSTEM.  459 

and  constitute  what  is  widely  known  as  the  Siwalik  System,  which 
is  extensively  developed  in  the  Punjab,  Baluchistan,  Burma,  and 
Assam.  In  Burma  the  Siwalik  System  is  represented  by  the 
Iraivadi  Beds. 

The  sediments  of  this  system  were  deposited  by  the  streams  and 
rivers  flowing  into  the  enclosed  basins,  their  mode  of  formation 
in  some  respects  resembling  that  of  the  Flysch  deposits. 

The  variable  salinity  of  the  waters  of  these  detached  basins  was 
unfavourable  for  the  development  of  a  prolific  or  vigorous  organic 
life,  the  remains  of  which  are  consequently  scanty.  The  molluscs 
are  mostly  living  species  ;  but  the  chief  interest  of  the  Siwalik 
System  lies  in  the  extraordinary  number  of  fossil  vertebrates  which 
it  contains.  The  mammals  include  many  apes,  the  elephant  and 
mastodon,  numerous  ungulates  and  carnivores.  Fishes,  reptiles, 
and  birds  are  also  well  represented. 

The  living  mammals  include  the  elephant,  horse,  and  bear. 

North  America. — Marine  Pliocene  beds  play  an  unimportant 
part  in  the  geological  structure  of  the  North  American  continent. 
They  are  sparingly  developed  along  the  Atlantic  and  Gulf  coasts, 
occurring  as  isolated  patches  that  are  probably  the  remnants  of  a 
continuous  sheet.  The  strata  dip  seaward  under  the  accumula- 
tions of  later  date. 

The  bulk  of  the  Pliocene  deposits  are  lacustrine  and  fluviatile 
drifts  that  have  accumulated  in  inland  lake-basins. 

The  Lafayette  Series,  which  consists  of  a  series  of  sands,  clays, 
and  silts,  is  extensively  developed  between  the  Appalachians  and 
the  Atlantic,  whence  it  sweeps  around  the  Atlantic  border  to  the 
Mexican  Gulf  coastal  region.  It  spreads  over  a  large  tract  in  the 
lower  Mississippi  Basin  and  stretches  into  the  coastal  plains  of 
Texas.  Altogether  this  series  covers  an  area  exceeding  200,000 
square  miles. 

The  beds  rest  unconformably  on  the  eroded  surfaces  of  the 
Miocene  and  older  rocks,  and  seldom  rise  to  a  height  exceeding 
200  feet  above  the  sea. 

The  thickness  of  the  sediments  seldom  exceeds  50  feet,  and  is 
usually  less  than  30  feet,  but  in  a  few  places  it  reaches  200  feet. 

This  remarkable  and  widespread  maritime  formation  contains 
no  marine  fossils,  and  the  remains  of  land  animals  and  plants  are 
rare.  Hence  its  age  is  still  uncertain.  Although  obviously  of 
terrestrial  origin,  its  mode  of  formation  is  still  obscure. 

•Australia. — The  gold-bearing  drifts  of  Victoria  and  New  South 
Wales  that  lie  buried  under  sheets  of  basaltic  lava  range  in  age 
from  Miocene  to  Pliocene.  These  buried  deep-leads  frequently 
contain  lignite  beds,  and  the  trunks,  branches,  and  leaves  of  trees, 
also  freshwater  shells  and  the  remains  of  extinct  marsupials, 


460  A  TEXT-BOOK  OF  GEOLOGY. 

some  of  which  attained  a  gigantic  size.  The  most  remarkable 
of  these  primitive  mammals  are  the  Diprotodon,  the  tapir-like 
Nototherium,  a  giant  kangaroo,  a  marsupial  lion,  and  a  marsupial 
hyaena. 

The  Pliocene  drifts  also  contain  the  remains  of  a  large  extinct 
bird,  Dromornis,  related  to  the  existing  emu. 

Marine  beds  of  Tertiary  date  occur  in  the  State  of  Victoria  up 
to  an  altitude  of  1000  feet  above  sea-level.  The  fossils  include  the 
brachiopod  Magellania,  the  Lamellibranch  Trigonia,  and  the 
Gasteropods  Haliotis  and  Cerithium. 

New  Zealand. — Marine  deposits  of  Pliocene  age  cover  a  wide 
tract  in  the  North  Island,  where  they  rise  as  a  gentle  sloping  plane 
from  sea-level  to  a  height  of  4000  feet  above  the  sea.  They  rest 
conformably  on  the  Miocene  strata,  but  overlap  these  and  spread 
on  to  older  rocks.  Their  uplift,  like  that  of  the  Pliocene  of  Sicily, 
was  connected  with  volcanic  outbursts  which  piled  up  the  gigantic 
volcanoes  Ruapehu  and  Tongariro,  both  situated  in  the  centre  of 
a  great  dome  of  elevation,  from  which  the  Tertiary  strata  dip 
towards  the  sea  on  the  west,  south,  and  east. 

The  molluscous  fauna  is  related  to  that  in  the  surrounding  seas. 

Some  of  the  older  gold-bearing  drifts  of  Otago  and  Westland  are 
probably  Pliocene. 


CHAPTER  XXXIII. 
PLEISTOCENE  AND  RECENT.1 

Pleistocene  or  Glacial  Period. 

THIS  period  covers  the  interval  between  the  close  of  the  Pliocene 
and  the  advent  of  recent  times.  Its  downward  limit  is  not  always 
very  sharply  defined  from  the  Pliocene,  and  passing  upward  it 
merges  imperceptibly  into  the  Recent. 

The  duration  of  this  period  has  been  variously  estimated  at  from 
scores  of  thousands  to  hundreds  of  thousands  of  years,  which  is  a 
good  reason  for  saying  that  we  possess  no  sufficient  data  to 
enable  us  to  form  even  an  approximate  estimate.  Whatever  its 
length  expressed  in  years  may  be,  it  is  generally  agreed  that  the 
Pleistocene  represents  a  shorter  interval  of  geological  time  than  the 
Pliocene  or  other  Cainozoic  periods. 

The  time  that  has  elapsed  since  the  close  of  the  Glacial  Period 
is  estimated  at  from  10,000  to  50,000  years. 

During  the  Pliocene,  the  Alps,  Carpathians,  Himalayas,  and  other 
great  chains  attained  their  full  height,  and  at  the  close  of  that 
period  the  Earth  finally  assumed  its  present  form.  Since  that 
date  there  have  been  no  great  earth-movements  except  those 
caused  by  local  volcanic  disturbances,  but,  as  in  past  ages,  the 
various  processes  of  denudation  have  been  unceasingly  wearing 
away  and  modifying  the  surface  of  the  dry  land. 

Pleistocene  Glaciation. — The  dominant  feature  of  this  period 
was  the  phenomenal  increase  of  cold  in  both  hemispheres,  which 
permitted  an  extraordinary  invasion  of  the  temperate  latitudes 
by  gigantic  ice-sheets  moving  down  from  the  higher  latitudes, 
and  allowed  glaciers  to  accumulate  in  regions  where  permanent 
ice  did  not  formerly  exist. 

A  vast  ice-sheet  radiated  from  the  mountains  of  Scandinavia 
and  spread  over  the  whole  of  Northern  Europe.  It  extended  from 
the  Ural  Mountains  to  the  Atlantic  Ocean,  and  reached  as  far 
south  as  Central  Germany  and  the  basins  of  the  rivers  draining 

1  Pleistocene + Recent  ==  Post-Pliocene  ==  Quaternary,  when  the  geological 
record  was  divided  into  Primary,  Secondary,  Tertiary,  and  Quaternary  eras. 

461 


462  A  TEXT-BOOK  OF  GEOLOGY. 

into  the  Black  Sea.  This  gigantic  sheet  bridged  the  Baltic  Sea 
and  filled  the  North  Sea  basin,  in  which  it  flowed  southward  as  far 
as  the  English  Channel.  Its  southmost  limit  in  Europe  was  about 
50°  N.  latitude. 

At  the  same  time  enormous  glaciers  radiated  from  the  Pyrenees, 
Alps,  Carpathians,  Urals,  and  Caucasus,  and  spread  over  the 
foothills  and  neighbouring  plains. 

The  whole  of  Scotland  and  practically  the  whole  of  Ireland  were 
covered  with  ice,  and  in  England  the  invading  sheet  reached  as 
far  as  the  basin  of  the  Thames. 

The  magnitude  of  the  Pleistocene  glaciation  was  even  greater 
in  North  America  than  in  Europe.  Glacial  drifts  were  spread 
over  the  United  States  as  far  south  as  37°  N.  latitude,  or  13° 
further  south  than  in  Europe. 

In  South  America  and  New  Zealand  the  glaciers  reached  sea- 
level  in  39°  S.  latitude,  and  in  the  Antarctic  Continent  the  extent 
of  the  Barrier  Ice-sheet  was  vastly  greater  than  at  present. 

The  trend  of  the  later  researches  in  glaciology  is  to  show  that 
Europe  and  America  were  not  glaciated  by  an  invasion  of  the 
polar  ice-cap,  but  by  the  accumulation  of  vast  masses  of  ice  in 
certain  regions  situated  in  lower  latitudes.  In  Europe  the  centre 
of  dispersion  is  placed  in  Northern  Scandinavia ;  and  in  North 
America,  in  Canada  along  the  sixtieth  parallel  of  latitude.  The 
evidences  of  regional  glaciation  are  not  sufficiently  conclusive  for 
general  acceptance,  and,  after  all,  may  be  deceptive  ;  and  perhaps 
too  much  weight  has  been  attached  to  the  apparent  absence  of 
glaciation  in  Southern  Siberia,  and  to  the  radial  dispersion  of  the 
Scandinavian  ice-sheet. 

At  each  independent  local  centre  of  glaciation  the  ice-cap  will 
naturally  flow  outward  in  all  directions  from  the  gathering  ground 
independently  of  the  surface  configuration.  In  the  case  of  valley 
glaciers  the  direction  of  flow  will  always  be  determined  by  the 
trend  of  the  valley- walls. 

In  Scotland  the  local  ice-cap  radiated  north,  east,  south,  and 
west.  It  flowed  north  and  east  until  it  became  engulfed  in  the 
superior  mass  of  the  southward-flowing  Scandinavian  ice-sheet. 

Similarly  the  Scandinavian  land-ice  radiated  outward  towards 
all  the  cardinal  points  of  the  compass,  and  there  is  no  evidence  to 
show  that  it  did  not  meet  and  merge  into  the  advancing  polar  ice- 
sheet. 

The  great  glaciation  of  North  and  North- West  Europe  may 
have  been  directly  due  to  the  existence  of  the  superior  gathering 
ground  in  Scandinavia.  But  there  is  no  present  evidence  to  show 
that  the  existence  of  the  Scandinavian  ice-sheet  was  independent 
of  an  advance  of  the  polar  ice-cap. 


CAINOZOIC   ERA  :     GLACIAL   PERIOD.  463 

In  North  America  the  Pleistocene  ice-cap  covered  Greenland 
and  the  whole  of  the  northern  portion  of  the  continent,  with  perhaps 
the  exception  of  North- West  Alaska,  as  to  which  the  evidence  is 
too  scanty  for  final  pronouncement.  The  centres  of  accumulation 
and  dispersion  of  the  land-ice  were  localised  in  certain  regions 
from  which  the  confluent  ice-sheets  radiated  in  all  directions,  but 
there  is  no  evidence  to  support  the  view  that  these  accumulations 
of  ice  could  have  existed  apart  from  a  general  refrigeration  and 
advance  of  the  polar  ice-sheet. 

The  extreme  cold  which  characterised  the  older  Pleistocene  did 
not  come  on  suddenly.  On  the  contrary,  the  effects  of  boreal  cold 
began  to  be  manifest  as  far  back  as  the  Middle  Pliocene.  The 
wholesale  migration  of  Arctic  forms  into  the  East  Anglian  waters 
in  the  Newer  Pliocene  showed  that  the  Scandinavian  region  was 
already  in  the  grip  of  the  ice-cap,  and  the  advent  of  the  boreal 
flora  as  contained  in  the  Arctic  Freshwater  Bed  denoted  that  the 
refrigeration  was  approaching  a  climax.  After  this  stage,  the 
cold  continued  to  increase  until  the  glaciation  culminated  about 
the  Middle  Pleistocene. 

Subdivisions. — It  is  maintained  by  some  eminent  geologists 
that  there  were  two  periods  of  glaciation  separated  by  a  warmer 
interval,  during  which  the  ice-sheets  and  glaciers  retreated.  This 
warmer  interval  is  called  the  Interglacial  Period.  Its  supposed 
existence  is  based  on  the  occurrence  of  certain  drifts  with  plant 
and  peaty  deposits  and  mammalian  remains  intercalated  in  the 
Boulder-Clay.  A  better  knowledge  of  the  work  of  glaciers  and 
ice-sheets  does  not  support  this  view. 

It  is  almost  certain  that  the  advance  and  retreat  of  the  northern 
ice  was  sufficiently  slow  to  permit  forests  to  flourish  and  peat-bogs 
to  accumulate  on  the  drift-covered  lands  close  to  the  edge  of  the 
ice.  During  a  temporary  advance  of  the  ice,  the  forests  might 
well  be  covered  over  by  fluvio-glacial  sands,  gravels,  and  morainic 
debris. 

Three  principal  stages  of  glaciation  may  be  distinguished  in  the 
Glacial  Period,  and  they  pass  imperceptibly  into  one  another. 
They  are  the  Advancing  Stage,  the  Maximum  Stage,  and  the 
Retreating  Stage. 

The  Ice  Age  in  temperate  latitudes  began  and  ended  in  local 
glaciers  which  became  confluent  during  the  maximum  refrigeration. 

The  Advancing  Stage  is  characterised  by  the  gathering  of  local 
glaciers — the  outposts  of  the  advancing  northern  ice.  The  Maxi- 
mum Stage  is  distinguished  by  ice-sheets  of  great  magnitude,  and 
the  Retreating  Stage  by  local  glaciers  that  cover  the  retreat  of  the 
main  sheet  and  finally  disappear  or  shrink  back  among  the  deep 
mountain  valleys.  Thus  we  have  : — 


464  A  TEXT-BOOK  OF  GEOLOGY. 

p,     .  ,  f3.  Retreating  Stage  =  Local  glaciers. 
Ulacial  I  2    ]\|aximum  Stage  =  Confluent  glaciers  and  ice-sheets. 
[I.  Advancing  Stage  =  Local  glaciers. 

In  the  Advancing  Stage  the  glaciers  that  already  existed  in  the 
Alpine  chains  began  to  advance  and  grow  in  thickness.  At  the 
same  time  the  seasonal  snows  on  the  lower  ranges  became  perma- 
nent, and  glaciers  appeared  where  none  existed  before.  A  wintry 
boreal  aspect  now  took  possession  of  the  land,  and  the  Arctic 
plants  and  animals  slowly  retreated  southward  before  the  advancing 
ice,  always  keeping  within  the  limits  of  climatic  conditions  corre- 
sponding to  their  natural  habitat. 

With  the  increasing  refrigeration,  the  glaciers  grew  in  size  until 
they  filled  up  the  valleys  and  basins  in  which  they  lay. 

In  the  Maximum  Stage  the  advancing  glaciers  overflowed  the 
valley- walls  and  deployed  on  to  the  foothills  and  plains,  where  they 
formed  piedmont  ice-sheets  that  slowly  crept  onwards  until  in 
some  cases  their  terminal  face  was  hundreds  of  miles  from  the 
centre  of  dispersion.  In  their  onward  course  they  passed  over 
hill  and  dale,  filled  up  lake-basins,  and  even  bridged  wide  seas. 
In  this  stage  the  ice-sheets  derived  little  or  no  rocky  debris  from 
projecting  peaks  or  nunataks  ;  nevertheless,  they  carried  an  immense 
load  of  soil,  clay,  sand,  and  broken  rock  scooped  up  from  the  floor 
over  which  they  flowed.  The  conditions  now  resembled  those 
prevailing  in  Greenland  at  the  present  time. 

In  the  Retreating  or  Waning  Stage,  as  the  result  of  the  gradual 
approach  of  milder  climatic  conditions,  the  ice-sheets  began  to  shrink 
and  retreat,  and  in  time  they  disappeared  from  the  coastal  plains 
and  foothills.  Shrunken  in  thickness  and  no  longer  able  to  override 
the  valley-walls,  the  ice  now  began  its  long  retreat  up  the  mountain 
valleys. 

When  half-way  back  to  the  Alpine  chain,  the  glaciers  in  the 
temperate  regions  halted  and  entrenched  themselves  behind  piles 
of  morainic  debris.  Behind  these  temporary  fortresses  they  held 
their  ground  for  a  time,  and  on  two  or  more  occasions  made 
desperate  sallies  beyond  the  barriers.  Beaten  back  by  the  increas- 
ing and  relentless  waimth,  they  soon  began  the  final  retreat  which 
ended  in  the  disappearance  of  all  but  those  which  took  their  rise 
in  the  higher  Alpine  chains. 

Glacial  Evidences. — Glaciers  and  ice-sheets  leave  behind  them 
a  twofold  evidence  of  their  former  existence.  By  their  erosive 
effects,  they  modify  the  configuration  of  the  surfaces  over  which 
they  pass,  and  they  leave  behind  them  piles  of  detrital  material 
of  various  kinds. 

A  glacier  or  moving  sheet  of  ice  by  the  sheer  weight  of  its  mass 


CAINOZOIC    ERA  I     GLACIAL   PERIOD.  465 

removes  all  the  irregularities  of  the  surfaces  over  whichiit  flows, 
with  the  result  that  the  contours  become  rounded  and  smooth. 
Rough  rocky  hills  lying  in  the  path  of  the  moving  ice  are  worn 
down  into  rounded,  hummocky,  or  whale-backed  mounds  or 
roches  moutonnees,  and  the  surfaces  of  the  rocks  are  scored, 
scratched,  and  polished  by  the  cutting  effect  of  the  blocks 
embedded  in  the  bottom  of  the  ice.  Protruding  spurs  are  trun- 
cated, and  benches,  steps,  and  broad  platforms  frequently  excavated 
on  the  mountain  slopes.  Prominent  peaks  and  ridges  that  are 
overridden  by  a  stream  of  ice  are  worn  down  into  rounded  domes. 

A  region  that  has  suffered  intense  glaciation  usually  presents 
smooth  flowing  contours  and  soft  outlines. 

The  detrital  material  consists  mainly  of  fluvio-glacial  drifts, 
terminal  morainic  piles  often  arranged  in  crescent-shaped  mounds, 
and  ground-moraines  called  till  or  boulder-clay. 

Fluvio-Glacial  Drifts. — Glaciers  and  ice-sheets  in  all  the  stages 
of  their  existence  are  drained  by  streams  and  rivers  which  pick 
up  and  re-sort  the  detritus  discharged  at  the  terminal  edge,  and 
spread  it  out  as  a  wide  sheet  or  apron  of  rudely-sorted,  water- 
worn,  and  semi-angular  drift  in  front  of  the  ice-sheet.  In  this 
way  glacial  valley-trains  are  formed  (fig.  29). 

Older  Drifts. — The  drifts  formed  during  the  advancing  stage 
are  obviously  the  oldest.  In  many  places  they  were  cut  up  by 
the  advancing  ice,  carried  forward,  and  again  deposited  at  the 
terminal  face. 

It  is  obvious  that,  where  the  ice-stream  travelled  far  from  its 
gathering  ground,  the  drifts  and  detritus  laid  down  in  the  earlier 
stages  of  the  advance  may  have  been  re-sorted  over  and  over  again 
before  the  ice-flow  reached  its  furthest  limit.  The  constituent 
particles  and  blocks  by  the  continuous  grinding  and  attrition  of 
the  moving  ice  and  the  wear  and  tear  of  the  glacial  streams  and 
rivers  become  smaller  and  more  rounded.  Hence,  in  a  long  journey 
only  the  harder  rocks  are  able  to  survive  in  the  form  of  sand  and 
gravel.  The  softer  rocks  are  reduced  to  the  condition  of  silts 
and  muds,  much  of  which  is  carried  to  the  sea  by  the  glacial  streams. 

Morainic  Mounds. — These  are  formed  at  the  halting-places 
both  during  the  advance  and  retreat.  The  morainic  mounds 
formed  during  the  advance  are  overrun  by  the  ice  when  it  resumes 
its  forward  movement,  and  are  thereby  broken  up  and  re-deposited 
in  a  re-sorted  form.  The  morainic  mounds  formed  during  the 
retreat  remain  intact  except  where  they  have  been  attacked  by 
the  glacial  streams  and  rivers  issuing  from  the  ice-face. 

Terminal  moraines  are  formed  at  the  utmost  limits  reached  by 
the  ice,  provided  the  retreat  does  not  begin  as  soon  as  this  limit 
is  reached. 

30 


466  A  TEXT-BOOK  OF  GEOLOGY. 

Older  Moraines. — When  the  ice-sheet  halted  for  a  time  at  the 
utmost  limits  reached  by  it,  the  rocky  load  of  debris  transported 
under,  in,  and  on  the  ice  is  piled  up  as  a  terminal  moraine.  Such 
moraines  are  of  great  antiquity,  and  are  obviously  older  than  those 
formed  during  the  retreat.  Hence  they  are  called  Older  Moraines 
to  distinguish  them  from  the  Newer  Moraines  formed  in  the  valleys 
and  old  alpine  lake-basins  during  the  later  stages  of  valley 
glaciation. 

Boulder-Clays  or  Ground-Moraines. — During  the  retreat  the  rocky 
debris  entangled  in  the  ice  is  shed  as  a  sheet  over  the  ground 
from  which  the  ice  has  disappeared.  In  places  the  deposit  may  be 
thick,  in  others  thin  or  altogether  absent  according  to  the  dis- 
tribution of  the  material  in  the  ice.  It  may  be  spread  over  valley, 
hill-top,  and  slope  alike,  but  is  usually  thickest  in  the  hollows, 
as  the  tendency  of  the  newly  fallen  material  is  to  gravitate 
downwards. 

At  certain  places  the  ground- moraine  may  be  attacked  and 
re-sorted  by  the  glacial  streams  issuing  from  the  ice,  and  spread 
out  as  an  apron  of  drift  and  silt  in  front  of  the  terminal  edge.  In 
this  way  a  boulder-clay  may  pass  gradually,  or  may  be  suddenly, 
into  rudely  stratified  sand  and  gravel  drifts. 

There  is  abundant  evidence  that  a  boreal  vegetation  and  land 
animals  followed  up  the  retreating  Pleistocene  ice  pretty  closely, 
and  in  this  situation  their  remains  would  be  liable  to  be  covered 
over  with  glacial  debris  during  minor  advances  of  the  ice  arising 
from  fluctuations  in  the  climatic  conditions. 

Thickness  of  Ice-Sheets. — The  thickness  of  the  Scottish  ice- 
sheet  during  the  period  of  maximum  refrigeration  as  determined 
by  the  height  at  which  ice-worn  rocks  are  met  with  has  been 
estimated  at  5000  feet ;  of  the  Scandinavian,  7000  feet ;  of  the 
New  Zealand,  7000  feet ;  and  of  the  North  American,  from  7000 
to  15,000  feet. 

Local  Glaciation. 

British  Isles. — During  the  period  of  maximum  glaciation  the 
whole  of  Scotland  was  covered  with  a  sheet  of  land-ice  which 
radiated  outwards  from  the  Highlands  in  all  directions.  On  the 
east  side  the  Scottish  ice  spread  some  distance  over  the  sea 
and  repelled  the  invading  Scandinavian  ice  which  now  occupied 
the  North  Sea,  and  on  the  west  it  covered  all  the  Western 
Isles  and  stretched  an  unknown  distance  into  the  Atlantic.  The 
portion  covering  the  Western  Lowlands  crossed  the  Irish  Sea  and 
invaded  the  north-east  corner  of  Ireland. 

Passing  southward  it  encountered  the  Welsh  buttress  with  its 
ice-cap,  and  was  diverted  into  two  main  streams,  the  eastern  stream 


CAINOZOIC    ERA  :     GLACIAL   PERIOD.  467 

flowing  southward  through  the  Lancashire  depression  between  the 
Pennine  Chain  and  Highlands  of  Wales,  the  western  or  main  stream 
pursuing  its  course  down  the  Irish  Sea  between  South-East  Ireland 
and  Wales. 

The  Irish  Sea  was  so  completely  filled  that  the  ice  rode  over  the 
summit  of  Snaefell,  the  highest  point  in  the  Isle  of  Man,  which  is 
2034  feet  above  the  sea. 

The  Lancashire  ice-stream  flowed  as  far  south  as  the  basin  of  the 
Severn,  and  covered  the  greater  portion  of  Lancashire,  Cheshire, 
and  Shropshire. 

The  western  stream  passed  through  St  George's  Channel,  chafed 
against  the  rocky  coasts  of  Pembroke,  and  advanced  so  far  south 
that  the  ice-face  peeped  into  the  Bristol  Channel. 

The  local  glaciers  of  Ireland  formed  a  sheet  of  land-ice  which 
covered  the  whole  of  the  island,  with  the  exception  of  Counties 
Antrim  and  Down,  and  some  adjacent  areas  in  the  north-east 
corner  already  occupied  by  the  Scottish  ice,  and  perhaps  a  fringe 
along  the  south  coast. 

The  Welsh  glaciers  formed  a  small  but  compact  wedge  of  ice 
lying  between  the  two  main  branches  of  the  Scottish  ice.  On  the 
west  side  they  descended  into  Cardigan  Bay,  and  fended  off  the 
Scottish  ice  ;  on  the  south  sent  long  tongues  of  ice  into  the  Bristol 
Channel ;  and  on  the  east  descended  into  the  basin  of  the  Severn. 

The  Scandinavian  ice  filled  the  North  Sea  and  reached  as  far 
south  as  the  English  Channel,  but  it  was  unable  to  encroach  on 
the  Scottish  mainland  on  account  of  the  superior  pressure  of  the 
land-ice  descending  from  the  Highlands. 

In  England,  where  the  land-ice  was  thinner  and  its  pressure  less, 
the  Scandinavian  ice  was  able  to  invade  the  coastal  fringe  from 
North  Durham  to  the  Humber.  South  of  the  Humber  it  spread 
over  a  large  tract,  covering  practically  the  whole  of  Lincolnshire 
lying  east  of  the  Trent,  the  whole  of  the  counties  of  Norfolk,  Stafford, 
and  Cambridge,  and  portions  of  the  adjoining  Midlands,  as  far  south 
as  the  north  side  of  the  Thames  Valley. 

Local  glaciers  held  possession  of  the  Cheviot  Hills  on  the  border, 
and  the  highlands  of  Cumberland  and  Westmoreland. 

The  former  extent  of  the  local  glaciers  and  invading  Scottish  and 
Scandinavian  ice  is  shown  by  the  distribution  of  the  rocky  debris 
and  erratics  scattered  over  the  land,  and  by  the  direction  of  the  ice- 
striated  rock-surfaces. 

The  main  struggle  for  supremacy  between  the  Scottish  and 
Scandinavian  ice-sheets  seems  to  have  centred  about  the  north- 
east corner  of  England,  and  partly  as  a  result  of  this  struggle,  and 
partly  as  the  result  of  the  check  the  Scottish  ice  received  from  the 
Cheviot  barrier,  and  the  resistance  of  Northumberland  and  West- 


468  A  TEXT-BOOK  OF  GEOLOGY. 

moreland  glaciers,  there  appears  to  have  remained  a  neutral  ground 
—  a  kind  of  no-man's-land  —  embracing  a  large  portion  of  the 
North,  East,  and  West  Ridings  of  Yorkshire,  and  the  greater 
part  of  Nottinghamshire  lying  west  of  the  Trent,  where  no  ice 
intruded. 

Glacial  Deposits. — The  character  of  the  glacial  deposits  varies 
from  place  to  place,  and  is  largely  dependent  on  the  character  of  the 
rocks  and  the  local  conditions  of  glaciation. 

Generally  the  glacial  deposits  of  a  region  may  be  classified  as  (a) 
those  formed  during  the  Advancing  Stage  ;  (6)  those  that  accumu- 
lated at  the  utmost  limits  reached  by  the  ice ;  and  (c)  those  formed 
during  the  Retreating  Stage. 

The  pre-glacial  deposits  are  mainly  fluvio-glacial  drifts,  and  from 
their  nature  are  mostly  composed  of  local  detritus. 

The  deposits  formed  during  the  maximum  refrigeration  are 
mainly  terminal  moraines  which  may  contain  erratics  mingled 
with  the  local  debris,  and  widespreading  aprons  and  trains  of 
fluvio-glacial  drift  formed  by  the  streams  issuing  from  the  ice-face. 

The  glacial  deposits  of  the  Retreating  Stage  are  mainly  boulder- 
clays  intercalated  with  fluvio-glacial  drifts.  It  is  in  this  stage 
that  eskers  of  sand  and  drift  are  formed  in  subglacial  channels  and 
ice- tunnels. 

There  is  no  general  agreement  as  to  the  succession  of  the  different 
glacial  deposits  scattered  throughout  the  British  Isles,  and  much 
diversity  of  opinion  exists  as  to  how  some  of  them  were  formed. 
And  the  difficulty  is  complicated  by  the  presence  of  organic  remains 
in  some  of  the  deposits.  But  perhaps  this  difficulty  is  not  so  great 
as  it  appears.  The  ancient  belief  that  the  advance  of  the  northern 
ice  necessarily  involved  the  destruction  of  all  animal  and  plant  life 
in  its  neighbourhood  is  now  known  to  be  fallacious.  Recent 
research  has  shown  that  forests  may  flourish  and  peat-bogs  grow 
on  the  moraines  and  valley-trains  of  a  glacier  up  to  the  edge  of 
the  ice.  Forests  may  even  establish  themselves  on  the  clays  and 
rocky  debris  carried  on  the  back  of  a  glacier. 

In  New  Zealand  there  is  the  instructive  spectacle  of  the  famous 
Franz  Josef  Glacier  embowered  in  a  luxurious  evergreen  forest  at 
a  height  of  670  feet  above  the  sea,  in  43°  S.  latitude. 

It  is  certain  that  where  forests  could  grow,  the  woolly  mammoth 
and  woolly  rhinoceros,  the  Arctic- reindeer,  the  moose,  and  bear 
would  find  a  genial  habitat. 

Forests  and  peat-bogs  in  front  of  a  glacier  are  always  liable  to  be 
covered  over  by  fluvio-glacial  drifts  or  overwhelmed  by  ice  during 
a  temporary  advance  of  the  ice. 

The  rapid  advance  of  the  Malaspina  Glacier,  in  Alaska,  which 
followed  the  great  earthquakes  at  Yakutat  Bay  in  1899,  caused  a 


CAINOZOIC   ERA  :     GLACIAL   PERIOD.  469 

wholesale   destruction   of  the   forests  lying  in  front  of  the  ice- 
face. 

The  till  or  Boulder-Clay  of  Scotland  is  spread  over  the  lowlands 
and  mountain  valleys,  and  usually  rests  on  ice- worn  rocks.  It 
consists  mainly  of  stiff,  unstratified  clay  mingled  with  semi-angular 
blocks  of  stone,  and  varies  from  0  to  100  feet  thick. 

In  some  places  near  the  coast  the  till  overlies  beds  containing 
Arctic  shells,  and  in  other  places  it  is  intercalated  with  sand,  gravel, 
laminated  clay,  and  layers  of  peaty  material  with  plant  remains, 
and  the  teeth  and  bones  of  the  mammoth  and  reindeer. 

The  Boulder-Clay  of  England  is  well  developed  in  East  Anglia 
and  the  countries  around  the  Wash,  where  four  local  subdivisions 
are  recognised  : — 

4.  The  Chalky  Boulder-Clay. 

3.  Mid-glacial  Drift. 

2.  The  Contorted  Drift. 

1.  Cromer  Till. 

The  Cromer  Till  consists  mainly  of  stiff  glacial  clays  with 
striated  fragmen^  of  chalk,  flint,  and  an  assortment  of  Jurassic 
rocks,  Carboniferous  limestones,  and  various  igneous  and  meta- 
morphic  rocks.  Some  of  the  boulders  are  clearly  erratics  from  the 
north.  The  rhomb -porphyry  is  believed  to  have  come  from  the 
neighbourhood  of  Christiania,  and  also  the  boulders  of  the  rock 
called  Laurvikite. 

The  Contorted  Drift,  which  is  well  exposed  in  the  Cromer  Cliffs 
on  the  north  coast  of  Norfolk,  is  a  yellowish-brown  loam,  with 
irregular  layers  of  gravel,  sand,  and  clay.  It  contains  many 
boulders  and  some  enormous  blocks  of  chalk.  This  deposit  is 
rudely  stratified,  and  on  the  north  coast  sharply  contorted,  a 
feature  resulting  probably  from  ice-thrust. 

The  Mid-glacial  Sands  contain  marine  shells  and  ostracod  crus- 
taceans of  a  northern  type. 

The  Chalky  Boulder- Clay  rests  on  the  Contorted  Drift,  but 
also  extends  far  beyond  the  limits  of  that  deposit,  being  found 
as  far  south  as  the  Thames  Basin.  Generally  it  does  not  differ 
much  from  the  Cromer  Till,  but  contains  fewer  Scandinavian 
boulders. 

The  Chalky  Boulder-Clay  is  so  named  from  the  prevailing  colour 
and  the  presence  of  numerous  fragments  and  blocks  of  chalk.  It 
passes  north  of  the  Wash  into  Lincolnshire  and  East  Yorkshire. 
The  infra-glacial  beds  at  Speeton  contain  land  and  marine  shells 
and  the  remains  of  mammals,  among  them  being  Elephas  antiquus, 
Rhinoceros,  and  Hippopotamus. 

In  East  Anglia  Scandinavian  blocks  are  comparatively  common, 


470  A  TEXT-BOOK  OF  GEOLOGY. 

in  East  Yorkshire  they  are  rare,  and  further  north  in  Scotland 
they  are  practically  unknown,  the  pressure  of  the  Scottish  land- 
ice  having  thrust  the  Scandinavian  North  Sea  ice  away  from  the 
mainland. 

Most  of  the  erratics  in  the  Boulder-Clay  of  England  are  from 
Scotland  and  North  England.  For  the  most  part  they  are  igneous 
rocks  of  limited  outcrop  and  distinctive  character,  and  hence  easily 
traced  to  their  original  source. 

The  Scottish  ice  flowing  down  the  Irish  Sea  transported  blocks 
of  the  riebeckite-granite  from  its  source  at  Ailsa  Craig,  on  the  Firth 
of  Clyde,  to  the  Isle  of  Man,  Anglesey,  and  St  David's  Head  in 
Pembrokeshire . 

Among  other  erratics  carried  southward  are  the  grey  granites  of 
Galloway,  the  pink  granophyre  and  granites  of  the  Lake  District, 
and  the  andesites  of  the  Borrowdale  Volcanic  Series.  Boulders 
of  the  famous  Shap  granite  were  carried  from  the  Lake  District 
eastward  into  Yorkshire  by  way  of  Teesdale. 

In  Lancashire,  Cheshire,  and  north  coast  of  Wales,  the  Boulder- 
Clay  is  irregularly  intercalated  with  shelly  sands  and  gravels  that 
do  not  occur  in  a  constant  horizon. 

At  Macclesfield,  in  Cheshire,  these  shelly  deposits  occur  at  a 
height  of  1200  feet  above  the  sea,  and  are  held  by  some  writers  to 
be  a  proof  of  submergence.  The  shells,  however,  are  often  striated, 
and  comprise  a  mixture  of  deep  and  shallow  water  forms,  and 
though  embedded  in  clay  they  are  frequently  filled  with  sand. 
The  shells  are  always  associated  with  erratics  transported  across 
an  arm  of  the  sea,  and  were  doubtless  scooped  up  from  the 
sea-floor  and  transported  in  the  body  of  the  ice  to  their  present 
situations. 

Vast  numbers  of  shells,  mostly  unbroken,  were  discovered  by 
Lamplugh  in  the  terminal  moraine  of  the  Sefstrom  Glacier,  Spitz- 
bergen,  after  it  had  crossed  an  arm  of  the  sea.  This  occurrence 
is  of  great  significance.  It  is  corroborated  by  similar  evidence 
from  Alaska  ;  and  shows  how  readily  marine  material  can  be 
lifted  from  the  sea-floor  and  transported  by  an  advancing  sheet 
of  land-ice. 

The  Lancashire  branch  of  the  Scottish  ice-sheet  from  the  Irish 
Sea  reaches  as  far  south  as  Wolverhampton,  in  South  Staffordshire, 
where  it  dropped  a  vast  number  of  erratic  boulders.  The  southern 
limits  of  the  ice-sheet  in  the  Thames  Basin  are  not  marked  by  a 
terminal  moraine,  which  would  indicate  that  the  ice  did  not  halt 
when  it  reached  these  limits,  but  began  the  northerly  retreat  almost 
at  once,  scattering  an  irregular  sheet  of  boulder-clay  in  its  wake. 

The  city  of  York  is  built  on  the  terminal  moraine  of  one  of  the 
tongues  of  ice  that  protruded  from  the  Scottish  ice-sheet. 


CAINOZOIC    ERA  :     GLACIAL   PERIOD.  471 

Continental  Europe. 

The  Scandinavian  ice-sheet  passed  over  Finland  and  spread  into 
North-East  Kussia,  reaching  as  far  as  the  Ural  Mountains.  It 
bridged  the  Baltic  Sea,  advanced  southward  across  the  great 
Germanic  Plain,  and  even  reached  the  northern  slopes  of  the  Harz 
Mountains  and  Riesengebirge,  where  it  scattered  Scandinavian 
erratics  of  gneiss,  granite,  etc.,  up  to  a  height  of  almost  1500  feet 
above  the  sea. 

The  maximum  thickness  of  this  gigantic  ice-sheet  is  estimated  to 
have  been  not  less  than  7000  feet. 

The  glacial  detritus  scattered  over  Northern  Europe  varies  from 
0  to  670  feet  thick,  and  generally  decreases  in  thickness  from  north 
to  south. 

The  deposits  exhibit  many  local  variations,  which  frequently  take 
place  with  startling  suddenness.  But  as  in  other  glaciated  regions, 
the  succession  is  difficult  to  unravel. 

The  lowest  deposits  are  fluvio-glacial  drifts  composed  of  well- 
worn  sands  and  gravels  formed  by  the  streams  and  rivers  that 
issued  from  the  front  of  the  advancing  ice-sheet.  These  Pre- 
glacial  Drifts,  as  they  are  sometimes  called,  are  followed  by  boulder- 
clays,  which  consist  of  stiff  clays,  with  numerous  blocks  of  stone 
only  slightly  rounded  and  frequently  scratched,  grooved,  and 
polished.  The  lower  portion  of  these  glacial  clays  is  a  bluish-grey 
colour  which  weathers  to  a  yellowish-brown  near  the  surface. 
Furthermore,  as  in  England  and  Scotland,  the  boulder-clays  are 
intercalated  with  irregular  deposits  of  fluvio-glacial  drift  com- 
posed of  sand,  gravel,  and  silt,  often  rudely  stratified.  In  these  so- 
called  Inter-glacial  Drifts  are  found  the  teeth  and  bones  of  mammals, 
peaty  matter,  and  plant  remains.  Among  the  mammals  are  the 
mammoth,  rhinoceros,  giant  elk,  reindeer,  ox,  bear,  etc.,  which  are 
common  in  the  neighbourhood  of  Berlin. 

These  animals  probably  followed  the  retreating  ice-sheet,  and 
frequented  the  broad  reed  and  moss-covered  plains  spread  out  in 
front  of  the  ice.  There  they  lived  and  died  in  great  numbers. 
When  the  ice-sheet  made  minor  advances,  the  drift  with  their 
remains  became  covered  over  with  a  sheet  of  boulder-clay. 

Along  the  Baltic  fringe  the  glacial  clays  contain  many  marine 
molluscs,  among  which  Leda  (Yoldia)  arctica,  Cyprina  islandica, 
and  Corbula  gibba  are  prominent. 

The  northern  glaciation  was  accompanied  by  a  great  extension 
of  the  Alpine  glaciers,  and  glaciers  occupied  the  slopes  and  deep 
valleys  of  the  Pyrenees,  Vosges,  Black  Forest,  Harz,  Eiesengebirge, 
Urals,  and  Caucasus,  where  permanent  ice-fields  no  longer  exist. 

The  Pleistocene  was  a  period  of  great  fluviatile  activity.     Fluvio- 


472  A  TEXT-BOOK  OF  GEOLOGY. 

glacial  and  fluviatile  drifts  were  spread  over  the  valley-floors  to  a 
great  depth,  far  in  advance  of  the  limits  reached  by  the  ice,  forming 
high-level  flood-plains. 

These  drifts  were  deposited  during  both  the  advance  and  retreat 
of  the  ice,  and  hence  range  in  age  from  the  earliest  to  the  latest 
Pleistocene.  The  rivers,  in  the  process  of  excavating  their  present 
channels,  have  in  many  places  left  strips  or  remnants  of  these 
drifts  at  different  levels  along  the  valley- walls.  Obviously  the 
lowest  terraces  are  composed  of  the  oldest  drifts,  and  the  highest 
terraces  of  the  youngest. 

The  loess,  which  covers  a  large  portion  of  Northern  Europe,  and 
extends  from  the  English  Channel  to  Galicia,  Hungary,  and  Russia, 
is  probably  the  wind-borne  flood-silt  of  the  rivers  draining  the  front 
of  the  ice-sheet,  mingled  with  wind-blown  desert- dust.  It  spreads 
over  hill  and  dale,  and  varies  from  0  to  80  feet  thick. 

The  loess  is  an  excessively  fine,  yellowish,  powdery,  unbedded 
loam,  and  is  frequently  calcareous.  When  exposed  in  natural 
cliffs  or  artificial  cuttings,  it  shows  a  tendency  to  assume  a  vertical 
cleavage. 

Among  the  land  shells  found  in  this  remarkable  silt  are  Helix 
hispida,  Succinea  oblonga,  and  Pupa  muscorum,  which  are  char- 
acteristic and  widely  spread.  The  remains  of  the  mammoth  and 
rhinoceros  are  not  uncommon. 

Northern  Asia. — Of  the  Pleistocene  glaciation  of  this  region  very 
little  is  known.  It  is,  however,  quite  certain  that  the  glaciers  of 
the  Himalayas  extended  far  beyond  their  present  limits,  but  how 
far  has  not  yet  been  ascertained. 

The  great  piles  of  morainic  material  in  the  valley  of  the  Kotchurla 
River  show  that  the  ancient  glaciers  of  the  Altai  Mountains  at  one 
time  spread  northwards  many  hundreds  of  miles  from  their  gather- 
ing ground*  A  few  small  glaciers  still  cling  to  the  mountain  slopes 
at  the  sources  of  the  Mushtuaire  River  in  the  Obi  Basin.  Some 
of  the  valley-glaciers  in  alpine  Turkestan  are  of  gigantic  size. 

The  taigas  and  tundras  of  Northern  Siberia  are  covered  with  a 
vast  sheet  of  fluvio-glacial  drift,  and  as  in  Northern  Europe  and 
North  America,  the  mammoth  occupied  a  prominent  place  in  the 
fossil  fauna. 

The  mammoth  lived  in  extraordinary  numbers  along  the  northern 
border  of  that  region,  where  the  well-preserved  bodies  are  found  in 
the  permanently  frozen  soil. 

The  constant  companion  of  the  mammoth,  and  like  it  protected 
with  a  coat  of  woolly  hair,  is  the  boreal  rhinoceros  (Rhinoceros 
antiquitatis).  With  these  also  occur  the  bones  of  the  horse  (Equus 
fossilis)  and  Hippopotamus  major,  the  last  scarcely  distinguishable 
from  the  living  H.  amphibius. 


To  face  page  473.] 


[PLATE    LXIII. 


SECTION  OF  GLACIAL  DRIFT.     NORTH-EAST  PART  OF  NEWARK, 
NEW  JERSEY.     (N.J.  Geol.  Survey.) 


CAINOZOIC   ERA:     GLACIAL   PERIOD.  473 

North  America. — The  Pleistocene  glaciation  of  this  continent  was 
even  greater  and  more  intense  than  in  Northern  Europe. 

There  is  evidence  that  the  northern  half  of  the  continent  from 
the  Atlantic  to  the  Pacific  was  covered  with  a  continuous  ice-sheet, 
which  stretched  northward  toward  the  polar  regions  and  spread 
southward  into  relatively  low  latitudes. 

The  confluent  ice-sheets  are  believed  to  have  radiated  from  four 
main  gathering  grounds,  namely,  the  Greenlandian,  Labradorian, 
Keewatin,  and  Cordilleran,  the  last  three  situated  on  the  mainland 
between  the  parallels  52°  and  55°  N.  latitude.  The  Labradorian 
centre  lay  about  1800  miles  east  of  the  Keewatin  or  Central 
gathering  ground,  and  the  Cordilleran  about  1000  miles  west  of  the 
Keewatin.  The  ice-streams  from  these  centres,  though  so  far 
apart,  united  as  they  radiated  outward  into  a  gigantic  sheet,  which 
altogether  covered  an  area  of  about  4,000,000  square  miles. 

The  Cordilleran  or  Western  ice-sheet  crept  southward  to  47°  N. 
latitude,  and  the  Labradorian  to  37°  or  1600  miles  from  the  centre 
of  dispersion.  From  the  great  glacial  centres  the  confluent  ice- 
sheets  spread  northward,  andj  probably  joined  the  advancing 
polar  ice. 

The  glacial  drifts  spread  over  the  land  by  the  Pleistocene  ice- 
sheet  vary  from  0  to  500  feet  thick,  and  erratics  have  been  found 
over  1000  miles  away  from  the  parent  rock  in  the  north. 

The  mammalian  remains  in  the  glacial  drifts  include  the 
mammoth  and  mastodon  (Plate  LXIV.),  but  the  rhinoceros, 
hippopotamus,  and  hyaena,  so  common  in  the  drifts  of  Northern 
Europe,  are  absent  in  North  America. 

Fluvio-glacial  drifts  are  conspicuous  in  the  Western  States, 
particularly  in  the  Great  Basin  lying  between  the  Kocky  Moun- 
tains and  Sierra  Nevada  Chain.  In  this  region  there  exists  several 
large  lake-basins  that  have  been  partially  or  completely  filled  by 
glacial  drifts.  The  most  notable  of  these  glacial  lakes  are  Lake 
Bonneville,  of  which  the  Great  Salt  Lake  is  the  remnant,  and  Lake 
Lahontan.  The  drifts  in  the  former  are  mingled  with  a  considerable 
quantity  of  volcanic  ash,  the  product  of  eruptions  within  the  lake 
area. 

Southern  Hemisphere. 

The  evidences  of  intense  and  widespread  glaciation  are  plentiful 
in  South  America,  Falkland  Islands,  New  Zealand,  New  South 
Wales,  Tasmania,  and  Antarctic  Continent. 

In  1872  Agassiz,1  the  veteran  glaciologist,  announced  the 
discovery  of  evidence  that  South  America  in  the  Pleistocene 
Period  was  covered  with  a  continuous  ice-sheet  extending  from 
1  A.  Agassiz,  Am.  Jour.  Sc.,  1872,  vol.  iv.  p.  135, 


474  A  TEXT-BOOK  OF  GEOLOGY. 

the  Atlantic  to  the  Pacific  as  far  north  as  37°  S.  latitude,  or  1400 
miles  north  of  Cape  Horn.  But  long  prior  to  this,  Darwin  had 
called  attention  to  the  thick  masses  of  boulder-clay  and  other 
criteria  of  glaciation  in  Tierra  del  Fuego. 

The  Pleistocene  fauna  of  South  America  is  distinguished  from 
that  of  North  America  by  the  presence  of  gigantic  sloths  and 
armadillos,  which  were  indigenous  to  that  region. 

The  advance  of  the  southern  ice  was  accompanied  by  a  corre- 
sponding development  of  Alpine  glaciers  in  the  Central  Andes.  In 
Bolivia  glacial  deposits  cover  both  sides  of  the  Andes,  and  are 
particularly  well  displayed  along  the  western  slopes,  where  they 
are  piled  up  on  the  foothills  to  a  depth  of  many  thousand  feet.  In 
many  places  the  streams  draining  the  existing  glaciers  have 
excavated  profound  gorges  through  these  accumulations.  At  La 
Paz  the  glacial  drifts  are  intercalated  with  thick  deposits  of  volcanic 
tuff  and  breccia.  Obviously  the  Pleistocene  glaciers  of  this  region 
attained  gigantic  proportions. 

In  New  South  Wales  Mount  Kosciusko  was  covered  with  a  cap  of 
glacier-ice,  and  in  Tasmania  glaciers  of  considerable  magnitude 
descended  almost  to  sea-level. 

Owing  to  its  isolation  Australia  followed  its  own  lines  of  develop- 
ment. The  vertebrate  fauna  still  consists  exclusively  of  mar- 
supials and  monotremes,  the  last  represented  by  the  singular 
OrnithorJiynchus  provided  with  a  duck  bill  and  webbed  feet. 

In  New  Zealand,  with  its  massive  Alpine  Chain  as  a  gathering 
ground,  the  confluent  glaciers  descended  to  the  sea-coast  all  round 
the  South  Island,  and  covered  the  greater  portion  of  the  surface 
with  an  almost  continuous  ice-sheet,  through  which  only  the  higher 
peaks  projected  as  gigantic  nunataks. 

In  this  region,  where  the  evidences  of  intense  and  prolonged 
glaciation  are  remarkably  well  preserved,  there  is  nothing  to 
indicate  more  than  one  period  of  Pleistocene  refrigeration,  which, 
as  in  Europe,  may  be  divided  into  three  phases  or  stages,  each 
characterised  by  its  peculiar  glacial  accumulations. 

In  the  Advancing  Stage  the  glaciers  descended  the  valleys  and 
filled  up  the  great  lake-basins  lying  at  the  foot  of  the  Alpine  Chain 
with  fluvio-glacial  drifts.  The  pre-glacial  drifts  were  afterwards 
cut  up  and  deeply  eroded  by  the  advancing  glaciers,  which  con- 
tinued their  seaward  journey  until  they  emerged  on  the  foothills 
and  coastal  plains. 

On  the  west  coast  the  confluent  glaciers  extended  far  out  to  sea  ; 
but  on  the  east  coast  they  halted  at  the  present  coast-line,  where 
the  confluent  glaciers  formed  a  piedmont  ice-sheet,  on  the  terminal 
front  of  which  there  were  piled  up  vast  morainic  accumulations. 
Of  these  the  Taireri  Moraine  in  East  Otago  is  perhaps  the  largest 


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PLATE  LXV. 

POST-PLIOCENE  FOSSILS. 

1.  Pecten  islandicus  (Miill.).     Clyde  beds,  etc.,  reduced. 

2.  Saxicava  rugosa  (Penn.).     Clyde  beds,  etc. 

3.  Astarte  borealis  (Chem.).     Glacial  clays,  half  natural  size 

4.  Nucula  Cobboldice  (Sow.).     Glacial  and  Mam.  Crag. 

5.  Tellina  proxima  (F.  &  H.)  lata  (Gmelin). 

6.  Leda  oblonga  (Sow.)  (lanceolata).     Glacial  beds  and  Arctic  seas. 

7.  Trophon  clathratum  (Linn.).     Glacial  beds. 

8.  Natica  clausa  (Brod.).     Glacial  beds. 

9.  Pleurotoma  rotata  (Def.).     (Murex  Brocc.  Sub- Apennines.) 

10.  Pecten  pleuronectes  ? 

11.  Tooth  of  Mastodon,  one-fourth  natural  size. 


To  fact,  page  474. 


[PLATE    LXV. 


POST-PLIOCENE  FOSSILS. 


CAINOZOIC    ERA  :     GLACIAL   PERIOD.  475 

in  existence.  It  is  35  miles  long,  and  varies  from  a  few  hundred 
yards  to  three  miles  wide,  forming  a  coastal  range  of  hills  which 
rises  in  places  to  a  height  of  over  1000  feet  above  the  sea. 

The  great  Marlborough  Moraine,  which  occurs  300  miles  further 
north,  can  be  traced  along  the  east  coast  for  30  miles  up  to  latitude 
41°  30'  S. 

In  the  North  Island  of  New  Zealand  the  existing  Ruapehu 
glacier  sent  long  streams  of  ice  down  the  neighbouring  valleys. 

When  the  glaciers  in  their  retreat  had  reached  the  inland 
basins  sheltered  by  the  coastal  ranges,  they  halted  and  for  a  time 
entrenched  themselves  behind  barriers  of  morainic  material  which 
they  piled  up  to  a  great  height.  On  two  occasions  they  made 
minor  advances  of  ten  or  twelve  miles,  and  then  began  the  final 
retreat  which  ended  in  the  extermination  of  all  but  the  larger 
glaciers,  the  remains  of  which  still  occupy  the  Alpine  Chain. 

During  the  retreat  vast  quantities  of  fluvio-glacial  drift  were  shot 
into  the  inland  basins,  most  of  which  were  completely  filled  up. 
At  the  present  day  the  filling  up  of  the  remaining  lakes  is  proceeding 
with  almost  incredible  rapidity. 

The  Antarctic  Continent  is  covered  with  a  vast  polar  ice-sheet 
from  which  gigantic  glaciers  descend  to  the  sea  all  around  the 
continent. 

In  South  Victoria  Land  the  confluent  glaciers  descend  to  sea- 
level  and  form  the  famous  Ross  piedmont  ice-sheet  which 
stretches  northwards  across"the  deep  sea  for  hundreds  of  miles, 
its  outer  edge  forming  the '  well-known  Great  Ice  Barrier  first 
seen  by  Ross. 

The  gigantic  glaciers,  the  phenomenal  Ice  Barrier,  the  ice-worn 
domes,  and  scattered  erratics  found  on  both  sides  of  the  continent, 
are  sufficient  to  warrant  the  view  expressed  by  Scott  and  others  that 
the  Antarctic  glaciation  of  the  Pleistocene  must  have  been  vastly 
greater  than  at  present. 

During  the  Glacial  Period  the  Scandinavian  ice-sheet  extended 
across  the  North  Sea  many  hundreds  of  miles,  and  the  Antarctic 
ice-sheet  still  extends  over  the  sea  for  500  or  700  miles,  notwith- 
standing that  the  maximum  glaciation  is  now  long  past. 

How  far  the  ice  extended  northward  during  the  Glacial  Period 
is  unknown.  It  seems  not  improbable  that  the  ice-sheet  from 
Graham's  Land  crossed  the  intervening  sea  to  the  Falkland  Islands 
and  met  the  land-ice  which  spread  over  the  southern  portion  of 
South  America.  The  ability  of  land-ice  to  spread  over  deep 
seas,  so  long  denied,  is  now  generally  recognised  by  geologists. 

Causes  of  Glacial  Period. — Many  hypotheses  have  been  advanced 
to  account  for  the  Pleistocene  refrigeration,  but  as  yet  no  agreement 
has  been  arrived  at,  and  the  final  solution  seems  as  far  off  as  ever. 


476  A  TEXT-BOOK  OF  GEOLOGY. 

Among  the  more  probable  causes  that  have  been  suggested  we 
have  : — 

(1)  Variations  in  the  eccentricity  of  the  Earth's  orbit,  as  advanced 

by  Croll. 

(2)  Wandering  of  the  polar  axis. 

(3)  Depletion    of    the    carbonic    acid   in  the  atmosphere   first 

suggested  by  Herbert  Spencer   and  afterwards  urged  by 
Arrhenius. 

(4)  Climatic  changes  arising  from  Pliocene  uplift  of  great  chains. 

It  is  now  realised  that  powerful  climatic  changes  may  be  caused 
by  the  elevation  of  land  masses  such  as  great  mountain-chains, 
and  that  these  meteorological  changes  may  be  accentuated  by 
a  redistribution  of  land  and  sea,  causing  a  deflection  of  established 
sea-currents. 

The  present  trend  of  investigation  is  to  lay  less  stress  on  probable 
astronomical  causes,  and  to  devote  more  attention  to  the  analysis 
of  the  effects  of  land-masses,  air-  and  sea-currents,  variations  of 
precipitation,  etc. 

It  is  suggestive  that  the  great  mountain-building  of  the  Car- 
boniferous was  followed  by  widespread  glaciation  in  the  Permian 
Period  in  both  hemispheres. 

Recent,  Post-Glacial,  or  Human  Period. 

The  end  of  the  Pleistocene  or  Glacial  Period  is  not  very  clearly 
defined,  but  is  usually  placed  at  the  time  when  the  ice-sheets 
retreated  from  the  lowlands  in  temperate  latitudes. 

The  Glacial  Period  can  only  be  regarded  as  a  past  geological 
age  in  the  latitudes  from  which  ice-sheets  have  completely  dis- 
appeared. Greenland  is  still  in  the  Ice  Age.  The  climatic  condi- 
tions which  now  exist  in  that  northern  land  are  not  unlike  those 
prevailing  in  Northern  and  Central  Europe  during  the  period  of 
greatest  Pleistocene  glaciation.  Similarly  the  Antarctic  region 
has  passed  the  ice-cap  stage  and  is  now  in  the  waning  stage, 
characterised  by  gigantic  valley-glaciers  and  piedmont  ice-sheets. 

Deposits. — The  deposits  of  the  Kecent  Period  include  those  now 
in  process  of  formation,  such  as  river  silts,  sands,  and  gravels  ; 
beach  sands,  muds,  and  gravels  ;  desert  sands,  dust,  and  soils  ; 
growing  peat-bogs  ;  the  detritus  from  recent  volcanic  eruptions  ; 
the  shell-banks  and  coral  reefs  growing  on  the  sea-coasts.  They 
also  comprise  those  lately  formed,  such  as  river  flats,  old  fluviatile 
fans,  peat-bogs,  cave-deposits,  sand-dunes,  and  raised  beaches. 
The  growth  of  formation  of  some  sand-dunes,  peat-bogs,  and  fans 
has  been  continuous  from  the  close  of  the  Pleistocene  up  till  now. 


CAItfOZOIC    ERA  I     RECENT   PERIOD.  477 

Since  the  beginning  of  the  Human  Period,  the  streams  and 
rivers  have  cut  their  channels  a  few  feet  or  few  yards  deeper, 
and  the  sea  has  encroached  on  the  land,  or  the  land  on  the  sea  ; 
but  these  changes  are  relatively  insignificant. 

Fauna  and  Flora.  —  The  existing  fauna  and  flora  are  more 
prolific  and  varied  than  at  any  other  period  of  the  Earth's 
history. 

Foraminifera,  which  first  appeared  in  the  Cambrian,  now  attain 
their  greatest  development.  Nummulites,  which  were  so  numerous 
and  large  in  the  Eocene  and  Oligocene,  are  now  represented  by 
one  or  two  rare  species  found  in  subtropical  waters. 

The  single  corals,  which  were  almost  unknown  until  the  early 
Cainozoic,  are  exceedingly  numerous  ;  and  rock-building  corals, 
which  have  played  an  important  role  as  geological  agents  since 
the  remotest  times,  still  thrive  as  abundantly  as  ever  in  the 
warm  clear  waters  of  the  tropical  seas,  and,  as  in  former  ages, 
are  accompanied  by  sea-urchins  and  starfishes  in  great  numbers. 

Crinoids,  which  attained  their  greatest  development  in  the 
Silurian  Period,  are  now  rare. 

Brachiopods,  which  dominated  the  marine  faunas  of  some  of 
the  Palaeozoic  formations,  have  shown  a  steady  but  slow  decline 
since  the  Silurian,  and  at  the  present  day  are  represented  by  a 
mere  handful  of  genera,  among  which  we  still  have  Rhynchonella, 
Terebratula,  Magellania,  Crania,  Discina,  and  Lingula.  The  first 
and  last  three  are  forms  of  great  antiquity.  Although  only  a  few 
genera  survive,  the  individuals  of  some  species  exist  in  such  vast 
numbers  as  to  indicate  a  great  reserve  of  latent  vitality. 

Molluscs  are  represented  by  hordes  of  Lamellibranchs,  Gastero- 
pods,  and  Cephalopods,  the  geographical  distribution  of  which 
is  now  more  than  ever  dependent  on  climatic  conditions  and 
environment. 

The  Cephalopods,  with  chambered  shells  so  numerous  in  the 
Jurassic  and  Cretaceous  periods,  are  poorly  represented  in  the 
Cainozoic  era.  But  we  still  have  the  Nautilus,  the  fragile  Argonaut, 
and  the  beautiful  Spirula,  which  is  sometimes  cast  up  on  sandy 
shores  in  thousands. 

Cephalopods  of  the  octopus  and  cuttle-fish  kind  are  more 
plentiful  than  ever,  the  only  fossil  form  of  these  of  any  importance 
being  Belemnites,  of  which  there  were  scores  of  species  in  the 
Upper  Mesozoic. 

Crustaceans  are  still  represented  by  a  vast  number  of  Ostra- 
cods,  Cirripedes  (barnacles,  etc.),  and  Isopods  (lobsters,  cray-fish, 
crabs). 

Fishes,  birds,  reptiles,  and  mammals  now  attain  their  greatest 
development. 


478  A  TEXT-BOOK  OF  GEOLOGY, 

The  dominating  figure  among  the  mammals  is  man  (Homo 
sapiens)  ;  hence  the  name  Age  of  Man,  sometimes  applied  to  the 
Recent  Period. 

In  the  Australian  Continent,  on  account  of  its  isolation  and 
persistence  as  a  land-surface,  the  marsupial  mammals  and  the 
primitive  Eucalyptus  vegetation  still  dominate  the  fauna  and 
flora. 

The  plant  remains  discovered  in  Middle  Cainozoic  rocks  in 
Greenland,  Alaska,  and  Antarctic  regions,  supplemented  by  the 
abundant  evidence  provided  in  temperate  and  tropical  countries, 
shows  that  nearly  all  plant  families,  except  such  specialised  forms 
as  the  Orchidacece  among  the  Monocotyledons,  and  the  Composites 
and  their  allies  among  Dicotyledons,  were  at  one  time  more  widely 
distributed  than  at  present. 

Subdivisions.  —  The  remains  of  man  and  traces  of  his  handiwork 
have  been  found  in  drifts  ascribed  to  the  Pliocene  Period,  but  the 
evidence  has  been  challenged,  and  in  every  case  the  age  of  the 
deposits  is  open  to  doubt. 

Human  remains  or  weapons  are  seldom  found  except  in  drifts 
that  are  clearly  of  Post-  Glacial  age  ;  but  the  presence  of  these 
would  tend  to  show  that  man  already  existed  in  Southern  Europe 
in  the  Glacial  Period,  and  followed  the  ice  as  it  retreated  northward. 

There  are  no  unquestioned  evidences  of  man  in  the  glacial 
deposits  of  England  ;  and  since  the  land  connection  between 
Britain  and  the  Continent  was  probably  not  broken  until  the 
late  Pleistocene,  it  seems  unlikely  that  he  would  venture  into 
this  region  until  the  ice-sheet  had  disappeared  for  some  time 
from  the  lowlands.  It  is  almost  certain  that  man  would  follow 
and  not  precede  the  vegetation  and  land-animals  which  followed 
close  on  the  wake  of  the  retreating  ice. 

The  remains  of  man  that  mostly  occur  are  the  weapons  and 
implements  he  fashioned  and  used  ;  and  as  these  show  a  progressive 
development  of  skill  in  their  manufacture  the  nearer  we  approach 
our  own  time,  they  afford  a  means  of  dividing  the  Human  Period 
into  stages.  The  earliest  weapons  and  implements  were  made 
of  stone  and  bone  ;  hence  the  earliest  Human  Period  is  called  the 
Stone  Age,  which  was  successively  followed  by  the  Bronze  Age 
and  Iron  Age. 

(3)  Iron  Age. 
(2)  Bronze  Age. 


(1)  Stone  Age  p  (Kate  LXVI  } 

In  the  Palceolithic  l  or  Old  Stone  Age  the  weapons,  tools,  etc., 
1  Gr.  palaios  =  ancient,  and  lithos  =  a,  stone. 


To  face  page  478.] 


[PLATE    LXVI. 


PALAEOLITHIC  IMPLEMENTS.     (After  Holmes.) 


CAINOZOIC   EEA  :     RECENT   PERIOD.  479 

of  primitive  man  were  roughly  chipped  from  blocks  of  flint,  obsidian, 
chert,  quartzite,  aphariite,  and  other  fine-grained  rocks,  but  in  the 
Neolithic  l  or  New  Stone  Age  they  were  ground  and  polished  with 
much  skill  and  patience. 

(b)  Neolithic      =New  Stone  Age  =  Well- finished  implements. 

(a)  Palaeolithic  =01d  Stone  Age    =Koughly  fashioned  implements. 

The  Palaeolithic  and  Neolithic  represent  stages  of  art  development 
rather  than  periods  of  time.  Hence  the  Palaeolithic  of  one  region 
may  overlap  the  Neolithic  of  another.  At  the  advent  of  Europeans, 
the  Australian  aborigines  were  still  in  the  Palaeolithic  stage  of  art, 
and  the  Maori  of  New  Zealand  in  the  Neolithic.  Neither  of  these 
isolated  races  was  acquainted  with  the  manufacture  or  working 
of  the  metals. 

In  Continental  Europe  and  England  there  are  numerous  caves 
in  which  relics  of  Palaeolithic  man  are  associated  with  the  remains 
of  large  extinct  mammalia.  In  many  cases  the  relics  and  animal 
remains  are  protected  with  a  layer  of  calcareous  stalagmite.  This 
covering  cannot  always  be  regarded  as  an  evidence  of  great  anti- 
quity, as  calcareous  sinters  and  stalagmitic  deposits  are  known 
to  accumulate  with  great  rapidity  in  favourable  situations.  In  the 
rock-shelters  in  the  Waipara  district  in  New  Zealand,  the  bones  of 
the  sheep,  introduced  less  than  seventy  years  ago,  have  been  found 
buried  under  an  encrusting  layer  of  calcareous  stalagmite  four 
inches  thick.  The  evidence  furnished  by  cave-deposits  and  river 
alluvia  must  always  be  subjected  to  critical  examination  before 
using  it  as  the  basis  for  important  deductions  as  to  the  antiquity 
of  man. 

There  is  conclusive  evidence  that  Palseolithic  man  was  contem- 
porary with  several  extinct  mammals,  which  include  the  mammoth 
(Elephas  primigenius),  the  woolly  rhinoceros  (Rhinoceros  ticho- 
rhinus),  the  giant  Irish  elk  with  flattened  horns  (Megaceros  gigan- 
teus  =hibernicus),  long-faced  ox  (Bos  primigenius),  hippopotamus 
(Hippopotamus  major),  the  cave  hyaena  ( Hycena  spelcea),  and  the 
cave  bear  (Ursus  spelceus). 

This  association  may  mean  either  a  considerable  antiquity 
for  man,  or  the  existence  of  these  animals  up  to  a  time  not  so 
very  remote.  The  final  solution  of  this  problem  has  not  yet 
been  found. 

One  of  the  most  important  of  recent  discoveries  relating  to  the 

antiquity  of  man  was  made  by  Eugene  Dubois  in  a  fossil-bearing 

stratum  of  drift  on  the  left  bank  of  the  Solo,  or  Bergawan  stream, 

near  the  centre  of  Java.     The  human- like  remains,  Pithecanthropus 

1  Gr.  neos  =  new,  and  lithos=a,  stone. 


4SO  A  TEXT-BOOK  OF  GEOLOGY. 

erectus,  consist  of  the  roof  of  a  skull,  a  thigh  bone,  and  two 
teeth,  which  were  found  associated  with  a  rich  fauna  and  flora. 
Of  the  mammalian  fauna  no  less  than  19  genera  and  27  species 
were  discovered,  all  of  which  are  now  extinct  or  greatly  modified. 
But  87  per  cent,  of  the  Gasteropods  found  in  the  same  bed  are 
living  species.  Hence  the  drift  with  its  remains  cannot  be  older 
than  Pleistocene. 

Human  remains  were  found  in  1911  by  the  Yale  Peruvian 
Expedition  in  the  Cuzco  Valley  embedded  in  drifts  under  75  feet 
of  gravel.  They  were  associated  with  the  bones  of  several  lower 
animals,  and  are  believed  to  be  of  Pleistocene  age. 

In  1912  an  important  discovery  of  ancient  human  remains  was 
made  in  a  gravel  pit  near  Piltdown  Common,  Fletching,  Sussex. 
The  gravel  bed  lies  about  80  feet  above  the  river  Ouse,  and  less 
than  a  mile  to  the  north  of  the  existing  stream.  The  deposit  is 
about  4  feet  thick,  and  consists  mainly  of  water-worn  fragments 
of  Wealden  ironstone  and  sandstone,  with  a  few  chert  pebbles  and 
a  considerable  proportion  of  water-worn  flints  derived  from  the 
Chalk  of  the  South  Downs.  Portions  of  a  human  skull  were  found 
associated  with  a  jaw  of  simian  type,  and  the  remains  of  an  elephant, 
a  mastodon,  hippopotamus,  and  red  deer,  besides  flint  implements. 
The  skull  shows  a  high  cranial  development,  but  is  believed  by 
Keith  to  belong  to  a  man  of  greater  antiquity  than  the  Neanderthal 
flat-skulled  man  of  Germany,  or  the  Spy  man  of  Belgium  charac- 
terised by  enormous  brow-ridges. 

Among  the  most  prolific  bone  caves  or  hycena-dens  in 
England  are  the  famous  Kirkdale  Cave,  near  Kirkby  Moorside, 
in  Yorkshire  ;  Dream  Cave  in  Derbyshire  ;  Banwell  Cave  in  the 
Mendip  Hills ;  Kent's  Hole,  near  Torquay ;  and  Cefn,  near 
Denbigh. 

In  France  caves  rich  in  bones  have  been  found  near  Montpellier 
and  Narbonne ;  in  Germany,  between  the  Harz  and  Franconia  ; 
and  in  Austria,  in  Carniola  and  Hungary. 

No  Palaeolithic  remains  are  known  in  Scotland  or  the  far  north 
of  England. 

In  North  America  Palaeolithic  and  Neolithic  relics  are  common 
in  recent  deposits,  but  they  are  not  found  in  association  with  the 
large  Pleistocene  mammals  as  in  Europe. 

In  Britain  Neolithic  relics  are  found  in  sand-dunes,  caves,  peat- 
bogs, swamps,  and  in  tumuli  which  are  now  known  to  be  the  tombs 
of  Neolithic  man.  The  associated  fauna  is  quite  distinct  from 
that  of  the  Palaeolithic  age.  Most  of  the  large  mammals  have 
become  extinct,  but  the  Irish  elk,  reindeer,  and  bear,  which  no 
longer  survive  in  England,  were  present,  together  with  the  fauna 
of  early  historic  times. 


CAINOZOIC    ERA  :     RECENT    PERIOD.  481 

Raised-beaches  occur  around  all  the  continents,  and  may  be 
observed  on  the  shores  of  England,  Scotland,  Norway,  Finland, 
France,  Sicily,  Italy,  Egypt,  East  Africa,  North  Africa,  Arabia, 
India,  Malaysia,  Australia,  Tasmania,  New  Zealand,  North  and 
South  America.  The  presence  of  these  marine  benches  is  an 
evidence  of  universal  recession  of  the  sea  in  quite  recent  times. 

SUMMARY. 

From  the  earliest  Cambrian  when  the  first  assemblage  of 
organisms  appeared,  there  has  been  a  continuous  procession  of 
life,  receiving  accessions  of  new  forms  at  each  geological  stage 
until  it  grew  into  the  majestic  stream  which  now  floods  the  Earth 
in  such  amazing  wealth  of  animal  and  vegetable  life. 

(1)  The  Eocene  Period  is  specially  characterised  as  the  dawn  of 
existing  life,  and  it  is   sharply   separated   from  the   Cretaceous 
by    the    absence    of    the    Cephalopods     Ammonites,    Belemnites, 
Samites,  Turrilites,  Baculites,  etc.,  and  of  the  reptilians  Ichthyo- 
saurus, Plesiosaurus,  and  huge  Deinosaurs. 

The  stratigraphical  unconformity  between  the  Cretaceous  and 
Eocene  is  not  strongly  marked,  and  is  often  difficult  to  distinguish  ; 
but  the  palaeontological  break  is  the  greatest  and  most  abrupt  in 
the  whole  geological  record. 

The  great  Central  Sea,  Tethys,  still  stretched  from  the 
Atlantic  to  Further  India.  On  its  floor  were  accumulating 
thick  deposits  of  Nummulitic  Limestones  and  on  its  margin, 
piles  of  deltaic  sands  and  muds  of  the  Flysch  facies  of  detrital 
deposits. 

The  volcanic  forces  which  lay  dormant  during  the  whole  Mesozoic 
era,  but  revived  at  the  close  of  the  Cretaceous,  still  continued  to 
exhibit  great  activity  in  certain  regions. 

The  dominating  feature  of  the  Eocene  fauna  is  the  advent  of 
placental  mammals,  including  ancestral  forms  of  most  of  the  large 
mammals  of  the  present  day. 

The  angiosperms  or  flowering  plants  which  appeared  in  the 
Upper  Cretaceous  comprise  the  dominant  vegetation  in  the  Eocene 
forests,  being  represented  by  a  great  variety  of  Dicotyledons  and 
Monocotyledons. 

(2)  The    Oligocene    is    stratigraphically    and    palaeontologically 
related  to  the  Eocene,  to  which  it  properly  belongs.     It  is  mainly 
distinguished  by  the  vast  development  of  Foraminifera  and  reef- 
building  corals. 

The  Central  Sea  is  still  a  feature  of  vast  geographical,  geological, 
and  meteorological  importance  ;  and  on  its  northern  shores  the 
deposition  of  the  deltaic  sands  and  muds  of  the  Flysch  type 

31 


482         A  TEXT-BOOK  OF  GEOLOGY. 

continue  to  be  deposited  without  interruption.  At  their  outward 
limits  the  deltaic  detritus  is  intercalated  with  the  Nummulitic 
Limestone  deposits  formed  on  the  floor  of  the  deeper  clearer 
waters  of  this  great  inland  sea. 

(3)  The  Miocene  was  a  period  of   great  geographical  changes. 
It    witnessed    the    uplift    of    the    Pyrenees,    Alps,    Carpathians, 
Caucasus,  and  Himalayan  Chains  from  the  floor  of  the  Central  Sea, 
which  thereby  became  reduced  in  size  and  broken  up  into  large 
disconnected  seas  and  inland   salt-water  lakes.     It   was  at  this 
time  that  the  Mediterranean  Sea,  which  is  the  last  remnant  of  the 
great  Tethys,  was  cut  off  from  the  Indian  Ocean  by  the  uplift 
of  Syria,  Asia  Minor,  Arabia,  and  Persia. 

The  Miocene  fauna  and  flora  show  an  increasing  relationship 
to  the  life  of  the  present  time.  The  mammals  now  include  the 
Mastodon,  true  elephant,  the  huge  Deinotherium,  rhinoceros, 
hippopotamus,  deer,  whales,  dolphins,  etc. 

(4)  In  the  Pliocene  the  Alps,  Himalayas,  and  other  great  chains 
attained   their   full  height ;    and   the   continents   assumed   their 
present  forms. 

Up  till  the  middle  of  this  period  the  climate  of  Northern  Europe 
and  North  America  was  tropical  or  subtropical,  but  thereafter 
the  character  of  the  fauna  and  flora  shows  the  approach,  first  of 
temperate,  then  of  sub- Arctic  cold.  This  gradual  increase  of 
cold  was  heralded  by  the  southern  migration  of  boreal  forms  into 
the  temperate  zones,  both  in  Northern  Europe  and  North  America, 
and  the  migration  of  the  southern  forms  into  more  congenial 
latitudes. 

The  fauna  and  flora  of  the  Pliocene  had  already  assumed  a 
modern  appearance,  and  90  per  cent,  of  the  marine  molluscs  are 
living  forms. 

(5)  The    increasing    cold    culminated    in    the    Pleistocene    or 
Glacial  Period,  also  called  the  Great  Ice  Age.     In  the  early  Pleisto- 
cene Northern  Europe  and  North  America  were  invaded  by  the 
northern  ice-sheet.     In  Europe   the   ice-sheet  radiated  outwards 
in  all  directions  from  the   highlands  of   Scandinavia,  and  at  the 
same  time  the  Alpine  glaciers  crept  down  their  valleys  to  the 
foothills  and  plains. 

The  Scandinavian  ice-sheet  bridged  the  Baltic  Sea  and  filled 
up  the  North  Sea  as  far  south  as  the  English  Channel.  The  land- 
ice  radiating  from  the  Scottish  Highlands  flowed  into  the  North 
Sea  and  fended  the  Scandinavian  ice  from  the  mainland  ;  but  in 
England  the  Scandinavian  ice  rasped  the  north-east  coasts,  and, 
flowing  down  the  Wash  gulf,  overflowed  East  Anglia  and  the  adjoin- 
ing counties.  Wherever  it  touched  land  it  left  a  trail  of  Scandi- 
navian erratics. 


CAINOZOIC   ERA  :     RECENT   PERIOD.  483 

The  Scottish,  land-ice  flowed  southward  into  England  as  far 
as  York  ;  and  on  the  west  coast  filled  the  Irish  Sea,  which  it 
descended  till  abreast  of  the  Bristol  Channel.  It  surged  over  the 
highest  peaks  in  the  Isle  of  Man,  2000  feet  above  sea-level  ;  and 
sent  a  huge  stream  through  the  Lancashire  Plain  into  Central 
England  and  basin  of  the  Severn.  The  division  of  the  Scottish 
ice  on  the  west  coast  was  due  to  the  resistance  offered  by  the 
Welsh  mountains  and  their  cap  of  land-ice. 

The  Pleistocene  Period  is  divided  into  three  stages,  namely,  the 
Advancing  Stage,  Maximum  Stage,  and  Retreating  Stage.  That 
is,  the  Ice  Age  began  and  ended  in  local  glaciers,  which  became 
confluent  during  the  maximum  stage  of  refrigeration. 

The  Advancing  Stage  was  characterised  by  the  deposition  of 
vast  deposits  of  fluvio-glacial  drifts  formed  in  front  of  the  descend- 
ing Alpine  glaciers  and  advancing  northern  ice-sheet. 

At  the  extreme  limits  reached  by  the  ice  during  the  period  of 
maximum  refrigeration,  at  the  place  where  the  ice-edge  halted, 
there  was  frequently  piled  up  high  mounds  and  ridges  of  morainic 
material  and  widespreading  valley-trains. 

During  the  retreat,  the  ice  scattered  a  sheet  of  boulder-clay  or 
ground-moraine  in  its  wake,  forming  a  deposit  that  spread  over 
hill  and  dale.  Where  the  ice-face  was  drained  by  glacial  streams 
or  rivers,  the  boulder-clays  were  partially  re-sorted  and  associated 
with  fluvio-glacial  drifts  that  frequently  contain  the  remains  of 
large  extinct  mammals.  During  minor  advances  of  the  ice,  these 
drifts  were  sometimes  covered  over  with  morainic  detritus,  and 
thus  became  intercalated  in  the  ground- moraines. 

Throughout  the  retreat,  fluvio-glacial  drifts  were  continually 
deposited  in  front  of  the  ice-edge,  forming  what  are  called  the 
Newer  Glacial  Drifts  to  distinguish  them  from  the  Pre-  Glacial 
Drifts  formed  during  the  advancing  stage. 

(6)  The  Recent  or  Human  Period  is  specially  characterised  by 
the  advent  of  Man,  whose  relics  are  found  in  caves,  drifts,  and  peat- 
bogs associated  with  some  large  extinct  mammals,  such  as  the 
woolly  mammoth,  woolly  rhinoceros,  great  Irish  elk,  cave-hysena, 
and  cave-bear. 

The  Recent  Period  is  divided  into  three  main  stages,  namely  : — 

3.  Iron  Age. 
2.  Bronze  Age. 
1.  Stone  Age. 

The  Stone  Age  is  subdivided  into  two  sub-stages,  the  Palaeolithic 
and  Neolithic. 

The  Palceolithic  was  the  age  of  rough,  rudely  fashioned  imple- 


484  A  TEXT-BOOK  OF  GEOLOGY. 

ments,  and  the   Neolithic  the  age   of   well-finished   and  polished 
implements. 

Raised-beaches  occur  around  the  coasts  of  all  the  great  continents 
and  islands,  and  indicate  a  general  recession  of  the  sea  in  com- 
paratively recent  times. 


CHAPTER   XXXIV. 
DEVELOPMENT   OF   SURFACE   FEATURES. 

THE  development  of  surface  forms  is  mainly  dependent  (a)  on  the 
character  and  arrangement  of  the  rocks,  and  (6)  on  the  climatic 
conditions.  Of  these  the  last  is  perhaps  the  more  important. 

When  we  broadly  view  the  surface  configuration  of  the  Earth, 
we  have  no  difficulty  in  distinguishing  two  outstanding  types  of 
surface  form,  namely  : — 

I.    Arid  Erosion  type. 
II.  Pluvial  Erosion  type. 

Arid  Erosion  Type. — Arid  erosion  may  be  defined  as  the  degrada- 
tion of  the  land  by  subaerial  agencies  where  the  annual  rainfall 
is  less  than  18  or  20  inches.  Its  effects  are  best  seen  where  the 
rainfall  is  confined  to  a  few  months  in  the  year. 

Arid  erosion  acts  uniformly  on  all  the  surfaces  exposed  to  the 
action  of  the  atmosphere,  but,  owing  to  the  effects  of  desert  winds 
and  gravitation,  it  is  more  energetic  on  the  prominent  land- 
features  than  elsewhere.  Hence  its  general  effect  is  to  reduce 
the  whole  surface  of  the  land  to  a  plateau  or  base-level  of  low 
relief.  The  plateau  form  of  feature  is  typical  of  arid  erosion  in 
all  continental  areas. 

Good  examples  of  plateaux  of  arid  erosion  may  be  seen  in  the 
high  veldt  lands  of  South  and  Central  Africa,  in  the  desert  regions 
of  the  Western  States  of  North  America,  in  the  sandy  wastes  of 
Arabia,  Central  Asia,  and  Mongolia,  and  in  the  high  undulating 
forest-covered  interior  of  Australia. 

In  South  and  Western  Australia  the  edge  of  the  great  plateau 
is  buttressed  by  great  descending  spurs  and  ridges  frequently 
surmounted  by  what  appear  to  be  prominent  mountain-peaks. 
Hence,  when  viewed  from  the  sea-border,  the  edge  of  the  plateau 
presents  the  configuration  of  a  mountain-chain.  The  same  rugged 
outline  and  Alpine  effect  is  seen  on  the  edge  of  the  high  veldt-lands 
of  the  Orange  Free  State  and  Transvaal  when  viewed  from  the 
Natal  border. 

485 


486  A  TEXT-BOOK  OF  GEOLOGY. 

A  peculiarity  of  arid  and  semi-arid  regions  is  the  circumstance 
that  many  of  the  boulders  and  fragments  of  stone  lying  on  the 
surface  have  become  coated  on  the  outside  with  a  bluish-black 
shining  glaze  or  enamel,  consisting  of  manganese  and  iron  oxides, 
mostly  the  former.  The  stones  usually  glazed  in  this  way  are 
basic  and  semi-basic  igneous  rocks  and  greywackes  ;  and  where 
such  are  abundant,  the  black  stones  impart  a  burnt  aspect  to  the 
landscape.  The  exposed  surfaces  of  siliceous  cement  stones,  which 
consist  of  sands  that  have  been  cemented  into  a  solid  rock  by  the 
infiltration  of  siliceous  waters,  and  of  siliceous  sinters  frequently 
become  glazed  with  a  vitreous  enamel  of  silica.  The  formation 
of  these  enamels  is  apparently  the  result  of  chemico-capillary 
action. 

The  characteristic  colour  of  arid  landscapes  is  a  warm  yellowish- 
red  hue  arising  from  the  peroxidation  of  the  iron  contained  in  the 
rocks.  Desert  sands  and  dust  are  characteristically  red,  frequently 
brick-red. 

Pluvial  Erosion  Type. — The  general  tendency  of  rain,  frost, 
and  other  subaerial  agencies  of  denudation  is  to  degrade  the  whole 
surface  of  the  land  ;  but  the  erosive  effects  of  rain,  in  the  form 
of  brooks,  streams,  and  rivers,  is  to  wear  away  the  surface  faster 
in  one  place  than  in  another.  The  streams  will  naturally  follow 
fractured  and  faulted  zones  rather  than  excavate  channels  through 
solid  rock,  and  they  will  erode  soft  rock-formations  faster  than 
hard.  The  general  effect  of  this  differential  rate  of  denudation 
will  be  the  gradual  development  of  a  variety  of  surface  features, 
the  form  of  which  will  be  dependent  on  the  character  and  arrange- 
ment of  the  rocks,  the  amount  of  the  rainfall,  and  the  velocity 
of  the  streams  ;  and  this  last  will  be  governed  by  the  height  of  the 
land  relatively  to  its  base-level. 

As  erosion  proceeds,  the  harder  masses  of  rock  will  be  left  standing 
above  the  general  level  of  the  country,  and  they  may  form  hills, 
ridges,  or  even  mountains.  In  gently  folded  strata  the  harder 
bands  of  rock  will  form  prominent  lines  of  escarpment.  Where 
gently-inclined  strata  are  intersected  by  a  strike-fault,  the  harder 
bands  will  form  ridges  with  a  steep  descent  into  the  fault-valley 
on  the  one  side  and  a  long  gentle  dip-slope  on  the  other. 

In  a  previous  chapter  we  found  that  a  rock-formation  represent- 
ing a  cycle  of  deposition  is  generally  closed  by  a  bed  of  limestone. 
In  folded  or  faulted  strata  it  is  this  calcareous  member  which 
usually  forms  the  prominent  escarpments  or  declivities  of  a 
landscape. 

When  a  rock-formation  contains  a  number  of  hard  bands  of 
limestone,  conglomerate,  or  sandstone  separated  by  clays,  marls, 
or  other  soft  rock,  the  outcrops  of  the  hard  bands  not  infrequently 


DEVELOPMENT  OF  SURFACE  FEATURES. 


487 


stand  out  as  conspicuous  escarpments  that  can  be  traced  by  the 
eye  for  many  miles  as  they  contour  around  the  ridges  and  mountain 
slopes. 

Generally  speaking,  the  denudation  of  formations  composed  of 
clays,  marls,  shales,  chalk,  or  soft  sandstones  produces  gentle 
slopes  and  smooth  outlines,  even  when  the  beds  are  steeply 
inclined.  Conversely,  the  denudation  of  hard  rocks,  and  particu- 
larly of  hard  rocks  alternating  with  soft,  usually  develops  rugged 


FIG.  221.— Showing 
(C.  ] 


erosion  of  plateaux  in  cretaceous  rocks. 
!.  Dutton,  U.S.  GeoL  Survey.) 


outlines,  more  particularly  where  the  strata  are  tilted  at  high 
angles.  But  a  cycle  of  denudation  has  its  stages  of  infancy, 
maturity,  and  old  age  ;  hence  the  character  of  the  sculpturing 
as  presented  to  the  eye  will  depend  on  the  progress  made  towards 
maturity  or  old  age. 

In  a  region  occupied  by  a  great  thickness  of  mica-schist  occurring 
in  isoclinal  folds,  the  rock  in  the  infantile  stage  of  erosion  will  be 
carved  into  V-shaped  valleys  and  tent-shaped  ridges.  At  a  later 
stage  the  valleys  will  be  widened  and  the  ridges  rounded  ;  and 
in  the  decadent  stage  we  shall  get  an  area  of  gentle  slopes  and  low 
relief,  not  distinguishable  in  configuration  from  the  rolling  downs 
composed  of  clays,  marls,  or  shales. 


488  A  TEXT-BOOK  OF  GEOLOGY. 

As  with  mica-schist,  so  it  is  with  slate  or  gneiss.  Even  a  granite 
massif  may  form  a  bold  mountain  dominated  with  gigantic  tors 
or  a  flat  swampy  moorland. 

In  regions  that  have  been  overrun  with  land-ice,  the  contours 
are  softened  and  rounded.  In  Alpine  valleys  lakes  may  be  formed 
by  morainic  barriers,  and  crescent-shaped  piles  of  glacial  debris 
scattered  over  the  plains  and  foothills. 

But  pluvial  denudation  is  not  always  destructive.  When 
constructive,  it  is  responsible  for  the  development  of  many  minor 
surface  features,  among  which  may  be  named  the  great  alluvia] 
plains  and  deltas  that  border  the  sea. 

Mountains. 

A  mountain  may  be  denned  as  a  hill  or  ridge  that  rises  con- 
spicuously above  the  surrounding  country.  The  term  is  in  some 
respects  a  relative  one,  for  the  ridge  that  would  form  a  conspicuous 
mountain  on  the  plains  of  Prussia  might  sink  into  insignificance 
if  placed  among  Alpine  surroundings. 

A  mountain-chain  is  a  narrow  ridge  or  a  succession  of  narrow 
ridges  running  more  or  less  parallel  with  one  another.  The  pro- 
minent peaks  on  a  mountain-chain  are  often  called  mounts  or 
mountains. 

If  we  look  at  a  physical  map  of  the  Earth,  we  shall  find  that  (a) 
the  continents  are  in  general  bordered  with  mountain-chains, 
and  (6)  that  the  highest  border  faces  the  larger  ocean.  The  girdle 
of  mountain-chains  that  encircles  the  Pacific  Ocean  is  a  striking 
illustration  of  this  type  of  continental  fringe,  and,  moreover,  it 
faces  the  greater  ocean. 

Origin  of  Mountains  and  Mountain- Chains. — Mountains  and 
mountain-chains  have  originated  in  various  ways,  and,  according 
to  their  origin,  they  may  be  divided  into  four  classes  : — 

(1)  Folded  Mountains,  i.e.  the  Alpine  type. 

(2)  Volcanic  Mountains. 

(3)  Plateau- Mountains. 

(4)  Residual  Mountains. 

Folded  Mountains. — This  type  includes  all  the  great  mountain- 
chains  of  the  globe  which  are  now  known  to  consist  of  uplifted 
crustal  folds  ;  hence  the  origin  of  the  distinctive  name  Alpine 
by  which  this  type  is  sometimes  not  inappropriately  designated. 

To  this  type  of  folded  chain  belong  the  Andes,  Rockies,  Sierras, 
Appalachians,  Alps,  Pyrenees,  Carpathians,  Urals,  Himalayas, 
and  New  Zealand  Alps. 

The  folding  has  been  caused  by  lateral  compression  or  thrust 


II 

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To  face  page  489.] 


[PLATE    LXVIII. 


REPRESENTATION  OF  WILLIS'  EXPERIMENTS  IN  THE  ARTIFICIAL  PRODUCTION 
OF  MOUNTAIN  FOLDS,  WITH  LAYERS  OF  WAX  OF  DIFFERENT  COLOURS. 
(U.S.  Geol.  Survey.) 


DEVELOPMENT  OF  SURFACE  FEATURES.        489 

arising  from  the  contraction  of  the  Earth's  crust.  The  most 
obvious  result  of  compression  is  the  shortening  of  the  area  occupied 
by  the  strata. 

The  geological  investigations  of  Nicol,  Lap  worth,  Peach,  Home, 
and  others  in  the  North- West  Highlands  of  Scotland  have  proved 
conclusively  that  sharp  folding  is  always  accompanied  by  fracture 
and  faulting  ;  and  sometimes  by  extraordinary  horizontal  shear, 
whereby  overriding  sheets  of  rock  may  be  overthrust  many  miles 
from  the  place  where  they  were  formed.  By  a  series  of  pressure 
experiments  in  1888,  Cadell  obtained  instructive  imitations  of 
the  tectonics 1  of  mountain-building,  overthrust,  and  infolding  of 
strata. 

The  forms  of  folded  or  tectonic  mountains  in  their  juvenile  stages 
of  existence  are  in  a  measure  an  expression  of  their  structure. 
The  ridges  coincide  with  the  anticlines,  and  the  valleys  with  the 
synclines  or  downward  folds.  This  juvenile  structure  is  well 
exemplified  in  the  Swiss  Jura,  which  consist  of  four  main  parallel 
ridges,  each  dominated  by  an  anticlinal  fold,  as  shown  in  fig.  222. 


N.W. 
FIG.  222. — Showing  symmetrical  flexures  of  Swiss  Jura.     (After  Clerc.) 

With  increasing  age,  and  as  the  effects  of  denudation  become 
greater  and  greater,  the  coincidence  between  folding  and  configura- 
tion becomes  less  and  less,  and  finally  disappears. 

In  the  Appalachian  Mountains  in  Pennsylvania,  denudation  has 
progressed  so  far  that  the  valleys  follow  the  anticlines,  while  the 
ridges  coincide  with  the  synclines,  which  resist  denudation  more 
effectually  than  anticlines. 

The  mountains  in  time  become  remodelled  by  erosion  ;  and  the 
new  configuration  is  determined  by  the  character  and  arrangement 
of  the  rocks,  and  the  climatic  conditions.  Such  ancient  tectonic 
mountains  may  therefore  be  so  modified  by  erosion  as  to  be  difficult 
to  distinguish  from  the  type  of  mountains  called  residual.  Many 
folded  mountain-chains  existed,  of  which  only  the  worn-down 
stumps  now  remain.  They  have  been  truncated  by  the  erosion 
in  past  geological  ages,  and  partly  buried  under  the  detritus 
derived  from  their  own  destruction. 

Volcanic  Mountains. — These  are  hills  or  ridges  piled  up  by  the 
accumulation  of  lavas  and  other  material  ejected  from  a  volcanic 
vent  or  fissure. 

Volcanoes  may  rise  from  the  floor  of  the  sea,  from  a  plain  or 
1  Gr.  tekton= builder. 


490 


A   TEXT-BOOK    OF   GEOLOGY. 


plateau,  or  from  the  crest  of  an  Alpine  chain,  as  many  do  in  the 
Andes  of  South  America.  They  may  occur  as  isolated  mountains 
or  in  groups  of  mountains,  each  with  its  own  crater,  or  in  consider- 
able chains. 

Volcanoes  of  late  Tertiary  date  generally  retain  their  original 
form,  modified  perhaps  to  a  small  extent  by  recent  fluviatile  erosion, 
but  the  older  piles  of  volcanic  rocks  have  in  most  cases  been  so 
deeply  eroded  that  they  now  present  all  the  features  and  sculpturing 
of  mountains  formed  by  erosion. 


FIG.  222A. — Types  of  plateau-mountain  structure. 
(After  Powell,  modified  by  Rastall.) 


(a)  Uinta  type. 

(6)  Park  Plateau  type. 


(c)  Kaibab  type. 

(d)  Block-mountain  type. 


Plateau-Mountains. — These  are  subordinate  in  extent  and 
grandeur  to  the  great  alpine  chains  composed  of  uplifted  folds,  but 
they  present  some  features  of  peculiar  interest  in  connection  with 
crustal  movements.  In  Colorado  and  Utah,  where  plateau-moun- 
tains are  well  developed,  four  types  of  structure  have  been  recognised 
by  Powell  and  other  members  of  the  Geological  Survey  of  the 
United  States. 

(1)  Uinta  Structure. — This  type  of  structure  is  typically  developed 
in  the  Uinta  Mountains  of  Utah  and  Wyoming.  It  consists  of 
two  large  monoclinal  flexures  bending  in  opposite  directions,  each 


DEVELOPMENT  OF  SURFACE  FEATURES.       491 

with  the  downthrow  on  the  external  side,  thus  leaving  a  broad 
plateau-like  mountain  between  them  (fig.  222A,  A).  In  some  places 
the  strata  in  the  line  of  maximum  flexure  have  given  way,  and  here 
the  bending  stress  has  been  relieved  by  fracturing,  accompanied 
by  profound  faulting. 

(2)  The  Park  Plateau  Structure  is  found  in  the  Yellowstone  Park 
region  of  Wyoming,  where  a  thick  series  of  Mesozoic  rocks  has  been 
bent  over  a  plateau-like  complex  of  granites  and  gneisses,  and  after- 
ward denuded,  thereby  leaving  their  truncated  ends  standing  up  at 
high  angles  against  the  flanks  of  the  central  mass  of  older  rocks 
(fig.  222A,  B). 

(3)  The  Kaibdb  Structure,  is  related  to  the  Uinta  structure,  of 
which  it  is  merely  a  modification.     It  consists  of  one  dominant 
monoclinal   flexure,   and   a   powerful  fault  which  dislocates  the 
upraised  horizontal  strata  (fig.  222A,  G). 


Old  Man  Range 


S.W. 


FIG.  223. — Showing  block -mountains  of  Central  Otago,  N.Z. 
(a)  Mica-schist.  (c)  Middle  tertiary  lacustrine  beds. 

(6)  Triassic  sandstones  and  shales.        (d)  Pleistocene  gravels. 

(4)  Block- Mountains. — This  type  owes  its  origin  to  the  displace- 
ment of  large  blocks,  or  segments,  of  the  crust  along  the  track  of 
gigantic  faults.  Typical  structures  of  this  class  are  found  in 
Colorado  and  Utah,  and  in  Central  Otago,  New  Zealand,  all  in 
arid  plateau  regions. 

This  structure  is  produced  when  a  broad  and  lofty  plateau,  or 
elevated  peneplain,  is  broken  into  blocks  by  profound  faults.  In 
the  Great  Basin  region,  lying  between  the  Sierras  and  .Rocky 
Mountains,  the  blocks  are  tilted  at  various  angles  (fig.  222A,  D)  ; 
but  in  Central  Otago  they  have  maintained  a  nearly  horizontal 
position,  and  are  separated  by  wide  rift- valleys  or  graben. 

Horsts. — For  well-defined  blocks  that  have  remained  as  elevated 
masses  while  the  areas  around  them  have  sunk,  Suess  has  used  the 
German  term  horst.  Such  fixed  blocks  are  closely  related  to  block- 
mountains.  Typical  examples  are  the  mountains  of  the  Vosges 
and  Black  Forest,  which  owe  their  existence  to  the  subsidence  of 
the  segment  which  now  forms  the  rift- valley  of  the  Rhine, 


492 


A   TEXT-BOOK    OF    GEOLOGY. 


Large  portions  of  Australia,  of  Central  and  South  Africa,  have 
been  land  areas  since  the  earliest  times,  and  may  be  looked  upon 
as  forming  horsts  of  continental  dimensions.  These  immovable 
blocks  are  mostly  composed  of  complex  masses  of  crystalline  rocks, 
and  in  a  large  measure  they  have  controlled  and  directed  the  lateral 
crust al  movements  of  younger  Palaeozoic  and  later  times.  They 
have  formed  the  thrust-blocks,  or  anvils,  against  which  the  unaltered 
sedimentary  formations  have  been  crushed,  folded,  and  sheared. 

Similar  unyielding  blocks  occur  in  the  Alaskan  and  Laurentian 
regions  ;  and  on  a  smaller  scale  we  have  the  crystalline  block 
forming  the  substructure  of  Great  Britain,  with  its  protruding 
domes  in  the  Southern  Uplands  and  Northern  Highlands  of  Scot- 
land. Nearly  antipodal  to  this  there  is  the  New  Zealand  crystal- 
line block,  to  the  existence  of  which  the  distant  Dominion  owes  its 
oceanic  character. 


Vosges 


Black  Forest, 


FIG.  223A. — Showing  the  Rhine  flowing  in  a  Rift-valley. 
1,  Granite.     2-7,  Mesozoic  rocks.     8,  9,  Tertiary  and  Recent. 

Residual  Mountains. — These  are  formed  by  the  removal  of  the 
surrounding  country  by  erosion.  Residual  mountains  and  ridges 
therefore  owe  their  existence  to  their  ability  to  resist  denudation. 

When  a  high  plateau  has  been  deeply  dissected  by  the  erosion 
of  considerable  streams  and  rivers,  the  portions  that  have  escaped 
destruction  may  sometimes  form  a  complex  of  ridges  and  mountains 
with  narrow  crests  and  projecting  peaks.  As  a  rule,  residual 
mountains  do  not  occur  in  continuous  chains  like  the  Alpine  type, 
nor  do  they  show  any  symmetrical  arrangement.  On  the  con- 
trary, they  generally  occur  as  a  tumble  of  isolated  ridges  and 
mountains. 

The  Highlands  of  North  Scotland  are  a  good  example  of  moun- 
tains of  the  residual  type.  A  study  of  their  structure  shows  that 
they  are  merely  the  remnants  of  the  ancient  Caledonian  plateau, 
that  itself  represented  the  stump  of  a  still  more  ancient  folded 
mountain  system. 

Origin  of  Plateaux. — Plateaux  are  elevated  tablelands  possessing 


DEVELOPMENT  OF  SURFACE  FEATURES. 


493 


a  more  or  less  undulating  surface.       Genetically  they  may  be 
arranged  in  three  dominant  groups  : — 

1.  Plateaux  of  arid  erosion. 

2.  Basaltic  plateaux. 

3.  Block-plateaux. 


FIG.  224. — Sketch  and  corresponding  contour  map,  U.S.  Geol.  Surv. 

Plateaux  of  Arid  Erosion. — An  elevated  region  that  has  been 
subjected  to  subaerial  erosion  in  an  arid  region  in  time  becomes 
worn  down  to  a  plateau  or  peneplain.  Typical  plateaux  of  this 
kind  occupy  nearly  the  whole  of  the  interior  of  the  Australian 
continent.  They  appear  almost  level  when  viewed  in  distant 
perspective,  but  when  travelled  over  are  found  to  be  diversified 
with  rocky  knobs,  minor  elevations,  river  gorges,  and  fault- 
escarpments. 

Many  of  the  prominent  mountains,  as  seen  from  the  coast,  are 
only  the  scarps  of  the  inland  plateaux. 


494  A  TEXT-BOOK  OF  GEOLOGY. 

The  plateaux  which  occupy  the  interior  of  South  and  Central 
Africa  are  high  peneplains  of  arid  erosion. 

Basaltic  Plateaux— These  have  been  formed  by  floods  of  basaltic 
lavas  that  have  issued  from  volcanic  vents  of  the  fissure  type. 
Familiar  examples  are  the  Snake  Biver  Plains  in  Western  North 
America,  Deccan  plateau  of  Central  India,  and  the  basaltic  plateau 
of  Victoria. 

Block- Plateaux. — These  are  platforms  arising  from  the  uplift 
of  crustal  segments.  They  are  bounded  by  powerful  faults. 

The  beautiful  table-topped  block-mountains  in  Central  Otago 
in  New  Zealand  are  remarkably  fine  examples  of  this  type  of 
plateau. 

Valleys. 

The  term  valley  is  applied  to  the  longitudinal  hollow  or  depression 
through  which  a  stream  or  river  flows.  In  arid  and  recently- 
glaciated  regions  many  valleys  and  gulches  are  not  drained  by  a 
stream,  and  hence  are  called  dry-valleys  or  dry-gullies. 

The  river-valley  proper  should  be  clearly  distinguished  from 
the  channel  in  which  the  river  pursues  its  downward  course  to 
the  sea. 

The  valley  proper  may  be  bounded  by  low  undulating  downs, 
low  foothills,  high  foothills,  ridges,  or  mountain-chains. 

When  the  ridges  bounding  a  valley  are  clearly  defined,  they  are 
called  the  valley-walls. 

Genesis  of  Valleys. — Practically  all  valleys  are  the  work  of 
subaerial  erosion  ;  but  their  initial  direction  or  course  may  arise 
from  different  causes.  Among  the  principal  of  these  determining 
causes  are  : — 

(a)  A  powerful  fault  or  dislocation  which  at  first  forms  the 

channel  of  a  rivulet. 

(b)  A  series  of  parallel  faults  giving  rise  to  longitudinal  strips 

of  sunken  areas,  forming  what  are  called  rift-valleys  or 
graben.  The  stationary  areas  forming  the  valley-walls 
have  already  been  described  as  horsts  or  block-mountains. 

(c)  A   shear-zone   along    which    the    country-rock    may    be    so 

shattered  and  broken  as  to  be  easily  attacked  by  fluviatile 
or  arid  erosion. 

(d)  Earthquake    and    volcanic    rents.  —  The    great    fissure-rent 

formed  by  the  Tarawera  eruption  in  1886  has  become  the 
channel  of  a  considerable  stream,  and  is  already  assuming 
the  dimensions  of  a  well-defined  immature  valley.  Earth- 
quake rents  that  coincide  with  old  fault-lines  have  in  many 
regions  become  stream  channels  that  will  in  time  develop 
into  valleys. 


DEVELOPMENT  OF  SURFACE  FEATURES.       495 

(e)  Zones  of  soft  rock  bounded  by  hard  rock,  giving  rise  to  what 
may  be  called  differential  erosion  by  pluvial,  glacial,  or 
arid  agencies.  While  the  direction  of  most  trunk- valleys 
is  probably  determined  by  faulting  or  some  other  structural 
cause,  it  is  certain  that  the  direction  and  existence  of  most 
of  the  numerous  lateral  valleys  is  due  to  differential  erosion. 
A  soft  series  of  strata  may  be  repeatedly  brought  to  the 
surface  by  acute  folding  or  faulting,  giving  rise  to  the 
formation  of  a  number  of  more  or  less  parallel  valleys 
bounded  by  steep  escarpments. 

Fault-scarps  may  become  so  indented  by  erosion  as  to 
be  no  longer  parallel  to  the  original  fault-line. 

(/)  Simple  synclinal  folding  in  recently-disturbed  areas.  It  is 
not  often  that  the  surface  configuration  coincides  with 
the  underground  arrangement  of  the  strata  ;  and  this  is 
seldom  seen  except  in  immature  valleys  where  the  amount 
of  erosion  that  has  taken  place  since  the  folding  of  the 
strata  is  insignificant. 

Soundings  undertaken  during  recent  years  have  brought  out 
the  striking  fact  that  the  submarine  continuations  of  many  rivers 
can  be  traced  to  the  edge  of  the  continental  platform,  whe're  they 
descend  through  narrow  gorges  into  the  abyssal  floor  of  the  ocean. 
Notable  examples  of  rivers  whose  courses  can  thus  be  traced  are 
the  Congo,  Tagus,  and  Shannon.  These  submarine  troughs  would 
indicate  a  considerable  subsidence  of  the  eastern  Atlantic  borders, 
always  provided  they  are  true  valleys  of  subaerial  erosion,  and 
not  the  seaward  continuation  of  rifts  or  graben  resulting  from 
faulting. 

The  deep  gutter  which  passes  through  the  Moray  Firth  probably 
marks  the  course  of  the  glacial  Spey  when  it  ran  northwards  and 
joined  the  older  and  greater  Rhine. 

A  deep  groove  surrounds  the  coast  of  Norway,  and  cuts  off  that 
region  from  the  shallow  plateau  of  the  North  Sea.  It  is  a  geo- 
graphical feature  of  great  interest,  but  its  meaning  and  origin  are 
not  yet  well  understood. 

Probably  all  great  valleys  are  connected  with  powerful  crustal 
dislocations. 

Rift-Valleys. — These  frequently  form  physiographical  features 
of  vast  importance.  Perhaps  the  most  remarkable  is  the  great 
rift-valley  of  Syria  and  the  Red  Sea.  This  depression  forms  the 
valley  of  the  Jordan  and  the  basin  of  the  Dead  Sea,  the  surface  of 
which  lies  1300  feet  below  the  level  of  the  Mediterranean.  From  the 
Dead  Sea  it  pursues  a  southerly  course  down  the  Gulf  of  Akaba, 
and  curving  to  the  south-east,  forms  the  trough  of  the  Red  Sea. 


496  A  TEXT-BOOK  OF  GEOLOGY. 

The  Shire  Valley  and  Lake  Nyasa  form  the  southern  portion  of 
one  of  the  greatest  rifts  in  the  African  Continent.  This  depression, 
with  a  break  at  the  north  end  of  Nyasa,  runs  northward  for  400 
miles  through  Lakes  Rukwa  and  Tanganyika  ;  and  with  a  few 
short  breaks  passes  through  Lakes  Kivu,  Albert  Edward,  and 
Albert,  and  thence  on  to  the  Nile. 

Fault-Valleys. — Most  Alpine  chains  are  intersected  by  deep 
transverse  valleys  that  cut  back  to  the  main  divide,  and  end  in  a 
pass,  saddle,  or  col.  Across  the  pass  there  is  frequently  another 
valley  which  intersects  the  range  on  the  opposite  side.  In  many 
cases  it  is  possible  to  cross  a  high  inaccessible  chain  by  ascending 
one  valley  and  descending  the  opposite  one. 

The  two  valleys  which  meet  at  the  pass  as  a  rule  follow  the  course 
of  a  powerful  fault  or  shear-zone  running  transversely  across  the 
main  chain.  Such  valleys  are  dislocation  rifts  widened  out  by 
subsequent  erosion,  and  hence  are  essentially  different  from  the 
graben  or  strips  of  subsidence  already  described. 

Canyons. — In  the  canyon  region  of  Colorado  the  horizontal, 
Cretaceous  and  older  strata  form  an  ancient  plateau  which  has 
become  intersected  by  numerous  deep  gorges  formed  by  fluviatile 
erosion  (see  Frontispiece).  The  canyons  do  not  follow  faults  or 
lines  of  dislocation.  In  the  beginning  they  were  probably  simple 
joint  fissures  that  gradually  became  enlarged  by  chemical  and 
mechanical  erosion.  . 

The  great  depth,  narrowness,  steepness,  and  U-form  of  the 
canyons  show  that  the  excavation  has  been  relatively  rapid. 
Moreover,  the  cycle  of  erosion  is  still  in  the  juvenile  or  torrential 
stage. 

Form  of  Valleys. — The  form  of  practically  all  valleys  has  been 
modified  by  pluvial  or  glacial  erosion,  or  by  the  united  activity 
of  both,  or  by  arid  erosion.  Subaerial  erosion  will  always  be 
greatest  where  the  rocks  are  softest,  or  where  they  are  so  broken 
as  to  offer  the  least  resistance. 

The  form  of  the  cross-section  of  a  valley  at  any  point  will  depend 
on  the  maturity  of  the  valley,  the  resistance  offered  by  the  rocks, 
and  the  character  of  the  eroding  agency. 

In  a  juvenile  valley  the  cross-section  will  be  V-shaped  ;  and  in 
the  uplands  the  river  will  occupy  the  whole  width  of  the  floor  of 
the  valley,  in  the  torrential  portion  of  its  course  running  in  a 
narrow  gutter  excavated  in  the  solid  rock,  and  in  the  middle 
portion  wandering  over  a  wide  shingle-bed  which  may  vary,  accord- 
ing to  local  circumstances,  from  a  few  yards  to  many  miles  wide. 

In  a  river-system  which  has  reached  maturity  the  valley  is  wide 
and  the  walls  are  not  very  clearly  defined.  In  this  portion  of  its 
course  the  river  usually  occupies  a  well-defined  channel. 


DEVELOPMENT  OF  SURFACE  FEATURES.       497 

A  typical  river-system  usually  exhibits  three  phases  or  tracts 
of  erosion,  namely:  (a)  a  torrent-tract,  where  erosion  is  at  a 
maximum  and  deposition  at  a  minimum  ;  (6)  a  valley-tract,  where 
the  gradients  are  flatter  and  erosion  consequently  slower — here 
also  the  valley-walls  are  flatter  and  deposition  of  detritus  is  taking 
place  in  favourable  situations ;  (c)  the  plain-tract,  where  erosion 
has  practically  ceased  and  deposition  is  at  a  maximum. 

Obviously  the  plain-tract,  which  has  been  subjected  to  erosion 
the  longest,  shows  the  flattest  gradients  and  lowest  relief.  In 
the  torrent-tract,  which  comprises  the  latest  territory  added  to  the 
river-system — the  frontier,  where  the  tributaries  are  continually 
extending  their  sphere  of  influence — the  contours  are  steep  and 
rugged. 

As  erosion  proceeds,  the  plain-tract  encroaches  on  the  valley- 


FIG.  225. — A,  Cross-section  in  hard  rock  with  corrie  lake  on  high 
shoulder  at  a  ;  B,  Gentle  slope  in  soft  rock. 

tract,  and  the  valley-tract  on  the  torrent-tract,  which  in  its  turn 
is  continually  reaching  out  into  new  territory.  It  is  in  the  plain- 
tract  that  a  river-system  first  reaches  its  base-level  of  erosion. 

Glaciated-Valleys. — Glaciated-valleys  are  usually  distinguished 
by  flat  bottoms  and  smooth  even  walls.  In  relatively  soft  rock 
the  projecting  spurs  or  ridges  are  faceted  so  as  to  present  tent- 
shaped  ends  ;  but  where  the  rock  is  hard,  the  ice  may  ride  over 
the  spurs,  the  crests  of  which  are  truncated  into  flat  platforms. 

Ridges  of  hard  rock  running  parallel  with  the  axis  of  the  valley 
are  usually  worn  into  rounded  whale-backed  ridges  or  roches 
moutonnees. 

The  form  of  the  cross-section  of  a  glaciated-valley  depends 
mainly  on  the  hardness  of  the  country-rock.  Where  the  rock  is 
granite,  gneiss,  or  hard  crystalline  schists,  the  form  approaches 
the  U-shape  ;  but  where  the  rocks  are  soft  schists,  shales,  or 
Tertiary  strata,  the  walls  are  worn  into  gentle  catenary  curves. 
These  forms  are  shown  in  fig.  225. 

32 


498  A  TEXT-BOOK  OF  GEOLOGY. 

Drowned-Valleys. — When  subsidence  takes  place  in  a  maritime 
region  intersected  by  deep  valleys,  the  sea  advances  and  floods 
the  valleys,  which  thereby  become  converted  into  sounds,  fiords,  and 
sea-lochs.  These  partially  submerged  or  drowned-valleys  as  they 
are  called,  possess  the  same  contour  forms  below  sea-level  as 
above. 

The  beautiful  fiords  of  Norway,  Alaska,  and  New  Zealand, 
with  their  numerous  ramifying  arms  and  inlets,  are  typical  examples 
of  drowned  valleys  that  have  been  modified  by  glacial  erosion. 

The  fiords  of  Norway  and  New  Zealand,  and  the  sea-lochs  of 
West  Scotland,  are  usually  deeper  inside  than  at  their  mouths. 
The  cause  of  this  deepening  may  arise  in  some  cases  from  accumu- 
lations of  glacial  debris  deposited  at  the  sea-face  of  the  tongues 
of  ice  that  occupied  the  valleys  at  the  close  of  the  Pleistocene ; 
in  others  from  the  presence  of  lake-basins  or  depressions  in  the 
floor  of  valleys  before  the  subsidence  took  place. 

Drowned-valleys  in  non-glaciated  regions  do  not  exhibit  this 
inside  deepening,  which  may  be  regarded  as  characteristic  of 
true  fiords. 

Fiords,  it  should  here  be  noted,  are  not  a  feature  peculiar  to  the 
sea-coast.  The  fresli-ioater  fiords  on  the  west  coast  of  Lake  Te 
Anau,  New  Zealand,  are  profoundly  deep,  narrow,  glaciated  valleys. 
that  extend  back  to  the  heart  of  the  Alpine  chain,  and  in  general 
outline  and  character  are  typical  of  the  well-known  sea-fiords  on 
the  opposite  side  of  the  chain. 

Lakes. 

A  lake  is  a  body  of  water  entirely  surrounded  by  land.  The 
term  is  usually  restricted  to  sheets  of  water  sufficiently  large  to 
form  physiographical  features  of  some  importance.  The  smaller 
bodies  of  water  are  called  tarns,  meres,  corrie  lakes,  and  pools. 

Genetically  considered,  lakes  and  tarns  may  be  classified  in  five 
main  groups  : — 

(1)  Tectonic  lakes,  or  lakes  due  to  differential  earth-movement. 

(2)  Glacial  lakes. 

(3)  Barrier  lakes. 

(4)  Crater  lakes. 

(5)  Dissolution  lakes. 

Lakes  due  to  Differential  Earth-Movement. — This  class  com- 
prises the  largest  and  most  important  sheets  of  water,  which, 
acording  to  local  conditions,  may  be  salt,  brackish,  or  fresh.  As 
a  rule  they  are  situated  in  plains  or  plateaux. 

The  Caspian  Sea  and  Sea  of  Aral,  which  occupy  portions  of  the 


DEVELOPMENT  OF  SURFACE  FEATURES.       499 

same  crustal  depression  and  were  at  one  time  united,  are  good 
examples  of  inland  salt-water  lakes  that  have  been  detached  from 
the  ocean  by  crustal  movement.  The  faunal  evidence  seems  to 
point  to  a  former  connection  with  the  Arctic  Ocean,  and  the  physio- 
graphical  evidence  to  a  connection  with  the  Black  Sea  depression. 
The  mean  water-leVel  of  the  Caspian  Sea  is  84  feet  below  that  of 
the  Black  Sea  ;  and  of  the  Sea  of  Aral,  128  feet  above  that  datum — 
differences  of  level  which  may  be  ascribed  to  crustal  warping. 

Estuaries  and  arms  of  the  sea  that  have  become  detached  by 
uplift  or  crustal  tilting  may  increase  in  salinity  in  arid  regions, 
or  become  fresher  in  regions  where  the  inflow  of  fresh  water  exceeds 
the  evaporation.  Conversely,  a  freshwater  lake  in  a  region  of  in- 
creasing aridity  may  become  saline  and  in  time  present  many 
of  the  features  of  a  salt-water  lake  that  has  been  cut  off  from  the 
ocean. 

The  Dead  Sea  lying  in  the  depression  of  the  Syrian  Rift- Valley 
is  extremely  saline,  while  the  great  lakes  situated  in  depressions 
along  the  course  of  the  East  African  Rift- Valley  are  fresh.  The 
Great  Salt  Lake  of  Utah  situated  in  the  Great  Basin  is  merely  the 
shrunken  remnant  of  a  large  inland  lake  that  owed  its  origin  to 
crustal  deformation. 

Glacial  Lakes. — These  are  situated  in  recently  glaciated  regions, 
and,  as  a  rule,  occupy  depressions  in  narrow  Alpine  valleys.  Good 
examples  are  Lakes  Como  and  Maggiore  on  the  Italian  side  of  the 
Alps,  and  Lake  Wakatipu  in  New  Zealand. 

Glacial  lakes  frequently  occupy  rock-basins  which  may  be  many 
hundred  feet  deep.;  but  the  depth  of  most  lakes  of  this  class  is 
increased  by  barriers  at  the  outlet  composed  of  morainic  and 
fluvio-glacial  detritus. 

The  erosive  effect  of  a  glacier  is  proportional  to  the  thickness  of 
the  ice.  A  stream  of  ice,  like  a  river,  tends  to  elongate  and  deepen 
the  depressions  in  its  bed.  In  this  way  certain  parts  of  glacial 
valleys  have  become  overdeepened,  and  now  form  lake-basins. 

The  erosive  effect  will  be  greatest  in  soft  strata,  or  in  zones  of 
rock  crushed  and  broken  by  faulting  or  shearing. 

Many  corrie  lakes  that  occupy  niche-like  indentations  on  the 
brow  of  mountain  slopes  and  on  the  flat  shoulders  of  valley- walls, 
rest  in  rock-basins  that  were  excavated  by  ice  during  the  Glacial 
Period. 

Barrier  Lakes. — Some  Alpine  lakes  have  been  formed  by  the 
blocking  up  of  the  valleys  by  glacial  detritus  ;  but  most  Alpine 
lakes,  as  mentioned  above,  are  partly  barrier  and  partly  rock-cut. 

The  circular  tarns  or  corrie  lakes  found  in  glacial  cirques  are 
frequently  held  up  by  accumulations  of  snow-piled  rocky  detritus, 
but  many  occupy  true  rock-basins. 


500  A  TEXT-BOOK  OF  GEOLOGY. 

Barrier  lakes  are  sometimes  formed  in  mountainous  regions  by 
extensive  land-slips,  avalanches,  or  ice- jams,  but  they  do  not 
form  permanent  geographical  features. 

Marginal  Glacial  Lakes. — When  a  continental  ice-sheet  advances 
across  the  divide  separating  one  watershed  from  another,  the 
drainage  from  the  ice-front  will  flow  down  the  invaded  valleys 
without  hindrance,  and  where  the  topographical  features  are 
favourable,  wide  valley-trains  of  fluvio-glacial  drift  may  be  formed. 
But  when  the  ice  retreats  behind  the  dividing  range,  the  drainage 
will  be  dammed  between  the  high  land  and  the  ice-front,  thereby 
forming  a  marginal  lake  which  will  increase  in  size  and  depth  as 
the  ice-recession  progresses,  till  a  natural  outlet  is  formed.  The 
site  of  such  glacial  lakes  will  be  marked  by  high-level  beaches  and 
lacustrine  deposits. 

Many  fine  examples  of  Pleistocene  glacial  lakes  existed  in  the 
Laurentian  Lake  Region  of  North  America,  along  the  ice-front  of 
the  Keewatin  and  Labradorian  ice-sheets. 

When  the  Mesabi  and  Giants  ranges  were  completely  covered 
with  ice,  and  the  ice-front  lay  to  the  south  of  these  transverse 
barriers,  the  drainage  passed  down  the  Mississippi  Valley  ;  but 
when  the  ice-sheet  retreated  into  the  Lake  Superior  basin,  on  the 
north  side  of  the  dividing  ranges,  the  drainage  was  held  up  between 
the  high  land  on  the  south  and  the  retreating  ice,  thereby  forming 
great  marginal  lakes,  the  largest  and  most  notable  of  which  has 
been  called  Glacial  Lake  Agassiz,  so  named  after  the  distinguished 
Swiss  naturalist  Agassiz,  who  was  the  first  to  recognise  the  evidences 
of  a  Pleistocene  extension  of  the  Arctic  ice-sheet  in  northern  con- 
tinental Europe  and  Scotland.  As  the  ice-sheet  receded  Lake 
Agassiz  grew  in  size,  till  the  whole  lacustrine  area,  as  estimated 
by  Warren  Upham,1  exceeded  110,000  square  miles  or  more  than 
the  united  area  of  all  the  present  Laurentian  lakes. 

The  three  detrital  terraces  or  benches,  forming  what  are  known 
as  the  Parallel  Roads  of  Glen  Roy  in  the  Highlands  of  Scotland,  are 
believed  by  some  writers  to  be  the  beaches  of  a  lake  formed  by 
a  barrier  of  glacier-ice. 

Rivers  are  sometimes  blocked  at  their  mouth  by  ridges  of  wind- 
blown sands,  whereby  large  shallow  lagoons  or  basins  are  formed 
near  the  sea.  In  some  maritime  regions  the  mouths  of  the  rivers 
are  blocked  by  wide  stretches  of  shingle  cast  up  by  powerful  tides. 
Many  capacious  lake-like  harbours  of  great  value  have  been  formed 
in  this  way  on  exposed  storm-beaten  coasts. 

Crater  Lakes. — These  occupy  the  craters  of  extinct  or  dormant 
volcanoes.  Good  examples  of  these  may  be  seen  in  the  Eifel, 
Auvergne,  Central  Italy,  and  North  New  Zealand. 

1  The  Glacial  Lake  Agassiz,  Monograph  xxv.,  U.S.  Geol.  Survey,  1894,  p.  214. 


DEVELOPMENT  OF  SURFACE  FEATURES.       501 

The  depressions  formed  by  explosive  volcanic  outbursts  fre- 
quently form  lakes  of  considerable  extent.  Lake  Rotomahana 
in  New  Zealand  occupies  a  portion  of  the  great  fissure -rent  formed 
by  the  Tarawera  eruption  in  1886.  In  the  same  region  many 
large  lakes,  notably  Lakes  Taupo  and  Rotorua,  have  been  formed 
by  local  subsidence  in  the  neighbourhood  of  old  centres  of  eruption 
situated  on  the  Central  Volcanic  Plateau. 

The  lavas  and  ejectamenta  of  volcanic  eruptions  have  in  some 

Sierras 

Rocky  flits.  Appalachians 

\Great  Basm    \ 

Mississippi  Basin 


w  £ 

FIG.  226.— Showing  profile  of  North  America. 

cases  formed  barriers  across  valleys  whereby  the  natural  drainage 
has  been   impounded,  thereby  forming   lakes.     The   Yellowstone 
Lake  is  confined  by  a  barrier  of  lava,  and  also  the  Lac  d'Aydat 
of  Auvergne. 
Dissolution   Lakes. — These    are   formed   in    limestone   regions," 


W  £ 

FIG.  227. — Showing  profile  of  South  America. 

and  arise  from  the  dissolution  of  the  limestone  by  the  carbonic 
acid  contained  in  the  ground  water.  It  is  probable  that  most  of 
the  lakes  in  the  limestone  districts  of  Ireland  belong  to  this  class. 

Continental  Forms. 

When  we  take  a  broad  survey  of  the  great  continents,  we  are 
impressed  with  certain  outstanding  physiographical  features  that 
seem  to  suggest  a  relationship  between  the  continents  and  the 
bordering  oceans.  This  relationship  has  been  expressed  by  Dana 
in  two  postulates  as  follows  : — 

(1)  The  continents  have  in  general  elevated  mountain  borders 
and  a  low  or  basin-like  interior. 

(2)  The  highest  border  faces  the  larger  ocean. 


502 


A    TEXT-BOOK    OF    GEOLOGY. 


America. — In  North  America  we  have  the  Kocky  Mountains 
on  the  Pacific  side  (the  side  of  the  greater  ocean),  and  the 
Appalachians  on  the  Atlantic  side.  Between  these  chains  lies  the 
great  interior  plain. 

In  South  America  the  great  Andes  Chain  faces  the  Pacific  (the 
larger  ocean),  and  a  low  coastal  range  faces  the  Atlantic. 

The  Bolivian  plateau  lies  between  the  Western  Cordillera  (a) 


'as 


Himalay* 

i  Huen-Lun 


Altai  Mts. 

Desert  of  Gobi 

Siberian  Plains 


N 


FIG.  228. — Showing  profile  of  Asia. 


and  the  Eastern  or  Bolivian  Cordillera  (6) ;  while  on  the  east  coast 
we  have  the  low  ranges  of  Venezuela  and  Guiana. 

Europe. — This  continent  does  not  present  the  well-defined 
coastal  arrangement  of  the  mountain-chains  of  North  America. 
Moreover,  the  great  chains,  as  in  Asia,  follow  a  general  east  and 
west  course. 


Western  Plateau 

\  Central  Basin 


Australian  Alps . 


FIG.  229. — Showing  profile  of  Australia. 


On  the  south  side  of  Europe  we  have  the  Pyrenees,  Alps.  Car- 
pathians, and  Caucasus ;  and  on  the  north  the  mountains  of 
Scandinavia.  Between  these  lies  the  Baltic  depression,  the  vast 
plains  of  North  Germany,  and  Baltic  provinces  of  Russia,  altogether 
occupying  three-fifths  of  the  area  of  all  Europe. 

Asia. — Facing  the  open  Indian  Ocean  stands  the  Himalayas  ; 
and  in  Central  Asia  the  Altai  Mountains  face  the  great  steppes 
and  tundras  of  Northern  Siberia,  which  extend  northwards  to  the 
Arctic  Ocean.  Between  these  chains  lie  the  plains  of  Mongolia 
and  the  Desert  of  Gobi. 

Africa. — This  continent  is  dominated  by  plateau  forms,  long 
continued  denudation  having  truncated  or  altogether  effaced  the 
mountain-chains. 


DEVELOPMENT  OF  SURFACE  FEATURES.       503 

A  high  rim  of  land  faces  the  Indian  Ocean,  and  as  a  result  the 
drainage  of  the  interior  is  westward  and  northward,  the  Zambesi 
being  the  only  river  to  break  through  to  the  Indian  Ocean. 

Australia. — The  plateau  form  of  Australia  is  even  more  con- 
spicuous than  that  of  Africa.  A  rim  of  mountains — the  Australian 
Alps — borders  the  eastern  side  of  the  continent,  as  shown  in 
fig.  229,  and  on  the  west  lies  the  Great  Western  Plateau. 


PART  III. 


CHAPTER   XXXV. 
ECONOMIC    GEOLOGY. 

ECONOMIC  Geology  is  mainly  concerned  with  the  occurrence  and 
genesis  of  ores  and  mineral  deposits  ;  with  coal  and  mineral  oil ; 
building-stones  and  roofing-slates  ;  flagstones  and  road-metal  ; 
limestones  and  cements  ;  grindstones  and  whetstones  ;  ornamental 
stones  and  marbles  ;  clays  for  brick-making  and  pottery  ;  sand 
for  glass-making  ;  soils  ;  and  with  the  supply  of  underground 
water  for  domestic,  manufacturing,  agricultural,  and  medicinal 
purposes. 

Ores  and  Mineral  Deposits. 

Mineral  deposits  mostly  occur  as  veins  or  lodes  traversing  rock- 
masses,  or  as  sheets,  layers,  or  beds  interbedded  with  and  forming 
part  of  rock-formations. 

In  regions  which  have  suffered  considerable  denudation,  the 
valuable  contents  of  veins  and  mineral  deposits  may  be  con- 
centrated in  the  sands,  gravels,  or  other  detritus  laid  down  along 
the  course  of  the  streams  and  rivers,  or  on  sea-beaches.  Such 
deposits  are  called  alluvial  or  detrital,  and  are  obviously  of  secondary 
origin. 

Definitions. — A  vein  or  lode  may  be  defined  as  a  more  or  less 
continuous  sheet-like  body  of  ore  enclosed  within  rocky  walls. 
It  may  be  horizontal  or  vertical,  or  inclined  at  any  angle. 

The  term  vein  is  frequently  restricted  to  designate  the  simple 
sheets  of  relatively  small  dimensions  possessing  well-defined  walls  ; 
while  the  term  lode  is  more  often  applied  to  the  larger  ore-bodies, 
or  to  zones  or  bands  of  mineralised  rock  that  contain  strings, 
bunches,  or  ramifying  veins  or  veinlets  of  valuable  ore. 

An  ore  may  be  defined  as  any  natural  metallic  or  non-metallic 
substance  that  can  be  turned  to  profitable  account  by  metallurgical 
manipulation.  It  therefore  excludes  such  substances  as  slate, 
chalk,  clay,  and  building-stones. 


ECONOMIC    GEOLOGY.  505 

A  lens  or  lenticel  of  ore  is  one  shaped  like  a  biconvex  or  plano- 
convex lens.  Such  deposits  taper  out  to  small  dimensions  in  all 
directions,  or  peter  out  altogether. 

Mineral  Deposits  Morphologically  Considered. 

Classification. — Mineral  deposits  are  found  in  many  different 
forms  and  under  many  varying  conditions.  Moreover,  they  present 
a  great  diversity  of  origin.  Hence  they  may  be  considered  morpho- 
logically, that  is,  according  to  their  outward  form,  or  genetically, 
according  to  their  origin.  Of  these,  outward  form  and  mode  of 
occurrence  offer  the  most  convenient  starting  place  for  the 
elementary  investigation. 

The  morphological  classification,  based  on  outward  form  and 
mode  of  occurrence,  but  entirely  independent  of  age  or  mineral 
character,  is  as  follows  : — 

Class      I. — Superficial  mineral  deposits. 
Class    II. — Stratified  mineral  deposits. 
Class  III. — Unstratified  mineral  deposits. 

For  convenience  of  study  and  description  these  classes  are  sub- 
divided into  groups  or  sub-classes  : — 

I. — SUPERFICIAL  DEPOSITS. 

(a)  Detrital — Forming  or  occurring  in  alluvial  drifts. 

(b)  Massive — Forming  superficial  layers  and  sheets. 

II. — STRATIFIED  DEPOSITS. 

(a)  Constituting     beds  —  Forming     members     of     a     stratified 

formation. 

(b)  Disseminated  through  a  bed. 

III. — UNSTRATIFIED  DEPOSITS. 

(a)  Deposits  of  volcanic  origin. 

(b)  Stockwork  deposits. 

(c)  Contact  and  replacement  deposits. 

(d)  Fahlbands. 

(e)  Impregnations. 
(/)  Segregated  veins, 
(g)   Gash-veins. 

(h)  Fissure-veins. 


506  A  TEXT-BOOK  OF  GEOLOGY. 

CLASS  I. 
Superficial  Deposits. 

(a)  Detrital. — Alluvial  or  placer  deposits,  as  they  are  usually 
called,  embrace  detrital  deposits  of  all  kinds,  whether  beach  sands, 
river  gravels,  lacustrine  gravels,  or  glacial  drifts,  containing 
particles  of  gold,  platinum,  tin-ore,  iron  ores,  emeralds,  rubies, 
diamonds,  or  other  precious  stones. 

The  alluvial  deposits  may  be  of  recent  date  or  of  great  antiquity. 
They  may  exist  as  sands  and  gravels  in  the  bed  or  bank  of  a  stream  ; 
or  form  terraces  ranging  in  age  from  the  Pleistocene  to  almost 
recent  times  ;  or  occur  as  so-called  deep-leads  which  are  merely 
river-drifts  of  Pliocene  and  later  date  covered  over  and  protected 
by  sheets  of  basaltic  lava  ;  or  occur  as  consolidated  gravels  or 


FTG.  230. — Section  of  gold-bearing  river- terrace. 
(a)  Terrace  gravels.         (6)  Pay-wash.         (c)  Slate  bed-rock. 

conglomerates  interbedded  with  rocks  ranging  in  age  from  the 
Silurian  to  the  late  Cainozoic. 

The  most  widely  distributed  and  valuable  of  alluvial  deposits 
are  those  of  gold.  The  alluvial  gold  originated  from  the  weathering 
and  denudation  of  country  containing  gold-bearing  veins  during 
countless  centuries,  followed  by  the  concentration  of  the  liberated 
particles  and  nuggets  of  gold  in  the  gravelly  bed  of  the  streams 
and  rivers  by  a  process  of  natural  sluicing. 

The  gold,  owing  to  its  great  specific  gravity,  gravitates  toward 
the  bottom  of  the  drifts  and  usually  lodges  on  or  near  the  bed- 
rock. The  crevices  in  the  bed-rock,  which  is  frequently  slate, 
sandstone,  or  mica-schist,  offer  a  convenient  and  safe  lodgment 
for  the  travelling  particles  of  gold. 

The  portion  of  the  gravel  drift  which  contains  the  gold  is  called 
the  pay-wash. 

The  pay-wash  usually  lies  on  the  bed-rock,  or  true-bottom  or 
reef-bottom  as  it  is  called,  but  in  thick  accumulations  of  river- 
drift,  two,  three,  or  more  layers  of  pay-wash  may  exist,  each 
resting  on  a  bed  of  clay  or  distinctive  bed  of  gravel,  which  is  called 
&  false-bottom. 


ECONOMIC    GEOLOGY. 


507 


The  most  valuable  gold-bearing  drifts  occur  in  California,  the 
State  of  Victoria  in  Australia,  and  New  Zealand. 

Deep-leads  of  great  value  occur  in  California,  and  at  Ballarat 
in  Victoria,  underlying  thick  sheets  of  basaltic  lava  of  late  Tertiary 
date. 

Tin-bearing  gravels  of  great  extent  and  value  occur  in  Malaysia, 
which  produces  the  bulk  of  the  world's  annual  supply  of  tin-ore. 

The  platinum-bearing  gravels  on  the  eastern  slopes  of  the  Ural 
Mountains  produce  90  per  cent,  of  the  platinum  of  commerce. 

Diamond-bearing  gravels  occur  on  the  banks  and  bed  of  the  Vaal 
River  in  South  Africa,  and  ruby  placers  have  been  worked  for 
centuries  in  Burma,  and  for  many  years  in  Brazil. 

(b)  Massive. — Deposits  of  this  kind  occur  in  superficial  sheets, 
layers,  or  masses  frequently  covered  with  soil,  clay,  or  other  recent 


DO V ETON  STKCCT 


HAIR   STRICT 


FIG-.  231. — Showing  deep-lead  underlying  sheet  of  basalt, 
Ballarat  Goldfield,  Victoria. 

accumulations.  They  include  deposits  of  bog  iron-ore,  laterite, 
rock-salt,  sulphur,  and  rock-phosphate. 

Bog  Iron-Ore  is  an  impure  hydrated  peroxide  of  iron  which 
frequently  forms  on  the  bottom  of  swamps  and  shallow  lagoons. 

Laterite  is  mainly  composed  of  alumina  and  iron  oxide,  with 
sometimes  manganese  and  titanium.  When  the  surface  of  a  flow 
of  basaltic  or  other  basic  lava  is  exposed  to  the  weathering  action 
of  the  atmosphere  in  a  warm  moist  climate,  the  carbonic  acid  in 
the  air,  in  conjunction  with  water,  attacks  the  felspars  and 
removes  the  silica,  lime,  magnesium,  potash,  and  soda  in  solution. 
The  alumina  and  iron  are  left  behind  as  a  reddish-brown  or  brick- 
red  sheet  of  earthy  ironstone. 

The  brick-red  layers  of  lateritic  clay  frequently  seen  in  ancient 
volcanic  regions  intercalated  among  lava-flows  and  beds  of  ash  are 
surfaces  of  lavas  that  have  become  weathered  and  decomposed 
during  periods  of  cessation  from  volcanic  activity. 


508  A  TEXT-BOOK  OF  GEOLOGY. 

Superficial  layers  of  rock-salt  occur  in  the  dried-up  swamps  and 
lagoons  in  the  arid  regions  of  Asia,  North  America,  and  Australia. 
Deposits  of  sulphur  occur  in  volcanic  regions,  and  rock-phosphates 
are  often  found  on  the  chemically  eroded  surfaces  of  limestones, 
where  they  have  accumulated  by  a  process  of  secondary  con- 
centration. 

CLASS  II. 
Stratified  Deposits. 

(a)  Constituting  Beds  or  Strata. — The  useful  minerals  which  occur 
in  beds  or  as  members  of  a  stratified  formation  are  coal,  oil-shale, 
iron-ore,  and  rock-salt. 

Coal  is  a  combustible  mineral  substance  resulting  from  the 
alteration  of  vegetable  matter.  It  occurs  in  beds  or  seams  which 
vary  from  a  mere  streak  to  90  feet  thick. 

The  progressive  stages  in  the  formation  of  coal  are  indicated 
by  the  numerous  varieties  which  occur,  ranging  from  peat  to 
anthracite : — 

1.  Peat. 

2.  Lignite. 

3.  Brown  coal. 

4.  Cannel  coal. 

5.  Bituminous  or  coking  coal. 

6.  Semi-anthracite. 

7.  Anthracite. 

The  principal  coal-bearing  formations  are  the  Carboniferous  in 
Britain,  Belgium,  and  North  America;  Permo- Carboniferous  in 
New  South  Wales  and  India  ;  and  Eocene  and  Miocene  in  New 
Zealand. 

Some  brown  coals  are  Upper  Cretaceous,  as  in  New  Zealand,  but 
in  most  countries  they  are  Lower  and  Middle  Cainozoic,  as  in  Texas, 
South  Hungary,  North  Germany,  and  New  Zealand. 

Lignites  are  mostly  of  Pliocene  and  Pleistocene  age. 

Coal-seams  may  be  horizontal,  or  inclined,  folded,  bent,  or  over- 
turned, according  to  the  amount  of  disturbance  suffered  by  the 
strata  in  which  they  are  enclosed. 

Oil-Shales  occur  in  Carboniferous  rocks  in  the  Lothians  of 
Scotland,  Permo- Carboniferous  of  New  South  Wales,  and  Lower 
Cainozoic  of  New  Zealand.  Oil  is  obtained  from  them  by  dis- 
tillation. 

Natural  Mineral  Oil  is  obtained  by  boring  in  California,  Texas, 
Ohio,  Pennsylvania,  Baku,  Burma,  Borneo,  and  other  regions.  Its 
origin  is  supposed  by  some  writers  to  be  igneous  or  inorganic,  but 


ECONOMIC   GEOLOGY. 


509 


the  bulk  of  the  evidence  seems  to  favour  the  view  that  it  is  organic, 
resulting  from  the  destructive  distillation  of  animal  and  vegetable 
organisms  buried  in  marine  and  lacustrine  sediments. 

The  oil  is  expelled  as  gases  from  the  muds,  shales,  and  calcareous 
deposits  in  which  the  organisms  were  buried,  and  is  condensed  into 
heavy  oil  in  the  overlying  porous  sandstones  or  strata,  where  it 
accumulates  under  great  pressure. 

When  the  impervious  strata  overlying  the  porous  sandstones  are 
penetrated  by  bore-holes,  the  oil  and  gases  rise  to  the  surface. 

Beds  of  rock-salt  occur  in  many  geological  formations.  Those  of 
the  Salt  Range  in  the  Punjab  are  Cambrian  ;  and  of  Cheshire, 
Triassic  ;  while  the  famous  salt-deposits  of  Wieliczka  in  Polish 
Austria  are  Miocene. 


FIG.  232.— Section  across  Shenandoah  Basin,  Pennsylvania 
anthracite  region,  showing  an  overturned  seam  of  coal. 

Other  Bedded  Deposits. — Among  other  bedded  deposits  of  great 
value  to  man  are  Building-stones,  Roofing-slates,  Limestones,  Flag- 
stones, and  Road-metals,  Clays,  and  Ornamental  stones. 

The  most  valuable  Building-stones  are  granite,  sandstones,  and 
'limestones.  Granite  is  hard,  durable,  and  takes  a  fine  polish.  The 
siliceous  sandstones  are  also  hard  and  durable,  while  the  softer 
calcareous  sandstones  or  freestones  are  free-cutting,  and  hence  of 
special  value  for  the  construction  of  front-walls  and  ornamental 
parts  of  buildings.  The  oolitic  limestones  of  England  and  Conti- 
nental Europe,  and  the  magnesian  limestones  common  in  many 
parts  of  the  globe,  are  valuable  for  all  kinds  of  buildings. 

The-  best  Roofing -slates  are  obtained  from  fine,  even-grained, 
muddy  sediments  in  which  slaty-cleavage  has  been  well  developed. 
They  are  found  in  air  the  older  Palaeozoic  formations  in  many  parts 
of  the  globe.  The  Welsh  roofing-slates  are  the  best  in  Britain,  and 
are  seldom  equalled  in  other  regions. 

Limestones  containing  80  per  cent,  or  more  of  carbonate  of  lime 
are  valuable  for  burning  into  lime  for  agricultural  purposes,  and 


510  A  TEXT-BOOK  OF  GEOLOGY. 

for  the  making  of  cement  and  mortar.  Such  limestones  are  found 
in  almost  all  the  rock-formations  from  the  Cambrian  to  the  late 
Cainozoic.  In  Britain  the  Wenlock  or  Dudley  Limestone,  the 
Carboniferous  and  Oolitic  Limestones,  and  the  Chalk  are  specially 
valuable.  The  impure,  earthy  limestones  of  the  Jurassic  and 
Cretaceous,  so  abundant  in  England,  France,  United  States,  and 
New  Zealand,  when  calcined  and  pulverised,  form  natural  hydraulic 
cements  of  great  value.  Portland  cement  is  extensively  manu- 
factured from  lime  mixed  with  a  suitable  quantity  of  sea-mud  or 
marl. 

Common  Clays  are  everywhere  used  for  brick-  and  tile-making. 
The  clays  of  the  Coal-Measures,  from  which  the  lime,  soda,  and 
potash  have  been  exhausted  by  the  growth  of  the  coal- vegetation, 
are  refractory  or  difficult  to  fuse,  and  hence  valuable  for  the  making 
of  fire-bricks  for  the  lining  of  furnaces,  etc. 

Pottery  clays  for  the  manufacture  of  china  and  porcelain  ware 
are  derived  from  the  disintegration  of  granitic  rocks.  Cornwall 
and  Devon  have  long  been  noted  for  their  production  of  Kaolin. 

Road-Metal  is  selected  from  the  hardest  and  toughest  rocks 
available,  and  those  not  subject  to  rapid  disintegration.  The  best 
rocks  for  road-metal  are  granites,  greywackes,  siliceous  sandstones, 
and  quartzites.  These  are  superior  to  such  igneous  rocks  as  dolerite, 
andesite,  phonolite,  and  diorite,  which  are  hard  and  tough,  but 
disintegrate  rapidly  into  mud  under  the  influence  of  the  organic 
acids  liberated  from  road-refuse. 

Flagstones  are  usually  sandstones,  schists,  or  limestones  that  can 
be  readily  split  into  thick  flags.  The  famous  calcareous  flagstones 
of  Caithness,  of  Old  Red  Sandstone  age,  are  exported  to  all  parts 
of  the  globe.  Flagstones  of  slate,  limestone,  gneiss,  and  quartz- 
schist  are  extensively  used,  but  natural  stones  are  being  largely 
replaced  by  artificial  slabs  of  concrete  and  other  stony  mixtures. 

Ornamental  Stones  are  usually  hewn  from  granites,  gabbros, 
marbles,  and  serpentine.  The  grey  granite  of  Cornwall,  the  grey 
and  pink  granites  of  Aberdeenshire,  the  Carboniferous  and  Devonian 
Limestones  of  Europe  and  America  are  much  used  as  ornamental 
stones.  The  statuary  marbles  of  Carrara,  in  the  Trias  of  the 
Apennines,  have  long  been  celebrated  for  their  pure  colour  and 
even  texture. 

Glass-making  Sands  are  plentiful  in  the  older  Cainozoic  forma- 
tions of  the  United  States,  England,  Brazil,  and  New  Zealand  at 
the  base  of  the  brown  Coal-Measures. 

Grindstones  and  Millstones  are  fashioned  from  gritstones  and 
siliceous  sandstones  like  the  Millstone  Grits  of  England. 

Whetstones  are  often  made  from  fine-grained  lavas  and  siliceous 
slates  or  hornstones. 


ECONOMIC   GEOLOGY.  511 

(6)  Disseminated  through  a  Bed. — Certain  beds  or  strata  of  sedi- 
mentary formations  are  sometimes  impregnated  with  valuable 
ores  or  minerals,  the  origin  of  which  is  uncertain.  The  metals  were 
either  introduced  contemporaneously  with  the  deposition  of  the 
sediments  in  which  they  occur,  or  after  the  consolidation  of  the 
sediments. 

Among  the  most  notable  examples  of  this  class  of  deposit  are 
the  famous  gold-bearing  banket-reefs  of  the  Rand,  in  the  Transvaal, 
which  are  merely  beds  of  quartzose  conglomerate  impregnated  with 
gold  and  pyrites  ;  the  celebrated  copper-bearing  shales  of  Mansfeld, 
in  Prussian  Saxony,  which  have  been  worked  for  eight  hundred 
years  ;  the  rich  copper-bearing  conglomerates  of  Lake  Superior  ; 
the  Silver  Sandstones  of  Utah  ;  and  the  Lead  Sandstones  of 
Commern,  in  Rhenish  Prussia. 


CLASS  III. 

Unstratified  Deposits. 

(a)  Deposits  of  Volcanic  Origin. — These  include  deposits  of 
sulphur  and  borax,  which  accumulate  in  and  around  fumaroles  in 
the  form  of  sublimates.  The  fumarolic  sulphur  of  Vesuvius,  Etna, 
and  volcanic  regions  of  Japan  and  New  Zealand  is  of  great  economic 
value.  The  steam  fumaroles  of  Pisa,  in  Italy,  yield  a  large  annual 
output  of  boric  acid. 

(6)  Stockwork  Deposit. — A  Stockwork  is  a  rock-mass  traversed  by 
numerous  small  veins  that  mutually  intersect  one  another,  but  are 
too  small  to  be  worked  separately.  The  valuable  ore  may  be  tin, 
gold,  or  any  metal  of  economic  importance.  With  such  deposits  it 
is  necessary  to  quarry  the  whole  of  the  rock-mass  in  order  to  extract 
the  valuable  mineral,  hence  the  name  Stockwork,  which  refers  to 
the  quarry-system  of  mining. 

The  celebrated  gold-bearing  ore-bodies  at  the  Treadwell  Mines 
in  Alaska  are  good  examples  of  this  class  of  deposit. 

(c)  Contact  and  Replacement  Deposits.— A  Contact  Deposit  is  one 
which  occurs  at  or  near  the  contact  of  a  sedimentary  rock  and  an 
intrusive  mass  or  dyke. 

Valuable  deposits  of  iron-ores,  and  of  copper,  lead,  and  zinc 
sulphates  are  frequently  found  in  the  vicinity  of  intrusive  dykes. 

The  famous  copper-bearing  pyritic  ore-bodies  at  Rio  Tinto,  in 
Spain,  are  typical  Contact  Deposits. 

(d)  Fahlbands. — These  are  bands  or  zones  of  crystalline  meta- 
morphic  rocks  so  highly  impregnated   with  ore  as  to  be  of  com- 
mercial value.     The  silver-bearing  Fahlbands  (grey  beds)  of  Norway 


512 


A   TEXT-BOOK    OF    GEOLOGY. 


are  among  the  best  known  examples.  They  follow  the  strike  and 
dip  of  the  strata  by  which  they  are  bounded.  The  thickness  may 
varv  from  a  few  inches  to  hundreds  of  feet. 


FIG.  233. — Showing  stockwork  of  magnesite  veins, 
U.S.  Ged.  Survey. 

Fahlbands  are  related  to  bed-impregnation,  and  probably  owe 
their  origin  to  aqueous  and  gaseous  emanations  expelled  from  a 
cooling  intrusive  magma. 


FIG.  234. — Showing  section  of  contact  deposits, 
(a)  Granite.         (6)  Slate.         (c)  Contact  deposit. 

(e)  Impregnations. — It  has  sometimes  happened  that  when  a  rock 
has  been  fissured,  a  portion  of  the  rock  on  one  or  both  walls  has 
become  impregnated  with  some  metallic  substance,  disseminated 


ECONOMIC   GEOLOGY. 


513 


as  grains,  bunches,  or  nests  throughout  the  mass  in  the  vicinity  of 
the  fissure. 

"  Such  an  occurrence  is  called  an  Impregnation,  implying  that  the 
mineral  has  been  introduced  as  a  secondary  product  by  mineralised 
waters,  superheated  steam,  or  gases. 

The  term  "  impregnation  "  refers  to  the  genesis  rather  than  the 
form  of  the  ore-body.     Genetically  the  majority  of  Stockworks, 


a 


FIG.  235. — Showing  fahlband  at  Dusky  Sound,  New  Zealand. 
(a)  Mica-schist.         (6)  Fahlband  of  pyrrholite,  etc. 

Contact  Deposits,  and  Fahlbands  may  be  regarded  as  impregna- 
tions, as  well  as  such  bedded  deposits  as  the  so-called  Banket- 
reefs  of  the  Rand,  the  copper-deposits  of  Mansfeld,  the  Silver 
Sandstones  of  Utah,  etc. 

Granite  and  other  acid  igneous  rocks  are  sometimes  impregnated 


Fm.  236.— Tin-impregnation, 
(a)  Granite.  (6)  Tin-ore. 

with  tin-ore,  and  such  tin-impregnations  may  be  regarded  as  typical 
of  this  class  of  deposits.  Among  the  most  famous  is  the  Mount 
Bischoff  tin-deposit  in  Tasmania. 

(/)  Segregated  Veins. — These  are  only,  found  in  sedimentary  rocks 
which  have  been  sharply  folded,  whereby  fissures  or  cavities  have 
been  formed  in  the  bent  portions  more  or  less  parallel  with  the 
bedding  planes.  That  is,  Segregated  Veins  mostly  occur  along  the 
crest  of  anticlinal  axes,  and  sometimes  along  the  axes  of  synclines. 

The  best  example  of  Segregated- Veins  are  Saddle-Reefs,  which 
are  typically  developed  at  Bendigo,  in  the  State  of  Victoria.  These 

33 


514 


A   TEXT-BOOK    OF    GEOLOGY. 


FIG.  237. — Sections  showing  formation  of  saddle-reefs.     (After  E.  J.  Dunn.) 

gold-bearing  ore-bodies  consist  of  arch-like  masses  of  quartz  that 
conform  to  the  bedding  planes  and  taper  out  going  downward. 


ECONOMIC   GEOLOGY. 


515 


In  what  are  called  Inverted  Saddle-Reefs  the  ore-bodies  occur 
in  the  troughs  of  the  folds.  Good  examples  of  Inverted  Saddle- 
Reefs  are  found  at  Mount  Boppy,  in  New  South  Wales. 


"PLAN 
North  A_ 


.B.  South 


Longitudinal  Section 


Transverse     Sections 
E 


Actual  Section  through 
North  Shaft 


Illustrates  how  the  reef 
may  occur  below  and  yet 
not  outcrop  at  the  surface 


Actual  Section  through 
Main  Shaft 


'feet  level 
termed/ate  level 


Section  through  No.  1  Winze 

Showing    bottom  of  Saddle  Reef 


300  FI 


FIG.  238. — Sections  showing  structure  of  inverted  saddle-reefs. 
(After  Jaquet.) 

(g)  Gash-Veins. — These  are  metalliferous  deposits  occupying  len- 
ticular or  lens-shaped  cavities  or  gashes  in  limestones.  They 
generally  occur  at  the  intersection  of  cross- joints  where  cavities 


516 


A    TEXT-BOOK    OF    GEOLOGY. 


have  been  formed  by  the  action  of  water,  and  simultaneously  or 
subsequently  filled  with  metallic  ores.  The  ores  that  usually 
occur  in  this  form  are  those  of  lead  and  zinc. 


FIG.  239. — Showing  gash-vein  in  limestone  at  Wangapeka,  New  Zealand. 

(a)  Silurian  limestone.  (6)  Silurian  slate. 

(c)  Gash-veins,  with  galena  and  blende. 

(h)  Fissure- Veins. — These  are  the  best  denned  and  most  persistent 
of  all  veins.  They  pass  through  all  kinds  of  rock  and  pursue  their 
course  independently  of  the  bedding  planes  or  stratification.  In 
some  parts  they  may  chance  to  coincide  with  the  strike  and  dip  of 
the  enclosing  rock-formation,  and  in  such  places  they  are  difficult 
to  distinguish  from  Segregated  Veins. 


FIG.  240. — Lodes  of  Botallic  Mine,  Cornwall. 
(a)  Lodes.         (6)  Slaty  shales.         (c)  Granite. 

All  veins  that  crop  out  at  the  surface  have  been  more  or  less 
truncated  by  denudation. 

The  mineral  contents  of  Fissure- Veins  were  in  most  cases 
deposited  by  ascending  aqueous  solutions  that  were  probably 
genetically  connected  with  some  deep-seated  igneous  intrusion. 

The  different  minerals  composing  the  vein-filling  are  frequently 


ECONOMIC   GEOLOGY.  517 

arranged  in  layers  parallel  with  the  walls.  When  the  layers  are 
made  up  of  crystalline  aggregates,  the  crystals  are  often  arranged 
with  the  longer  axis  at  right  angles  to  the  plane  of  the  walls,  thereby 
presenting  the  appearance  called  comb- structure. 

The  tin-lodes  of  Cornwall  are  typical  examples  of  Fissure- Veins. 

Filling  of  Cavities  and  Veins. — The  vein-filling  or  gangue  of  lodes, 
in  which  the  valuable  metal  or  ore  is  embedded,  is  in  most  cases 


FIG.  241. — Showing  comb-structure  of  lode-matter.     Section 
showing  arrangement  of  ore  in  vein. 

1,  Quartz,  with  galena  and  zinc  blende.       3,  Vug  quartz. 

2,  Country-rock.  4,  Iron  pyrite  disseminated  through  quartz. 

quartz,  which  may  be  crystallised  or  chalcedonic.  Ores  of  lead 
and  zinc  are,  however,  usually  enclosed  in  a  gangue  of  fluor-spar  or 
calcite. 

Origin  of  Vein- Cavities. — The  solutions  which  deposited  the  vein- 
matter  and  its  valuable  contents  either  found  the  cavities  and 
fissures  awaiting  them,  or  they  formed  their  own  channels  by  a 
process  of  slow  progressive  dissolution  and  replacement  of  the 
wall-rock  along  pre-existing  cracks  or  fractures. 

The  pre-existing  cavities  and  fractures  were  mechanically  formed 


518  A  TEXT-BOOK  OF  GEOLOGY. 

in  sedimentary  rocks  by  folding,  or  by  igneous  intrusions  ;  and 
in  eruptive  rocks  by  contraction  arising  from  cooling. 

Where  the  fracturing  of  the  rock  has  been  produced  by  igneous 
intrusions,  these  frequently  provide  the  mineralised  vapours  and 
solutions  that  corrode  the  shattered  rock  and  fill  the  fissures  with 
mineral  matter. 

Ore-bodies  are  often  formed  along  joint-  and  fault-planes,  and 
at  the  intersections  of  joints,  simple  fractures,  and  faults. 

Some  rock-fissures  are  known  to  be  of  great  antiquity  from  the 
presence  of  fossils  in  the  material  filling  them.  Dyke-like  masses 
of  sandstone  containing  Cambrian  brachiopods  occur  in  granite 
in  the  Aland  Islands  at  the  entrance  of  the  Gulf  of  Finland  ;  and 
narrow  veins  of  fossiliferous  sandstone  are  seen  in  the  sea-cliffs 
at  Oamaru,  New  Zealand,  traversing  a  sheet  of  basalt  intercalated 
with  Lower  Miocene  strata. 

Depth  of  Lodes. — Where  the  lodes  occupy  fissures  confined  to  a 
particular  rock,  or  rock-formation,  like  the  propylite-veins  of 
Cripple  Creek  and  Hauraki,  or  the  gash-veins  so  frequently  found 
in  limestone,  the  depth  to  which  the  lodes  may  descend  is  limited 
by  the  thickness  of  the  containing  rock.  But  where  the  original 
fissures  pass  down  through  the  crust  without  regard  to  the  character 
or  number  of  the  rock-formations,  the  lodes  may  descend  to  depths 
far  beyond  the  limits  of  deep  mining. 

Length  of  Lodes. — Lodes  may  vary  from  a  few  hundred  feet  to 
scores  of  miles  in  length.  In  Cornwall,  the  average  length  of  the 
lodes  is  about  a  mile  ;  in  Saxony,  three  or  four  miles ;  in  the  Harz 
Mountains,  eight  or  ten  miles.  The  Asch  Lode  in  the  Bavarian 
Forest  can  be  traced  for  25  miles  ;  the  Bohemian  Pfahl,  34  miles  ; 
and  the  Great  Pfahl,  92  miles. 

The  Mother  Lode  of  California  extends  through  five  counties, 
being  traceable  for  a  distance  of  70  miles. 

Width  of  Lodes. — Lodes  may  vary  in  width  botl^  along  their 
course  and  in  depth.  The  same  lode  may  vary  from  a  mere  clay 
parting  to  hundreds  of  feet  in  width.  The  celebrated  Comstock 
Lode,  Nevada,  varies  from  20  to  300  feet  wide.  The  Great  Pfahl, 
the  most  colossal  quartz-lode  in  the  globe,  maintains  an  average 
width  of  100  feet,  but  in  places  widens  out  to  370  feet. 

Most  great  lodes  occupy  fault-fractures.  The  three  gigantic 
lodes  of  the  Bavarian  Forest  are  regarded  by  Suess  as  the  greatest 
monuments  of  linear  dislocation  in  Europe. 

Age  of  Vein-Filling.— In  the  case  of  veins  and  lodes  traversing 
sedimentary  and  metamorphic  rocks,  it  is  natural  to  suppose  that 
the  formation  of  the  ore-bodies  would  follow  the  periods  of  great 
erogenic  movements,  two  of  which  are  specially  notable  in  geo- 
logical history,  namely,  the  late  Carboniferous  and  the  Middle 


ECONOMIC    GEOLOGY.  519 

Cainozoic.  Both  were  characterised  by  extraordinary  volcanic 
activity  and  igneous  intrusion. 

The  uplift  and  intense  folding  of  great  segments  of  the  earth's 
crust  cause  the  formation  of  powerful  fractures,  which  afterwards 
became  channels  for  the  circulation  of  mineral-bearing  waters. 

If  a  lode  traverses  a  pile  of  strata  ranging  in  age  from  the  Silurian 
to  the  Jurassic,  it  is  obvious  that  the  age  of  the  lode-matter  must 
be  post-Jurassic.  And  if  the  Jurassic  strata  are  overlain  by  an 
Eocene  formation  which  the  lode  does  not  penetrate,  then  we  know 


a 


FIG.  242.  —  Showing  formation  of  ore  -body  at  intersection  of  joint 
planes,  Bendigo  Goldfield,  Victoria.     (After  T.  A.  Rickard.) 

that  the  age  of  the  lode  is  post-Jurassic  and  pre-Eocene,  that  is, 
Cretaceous. 

In  the  Hauraki,  Cripple  Creek,  Tonopah,  Goldfield,  and  some 
Transylvanian  mining  areas,  the  gold-bearing  veins  are  confined 
to  andesitic  lavas  and  tuffs  of  Middle  Tertiary  age.  The  veins 
occupy  contraction  cracks,  and  their  filling  must  have  taken  place 
in  late  Tertiary  times.  Such  veins  are  called  Propylite  veins,  to 
distinguish  them  from  Tectonic  veins,  which  occupy  cracks  and 
cavities  formed  by  crustal  folding.  Fissure-veins  are  profound 
fractures  passing  from  one  formation  to  another  ;  and  Saddle-reefs 
are  typical  examples  of  the  tectonic  type  of  ore-body. 

Distribution  of  Valuable  Contents  in  Lodes.  —  The  matrix  or 


520  A  TEXT-BOOK  OF  GEOLOGY. 

gangue  of  most  metalliferous  lodes  is  quartz,  but  it  is  seldom  that 
the  valuable  ore  is  equally  distributed  throughout  the  whole  mass 
of  vein-stone.  Usually  it  occurs  in  isolated  bunches,  nests,  patches, 
pockets,  or  pipes,  to  which  the  general  term  pay-shoot  is  frequently 
applied. 

The  pay-shoot  or  commercially  valuable  ore  may  occur  on  the 
foot-wall,  hanging-wall,  or  middle  of  a  lode,  the  remaining  portion 
of  which  may  be  barren  or  too  lean  to  be  profitable.  Or  it  may 
occupy  the  full  width  of  that  portion  of  the  lode  in  which  it  occurs. 

Influence  Of  Country-Rock. — Frequently  a  lode,  when  it  passes 
from  one  kind  of  rock  to  another,  ceases  to  be  profitable,  or  the 
converse.  Moreover,  a  lode  which  yields  lead  and  zinc  where  it 
traverses  limestone,  may  contain  copper  in  slates  and  tin  in  granite. 

It  is  well  recognised  by  miners  that  certain  rocks  favour  the 
occurrence  of  particular  metals  and  minerals.  Thus  tin  has  a 
preference  for  granite  and  granitic  rocks  ;  copper  and  chrome  for 
serpentine  ;  lead,  zinc,  silver,  and  iron  for  limestones  and  calcareous 
rocks. 

Among  sedimentary  rocks  gold  has  a  preference  for  ancient 
sandstones  and  slates,  and  among  igneous  rocks  for  andesites  of 
older  Cainozoic  date. 

In  a  general  way  tin,  tungsten  ores,  and  gold  have  a  preference 
for  acid  rocks  ;  and  chrome,  nickel,  cobalt,  iron,  copper,  and 
platinum  for  basic  rocks. 

Paragenesis.1— -The  genetic  processes  which  have  led  to  the  for- 
mation or  deposition  of  metal  and  minerals  in  ore-bodies  have 
frequently  brought  about  the  association  of  certain  minerals  with 
one  another. 

Thus  tin  and  wolfram  are  constant  companions,  also  lead  and 
zinc,  gold  and  quartz,  cobalt  and  nickel,  iron  and  copper  (as 
sulphides),  chrome  and  serpentine. 

Secondary  Enrichment  of  Veins.— Sulphide  ore-bodies,  which 
crop  out  at  the  surface  in  non-glaciated  regions,  usually  consist  of 
two  distinct  portions,  namely,  the  oxidised  zone  or  zone  of  weathering , 
and  the  unoxidised  zone,  which  generally  lies  below  water-level. 

The  oxidation  of  the  upper  portion  of  the  lode  is  due  to  the 
action  of  rain-water,  usually  called  meteoric  water.  The  depth  to 
which  the  alteration  may  extend  is  dependent  on  local  topographical 
conditions  and  may  vary  from  a  few  feet  to  200  or  300  feet. 

Mining  operations  have  shown  that  most  oxidised  sulphide 
ore-bodies  contain  abnormally  rich  ore  in  the  oxidised  zone,  fre- 
quently in  the  lower  portion  of  it.  This  rich  ore  is  supposed  to 
arise  from  the  migration  of  the  valuable  metallic  contents  from  the 
higher  portion  of  the  vein  to  the  lower  portion  through  the  agency 
1  Gr.  para  =  along  side  of,  and  genesis  =  birth, 


ECONOMIC    GEOLOGY. 


521 


of  meteoric  waters.  In  some  cases  the  processes  of  dissolution, 
migration,  and  deposition  of  the  ore  may  have  taken  place  over 
and  over  again,  each  cycle  resulting  in  a  greater  concentration  of 
the  valuable  portion  of  the  ore. 

Secondary  enrichment  may  also  arise  through  the  removal  of 
the  worthless  metals,  thereby  leaving  the  valuable  ore  in  a  purer 
form. 

The  metals  held  in  solution  by  permeating  waters  may  be 
deposited  as  secondary  sulphides  on  the  primary  sulphides  in  the 
lower  portion  of  the  lode.  This  is  conspicuous  in  the  Mount  Lyell 
and  Rio  Tinto  sulphide  ore-bodies. 

The  first  operation  in  the  process  of  secondary  enrichment  is 


FIG.  242A. — Showing  Zones  of  Oxidisation  and  Secondary  Enrichment. 

(a)  Ironstone  cap  or  gossan.  (b)  Zone  of  oxidised  ore. 

(c)  Zone  of  Secondary  Enrichment.        (d)  Primary  sulphides. 


the  chemical  weathering  and  oxidation  of  the  sulphides  in  the  zone 
of  weathering.  The  surface  waters,  now  charged  with  minerals 
in  solution,  descend  through  the  body  of  the  lode  and  deposit  their 
valuable  contents  in  a  concentrated  or  purer  form  through  chemical 
or  electrolytic  action. 

Metasomatic  Replacement. — Until  lately  it  was  the  common 
belief  that  veins  and  all  ore-bodies  occupied  pre-existing  fissures 
and  cavities  in  the  country-rock.  In  recent  years  much  stress  has 
been  placed  on  what  is  called  metasomatic  replacement,  which  is 
now  regarded  as  an  important  process  of  lode-formation. 

According  to  this  hypothesis,  it  is  surmised  that  the  mineralised 
waters  percolating  through  the  rocks  dissolve  certain  tracks  or 
zones  which  they  partially  or  completely  replace  with  mineral 
matter  and  ores.  In  many  cases  the  altered  zone  follows  the 
track  of  a  fissure,  bedding  plane,  or  fault ;  but  the  replacement  may 


522 


A    TEXT-BOOK    OF    GEOLOGY. 


follow  a  particular  band  of  rock  without  the  aid  of  leading  cracks 
or  fissures. 


FIG.  244. — Showing  displacements  caused  by  strike-fault. 
(a-b)  Vertical  displacement  of  throw.         (b-c)  Horizontal  shift. 


A------ 


B 


Fia.  245. — Showing  repetition  of  coal-seam  by  strike-fault 

Metasomatic  replacement  may  take  place  among  certain  con- 
stituents of  rock-masses ,  no  matter  how  dense  ;  and  is  common  in 


[To  face  page  522. 


FIG.  243. Showing  iron  and  copper  pyrites  intergrown  with  garnets. 


ECONOMIC   GEOLOGY. 


523 


metamorphic    rocks    and    all    older    igneous    masses.     A    notable 
example  is  the  alteration  of  andesites  to  propylite. 

The  interchange  of  constituents  proceeds  molecule  by  molecule 
until  large  bodies  of  rock  along  certain  zones  are  altered  to  mineral- 


Fault. 


FIG.  246. — Showing  effect  of  dip-fault. 

ised  ore-bodies.  It  is  related  to  pseudomorphism,  in  which  a  mineral 
is  removed  and  replaced  by  another  mineral. 

There  was  always  a  difficulty  in  believing  that  such  huge  ore- 
bodies  as  the  Broken  Hill  Lode  in  New  South  Wales  or  the  Comstock 
Lode  in  Nevada  occupied  fissures  which  had  remained  open  as 
gaping  chasms  until  they  became  filled  with  mineral  matter. 

Thefsymmetrical  banding  of  the  lode-matter  on  the  opposite 
walls  of  many  lodes  and  the  comb-structure  are  conclusive  evidence 


524  A    TEXT-BOOK    OF    GEOLOGY. 

that  some  small  fissures  did  remain  as  open  channels  during  the 
deposition  of  the  vein-filling.  It  is  not  improbable  that  in  crustal 
areas  in  tensional  stress  the  width  of  the  cracks  and  fissures  might 
continue  to  increase  for  a  considerable  time  during  the  filling  process. 

Brecciated  Lodes. — Wall-movements  of  considerable  magnitude 
have  taken  place  on  the  course  of  many  lodes,  whereby  the  gangue 
has  become  crushed  and  broken.  Subsequently  when  re-cemented 
by  infiltration,  such  crushed  lode-matter  is  said  to  be  brecciated. 

Faulting  of  Lodes. — Lodes  and  bedded  mineral  deposits  may  be 
intersected  by  faults,  which  may  run  parallel  with  the  strike  or 
at  right  angles  to  it. 

Strike-faults  cause  vertical  and  horizontal  displacements  of  the 
veins  or  seams  they  intersect.' 

Strike-faults  also  cause  a  repetition  of  the  seam  or  vein  at  the 
surface. 

Dip-faults  produce  an  apparent  lateral  displacement  of  the  beds 
or  veins  which  they  cross.  When  the  faulting  takes  place,  the 
principal  movement  is  a  vertical  one.  Consequently,  when  the 
vein  is  vertical,  the  severed  ends  merely  slide  on  one  another. 

The  apparent  heave  or  lateral  displacement  is  produced  by  the 
denudation  of  the  elevated  portion  causing  the  outcrop  to  recede 
in  the  direction  of  the  dip  as  shown  in  figs.  95  and  96.  The  flatter 
the  dip,  the  greater  will  be  the  apparent  lateral  displacement.1 

Ores  and  Minerals  Genetically  Considered. 

The  constant  association  of  ore-deposits  and  igneous  rocks  has 
led  to  the  broad  generalisation—  That  ore-deposits  are  genetically 
connected  with  the  eruption  of  igneous  magmas. 

The  genetic  classification  which  satisfies  most  nearly  our  present 
knowledge  relating  to  the  origin  of  ores  and  minerals  comprises 
four  classes  as  follow  : — 

I.  Magmatic  Segregation. 
IT.  Eruptive  After- Actions  : — 

(a)  Solfataric. 

(b)  Fumarolic. 

(c)  Contact  metamorphic. 

(d)  Regional  metamorphic. 

III.  Meteoric  Waters  : — 

(a)  Chemical. 

(b)  Mechanical. 

IV.  Organic. 

1  The  recovery  of  faulted  lodes  and  coal-seams  by  graphic  projection  is 
fully  described  in  Park's  Mining  Geology,  Charles  Griffin  &  Co.,  Limited. 


[To  face  page  524. 


FIG.  247. — Showing  structure  ol  pyritic  quartz  in  thin  section, 
U.S.  Geol.  Survey.     (After  Lindgren.) 

(a)  Quartz.         (6)  Arseno-pyrite. 


\ 


ECONOMIC    GEOLOGY.  525 

I.  Magmatic  Segregation. 

This  class  includes  all  ores  and  minerals  which,  occur  as  primary 
constituents  of  igneous  rocks,  resulting  from  direct  segregation 
in  the  cooling  magma. 

The  most  important  ores  occurring  as  primary  constituents  of 
igneous  rocks  are  : — 

Chromite,  found  in  serpentine  and  peridotite. 
Copper  and  nickel-iron,  in  serpentine. 
Platinum,  in  peridotite  and  other  ultra-basic  eruptives. 
Magnetite  and  titanite,  in  basic  and  semi-basic  eruptives. 

II.  Eruptive  After-Actions. 

The  after-actions  resulting  from  an  igneous  intrusion  will  com- 
mence at  the  moment  of  intrusion  and  continue  with  waning 
intensity  until  the  magma  has  completely  cooled. 

Solfatarie.— Volcanic  eruptions  are  usually  accompanied  by  the 
emission  of  enormous  volumes  of  steam  ;  also  hydrogen  sulphide, 
sulphur  dioxide,  carbon  dioxide,  as  well  as  compounds  of  chlorine, 
fluorine,  and  boron. 

These  gaseous  and  aqueous  emanations  come  from  the  same 
source  as  the  igneous  magma,  accompany  the  magma  in  its  ascent, 
and  may  possibly  be  one  of  the  contributing  causes  of  the  eruption. 

In  the  expiring  stages  of  volcanic  activity  in  some  regions,  geysers, 
hot-springs,  and  fumaroles  occur  in  the  vicinity  of  the  volcanic 
vents. 

The  geysers  and  hot  springs  deposit  incrustations  of  silica  on  the 
walls  or  passages  through  which  the  waters  flow.  At  the  surface 
they  cover  large  areas  with  successive  layers  of  silica,  which  in 
time  form  circular  mounds  around  the  vent-holes. 

The  silica  when  first  deposited  is  soft  and  gelatinous,  but  it  soon 
becomes  hard  and  assumes  the  chalcedonic  form. 

The  siliceous  sinters  being  deposited  at  Rotorua,  in  New  Zealand, 
contain  both  gold  and  silver. 

.The  gold-bearing  chalcedonic  and  crypto-crystalline  lodes  at 
Waihi,  in  New  Zealand,  traversing  andesites  and  dacites,  are  of 
solfataric  origin,  as  also  are  the  great  umbrella-shaped  lodes 
scattered  throughout  the  same  region.  The  gold-bearing  lodes  at 
Cripple  Creek,  Goldfield,  and  Telluride,  which  occur  in  older  Tertiary 
andesites,  etc.,  are  also  solfataric. 

Fumarolie. — Deposits  of  sulphur  and  borax  are  formed  by  the 
aqueous  and  gaseous  emanations  in  the  waning  stages  of  volcanic 
activity  in  almost  all  volcanic  regions  where  eruptions  have  taken 
place  in  late  Tertiary  and  Recent  times. 


526  A  TEXT-BOOK  OF  GEOLOGY. 

The  sulphur  deposits  of  Italy,  Sicily,  Japan,  and  New  Zealand 
may  be  taken  as  typical  examples  of  the  fumarolic  class  of  mineral 
deposits. 

Contact  Metamorphic  Deposits. — An  intrusive  plutonic  magma 
tends  to  effect  considerable  changes  in  the  rocks  with  which  it 
comes  in  contact.  The  greater  the  mass  of  the  intrusion,  the 
slower  will  be  the  rate  of  cooling  ;  and  the  slower  the  rate  of 
cooling,  the  longer  will  the  adjacent  rocks  be  heated. 

The  rate  of  cooling  will  be  dependent  on  the  mass  of  the  intrusion, 
its  distance  from  the  surface,  and  the  relative  thermal  conductivity 
of  the  covering  and  enclosing  rocks. 

The  changes  effected  by  the  intrusive  magma  will  be  mainly 
mechanical,  thermal,  and  chemical. 

The  intruded  rocks  will  be  compressed,  bent,  and  more  or  less 
shattered  and  fissured  in  the  neighbourhood  of  the  intrusion. 

The  magma  will  part  with  its  heat  by  slow  radiation  into  the 
adjacent  rocks. 

The  steam  and  gases  occluded  in  the  cooling  magma,  together 
with  the  steam  generated  from  the  water  contained  in  the  rocks 
in  contact  with  the  magma,  will  pass  into  and  permeate  the  sur- 
rounding rocks,  and  thereby  cause  a  molecular  rearrangement  of 
the  constituent  minerals,  resulting  in  what  is  called  contact 
metamorphism. 

As  the  igneous  magma  and  the  intruded  sedimentaries  cool, 
they  will  contract  in  mass  ;  and  cracks,  fissures,  and  cavities 
will  be  developed  in  them  along  the  line  of  contact.  These 
fissures  and  cavities  will  form  channels  for  the  circulation  of  the 
mineralised  underground  solfataric  waters,  which  will  in  time 
fill  them  with  mineral  matter. 

Above  a  temperature  of  365°  C.  and  a  pressure  of  200  atmospheres, 
water  and  all  the  more  or  less  volatile  compounds  will  exist  as 
gases.  These  mineral-laden  gases,  being  under  enormous  pressure, 
will  permeate  the  shattered  intruded  rocks  in  all  directions ;  and 
when  their  temperature  falls,  will  condense  and  deposit  their 
load  of  metals  and  mineral  in  every  crack  and  crevice  to  which 
they  are  able  to  penetrate. 

In  this  way  rocks  may  become  impregnated  with  ores  near  the 
line  of  contact,  and  in  some  cases  ore-bodies  may  be  formed  at 
points  a  considerable  distance  from  the  intruding  magma. 

As  the  cooling  proceeds,  the  least  soluble  substances,  which  are 
obviously  the  last  to  be  dissolved,  will  be  the  first  to  be  deposited  ; 
and  as  the  temperature  and  pressure  continue  to  diminish,  the 
remaining  metals  and  minerals  will  be  deposited  in  the  inverse 
order  of  their  solubility. 

It  is  obvious  that  the  later  stage  of  eruptive  after-actions  will 


ECONOMIC    GEOLOGY.  527 

represent,  in  a  modified  form,  the  waning  effects  of  solfataric 
action.  The  deep-seated  conditions  with  the  greater  pressure 
and  temperature  will  also  accelerate  the  action  of  metasomatic 
processes,  whereby  lodes  may  be  formed  in  the  zone  of  contact  of 
metamorphism. 

It  is  almost  certain  that  the  formation  of  contact-deposits, 
fissure-veins,  and  rock-impregnation  may  be  traced  to  the  same 
genetic  causes,  the  actual  form  assumed  by  the  ore-body  being 
merely  the  result  of  some  local  geological  condition. 

Contact-deposits  frequently  lie  along  the  boundary-line  between 
the  eruptive  and  the  country-rock ;  also  at  variable  distances  from 
the  eruptive,  but  never  outside  the  zone  of  metamorphism. 

The  metallic  ores  which  are  the  most  common  in  contact-deposits 
are  sulphides  of  copper,  iron,  lead,  and  zinc,  also  magnetite  and 
specular  iron.  Typical  pyritic  contact-deposits  are  those  of 
Rio  Tinto  and  Tharsis  in  Spain,  Mount  Lyell  in  Tasmania,  and 
Broken  Hill  in  New  South  Wales. 

Among  the  minerals  developed  in  rocks  that  have  been  intruded 
by  igneous  dykes  or  introduced  by  vagrant-emanations  from  the 
intruding  magma  are  garnet,  vesuvianite,  scapolite,  wollastonite, 
augite,  biotite,  hornblende,  chiastolite,  etc. 

Regional  Metamorphie  Deposits. — To  this  group  belong  the 
massive  deposits  of  magnetite  and  specular  iron  that  occur  in 
crystalline  metamorphic  rocks  of  older  Palaeozoic  age.  The 
origin  of  the  iron  is  uncertain.  It  probably  existed  in  the  original 
sediments  and  became  concentrated  and  rearranged  under  the 
influence  of  heat,  pressure,  and  underground  solutions.  The 
concentration  of  the  iron  and  the  metamorphism  of  the  containing 
rocks  are  no  doubt  traceable  to  the  same  causes. 

The  valuable  bodies  of  magnetite  in  Sweden  and  the  greater 
masses  of  specular  iron  and  magnetite  in  Michigan  are  notable 
examples  of  regional  metamorphic  ores. 

Massive  aggregates  of  magnetite  and  specular  iron  are  common 
in  chlorite-schist  and  mica-schist  in  all  parts  of  the  globe. 

III.  Meteoric  Waters. 

Chemical. — This  group  includes  deposits  of  salt,  borax,  nitre, 
bog-iron  ore.  and  some  deposits  of  gypsum,  rock-phosphate,  and 
manganese. 

Mechanical. — This  embraces  all  sedimentary  rocks  and  their 
contents  ;  also  alluvial  drifts  containing  gold,  platinum,  tin, 
gems,  etc. ;  likewise  marine  sands  and  gravels  containing  gold  or 
other  valuable  metals,  etc. 


528  A  TEXT-BOOK  OF  GEOLOGY. 

IV.  Organic. 

Vegetable. — This  group  includes  all  varieties  of  mineral  fuel 
ranging  from  peat  to  anthracite  ;  also  graphite,  oil-shale,  mineral 
oils,  natural  gases,  and  infusorial  earth.1 

The  ultimate  source  of  carbon  is  undoubtedly  magmatic,  but  the 
immediate  source  of  most  carbon  compounds  is  organic. 

Animal.- — This  subdivision  includes  limestones,  coprolitic  phos- 
phates, infusorial  earths,  and  some  mineral  oils. 

Theories  of  Vein-Formation. 

The  two  theories  which  receive  the  most  support  are  : — 

1.  The  Ascensional  or  Eruptive  Processes  Theory. 

2.  Lateral  Secretion  Theory. 

Ascensional  Theory. — According  to  this  view,  all  ore-bodies  and 
ore-veins  owe  their  origin  directly  or  indirectly  to  the  intrusion 
of  igneous  magmas.  The  intrusive  shatters  the  rocks  and  provides 
the  metals  which  it  brings  up  from  the  barysphere.  The  metals 
are  expelled  from  the  cooling  magma  in  the  form  of  highly-heated 
vapours  which  are  deposited  in  the  cracks  and  fissures  in  the 
neighbouring  rocks.  Moreover,  the  steam  condenses  and  carries 
the  metals  upward  through  cracks  and  fissures  which  eventually 
become  filled  with  mineral  matter,  thereby  forming  mineral  veins. 
The  alteration  and  replacement  of  the  country-rock  is  also 
accelerated  by  the  steam,  gases,  and  heated  waters  emanating 
from  the  cooling  igneous  magma. 

The  almost  constant  association  of  ore-bodies  and  igneous 
intrusions  gives  powerful  support  to  the  Ascensional  theory,  which 
is  now  more  favoured  by  mining  geologists  than  any  other. 

Lateral  Secretion  Theory. — According  to  this  view,  it  is  assumed 
that  meteoric  waters  percolating  through  the  rocks,  by  the  aid 
of  carbonic  acid  and  alkalies,  dissolve  out  certain  constituents, 
which  are  afterwards  deposited  in  cracks,  fissures,  and  cavities, 
.thereby  forming  veins  and  ore-bodies. 

In  support  of  this  view  it  is  asserted  that  sedimentary  and 
igneous  rocks  alike  contain  all  the  constituents  formed  in  veins 
which  are  merely  regarded  as  local  concentrations. 

It  is  well  known  that  cracks  in  limestones  soon  become  filled 
with  calcite  deposited  by  water  slowly  percolating  through  the 
body  of  the  rock.  Similarly,  cracks  and  tension-rents  in  sandstones 
and  igneous  rocks  become  filled  with  quartz,  calcite,  pyrite,  or 

1  Most  infusorial  earths  (so-called)  consist  of  diatoms,  that  is,  of  plants  ; 
some  consist  of  polycystines  which  are  animal  ;  they  do  not  contain 
infusoria. 


ECONOMIC    GEOLOGY.  529 

other  minerals,  all  of  them  obviously  local  concentrations  of  mineral 
matter  derived  from  the  surrounding  rocks. 

The  frequent  association  of  igneous  intrusions  and  ore-bodies 
is  admitted  by  the  supporters  of  the  Lateral  Secretion  theory  ; 
but  they  contend  that  the  gases  and  steam  emanating  from  the 
magma  merely  accelerate  and  supplement  the  action  of  the  meteoric 
waters  in  the  concentration  of  the  metals  which  previously  existed 
in  the  intruded  rocks  as  primary  constituents. 

Summary. — The  Lateral  Secretion  theory  does  not  satisfactorily 
explain  the  formation  of  the  large  pyritic  replacement  ore-bodies  ; 
hence  at  present  the  Ascensional  theory  receives  the  most  support. 
At  the  same  time  it  is  acknowledged  that  the  filling  of  cracks, 
fissures,  and  cavities  with  mineral  matter  is  usually  the  work  of 
meteoric  waters,  as  also  is  the  weathering  and  oxidation  of  the  out- 
crops of  lodes  and  secondary  enrichment. 


FIG.  248. — Showing  natural  spring. 

(a)  Porous  stratum.  (c)  Upper  impervious  confining  bed. 

(6)  Lower  impervious  confining  bed.        (s)  Natural  spring. 

The  processes  of  lateral  secretion  by  meteoric  waters  may  there- 
fore be  regarded  as  supplementary  to  the  work  of  the  ascending 
magmatic  waters  and  gases. 

WATER  SUPPLY. 

Water  for  domestic,  manufacturing,  and  irrigation  purposes 
may  be  obtained  from  rivers,  lakes,  natural  springs,  and  artificial 
wells.  Medicinal  waters  are  usually  derived  from  natural  and 
artificial  springs. 

Natural  Springs.— The  requirements  of  a  natural  spring  are  : — 

(1)  A  constant  supply  of  water. 

(2)  A  porous  or  permeable  water-bearing  stratum. 

(3)  Confining  beds  below  and  above  the  water-bearing  stratum. 

(4)  A  natural  outlet  at  some  point  below  the  inlet  or  fountain- 

head. 

When  rain-water  falls  on  a  porous  or  pervious  stratum,  it  will 
travel  downwards  until  it  reaches  an  impervious  stratum  or  un- 
fissured  rock,  along  the  plane  of  which  it  will  travel  until  it  reaches 

34 


530  A  TEXT-BOOK  OF  GEOLOGY. 

the  surface,  where  it  will  form  a  natural  spring.  Obviously  such  a 
spring  can  only  be  formed  where  the  porous  water-bearing  stratum 
crops  out  at  the  surface  at  a  lower  level  than  the  fountain-head, 
as  in  a  gorge,  sea-cliff,  hill-slope,  or  artificial  cutting. 

A  discussion  of  what  forms  a  porous  stratum  and  adequate 
confining  beds  below  and  above  the  porous  stratum  will  be  found 
under  the  heading  Artesian  Wells. 

Artesian  Wells. 

When  underground  water  existing  under  hydraulic  pressure  is 
tapped  by  a  well  or  bore-hole,  it  forms  what  is  called  an  artesian 
well.  The  pressure  may  or  may  not  be  sufficient  to  cause  the  water 
to  overflow  at  the  surface. 

The  principle  underlying  the  flow  of  artesian  wells  is  based  on 
the  physical  law  that  imprisoned  water  always  tends  to  rise  to 
the  height  of  the  inlet  or  fountain-head.  In  other  words,  gravity 
is  the  main  cause  of  artesian  flow. 

The  main  requirements  for  an  artesian  flow  of  water  are  : — 

(1)  An  adequate  and  constant  supply  of  water. 

(2)  A  porous  stratum  to  act  as  a  reservoir  and  channel  for  the 

underground  flow. 

(3)  A  confining  stratum  below  and  above  the  porous  stratum 

or  water-bearing  bed. 

(4)  Absence  of  an  outlet  for  the  water  at  a  lower  level  than  the 

fountain-head. 

Porous  Stratum. — The  ideal  water-bearing  stratum  is  a  bed  of 
sand,  gravel,  or  porous  sandstone  ;  but  any  rock  that  is  crushed 
or  jointed,  or  possesses  distinct  bedding  planes,  pores,  vesicles, 
cracks,  openings,  or  passages  of  any  kind  whatever  may  form  an 
effective  reservoir  or  source  of  underground  water. 

Confining  Strata. — The  best  confining  stratum  is  a  bed  of  clay, 
marl,  or  shale  ;  but  any  compact  unfissured  rock,  or  even  any 
contantly  saturated  semi-porous  stratum,  may  act  as  an  effective 
confining  stratum  provided  it  is  less  porous  and  offers  a  greater 
frictional  resistance  to  the  flow  of  water  than  the  water-bearing 
stratum. 

Water  will  always  flow  with  the  greatest  freedom  through  the 
stratum  that  offers  the  least  frictional  resistance. 

Standing  water  will  always  rise  to  its  own  level  independently 
of  friction,  but  running  water  cannot  do  so  on  account  of  the  loss 
of  head  or  pressure  in  overcoming  the  resistance  offered  to  the 
flow  by  the  interstices  of  the  rock.  As  the  size  of  the  pores  or 
interstices  diminish,  the  frictional  resistance  increases  with  extra- 


ECONOMIC    GEOLOGY. 


531 


ordinary  rapidity,  being  inversely  proportional  to  the  diameter. 
For  example,  the  frictional  resistance  to  the  flow  of  water  in  a  half- 
inch  tube  is  four  times  that  of  an  inch  tube  of  the  same  length. 
Herein  we  discover  why  a  semi-porous  stratum  with  small  intersti- 
cial  pores  or  openings  may  be  an  effective  confining  stratum  for 
a  water-bearing  bed. 


FIG.  249. — Section  of  artesian  basin. 

(a-a)  Porous  strata. 

(6,  c,  and  d)  Confining  beds  above  and  below  a-a. 
(D  E)  Height  of  intake  or  fountain-head. 
(e,  f,  and  g)  Flowing  wells  ;  e  and  /  from  upper  water-bearing 
stratum,  g  from  lower. 

Arrangement  of  Strata. — -The  ideal  arrangement  of  the  strata 
to  give  the  necessary  pressure  or  head  for  an  artesian  well  is  the 
basin  or  trough,  and  next  to  that  the  sloping  plain. 


FIG.  250. — Section  of  artesian  sloping  plain. 

(a)  Porous  bed  resting  on  bed-rock. 

(6  and  d)  Confining  beds  of  finer  sand  and  clay. 

(c  and  e)  Porous  beds. 

One,  two,  or  more  water-bearing  beds  may  occur  in  the  same 
basin  or  sloping  plain. 

The  sloping  alluvial  plain  is  common  on  the  coasts  of  most 
countries,  and  nearly  always  contains  water-bearing  beds.  The 
alluvium  usually  consists  of  alternations  of  sand,  gravel,  and  clay, 
with  frequently  peaty  layers.  The  material  is  mostly  fluviatile, 
hence  the  beds  vary  greatly  in  thickness  and  extent.  Going 
seaward  the  material  usually  becomes  smaller  in  size  ;  and  a  bed 


532 


A   TEXT-BOOK    OF    GEOLOGY. 


of  gravel  may  pass  into  a  bed  of  sand,  and  a  bed  of  sand  into  clay 
or  silt. 

If  the  water-bearing  beds  crop  out  at  the  surface,  they  merely 
form  ordinary  surface  springs  ;  but  when  they  extend  beneath  the 
sea,  as  so  frequently  happens  with  maritime  plains,  the  fresh  water 
rises  against  the  pressure  of  the  sea  water,  which  seals  up  the  open 


FIG.  251. — Showing  fault-spring, 
(a)  Porous  stratum.         (6)  Impervious  bed.         (s)  Spring. 

ends  of  the  beds,  preventing  the  escape  of  the  fresh  water  wherever 
the  head  of  the  fresh  water  is  less  than  that  of  the  sea  water.  On 
account  of  its  greater  density,  a  column  of  sea  water  100  feet  high 
will  support  a  column  of  fresh  water  nearly  103  feet  high. 

If  the  water-bearing  bed  is  traversed  by  a  fault,  igneous  dyke, 


FIG.  252. — Showing  flow  along  bedding  planes. 
A,  Porous  strata.         B,  Artesian  well 

or  mineral  lode,  the  downward  course  of  the  water  may  be  stopped, 
and  if  the  wall-rock  is  pervious,  the  water  will  rise  upwards  until 
it  reaches  the  surface. 

Artesian  water  may  often  be  obtained  in  tilted  strata,  such  as 
slates  or  schists,  where  the  rain-water  follows  the  bedding  or 
foliation  planes. 

Source  of  Underground  Water. — The  main  source  of  artesian 
water  is  the  rain-water  which  percolates  into  the  ground.  The 
supplementary  sources  of  supply  are  :— 


ECONOMIC   GEOLOGY.  533 

(a)  The  residual  water  left  in  sedimentary  rocks  since  the  time 

of  their  deposition. 

(6)  Water  released  by  dehydration  of  rocks  and  minerals, 
(c)  Plutonic  or  juvenile  water  derived  from  the  interior  of  the 

earth. 

Sediments  laid  down  on  the  floor  of  the  sea  and  in  lakes  entangle 
from  10  to  40  per  cent,  of  their  volume  of  water.  Ordinary  sands 
hold  about  one-third  their  volume  of  water.  During  consolidation 
and  uplift,  the  greater  portion  of  the  water  will  escape,  except  where 
the  beds  are  arranged  in  a  trough  or  syncline.  Most  consolidated 
sedimentary  rocks  retain  from  5  to  10  per  cent,  of  water. 

Minerals  when  first  formed  are  in  the  hydrated  condition,  but 
through  the  influence  of  heat  and  pressure,  they  become  dehydrated 
particularly  during  the  process  of  crystallisation  or  metamorphism. 
Gelatinous  and  opaline  silica  becomes  changed  to  quartz  ;  amor- 
phous limestones  to  crystalline  limestones,  mainly  composed  of 
calcite  ;  peat  and  lignite  become  hard  anhydrous  coals. 

Probably  the  bulk  of  the  water  expelled  during  dehydration 
passes  upwards  and  is  absorbed  in  the  unaltered  porous  strata 
above,  where  it  may  remain  under  pressure  until  liberated  by 
artesian  wells. 

During  subsidence  resulting  from  crustal  movements,  sandstones 
and  other  porous  rocks  charged  with  meteoric  water,  or  with  the 
water  of  deposition,  may  reach  a  zone  where  the  water  is  expelled 
by  heat  and  rock-pressure.  The  tendency  of  such  expelled  water 
will  be  to  ascend  into  the  higher  unaltered  beds,  where  it  may 
accumulate,  or  it  may  rise  to  the  surface  and  escape  unobserved, 
or  as  natural  springs. 

The  main  source  of  the  artesian  water  derived  from  alluvial 
drifts  is  obviously  meteoric.  The  quantity  of  water  derived 
from  deep-seated  sources  is  in  this  case  probably  so  small  as  to 
be  negligible. 

The  artesian  water  derived  from  bore-holes  in  the  older  formations 
in  arid  regions  may  be  partly  meteoric  and  partly  uprising  deep- 
seated  water. 

Nearly  all  lavas  and  igneous  magmas  contain  a  considerable 
quantity  of  water,  but  whether  this  water  is  brought  up  from  the 
deep  interior  or  merely  derived  from  the  rocks  with  which  the 
uprising  magma  comes  in  contact,  is  unknown.  It  is  quite  certain 
that  a  cooling  magma  expels  enormous  volumes  of  steam,  much 
of  which  must  be  condensed  in  the  cooler  zones  of  sedimentary 
rocks,  where,  in  favourable  circumstances,  it  will  accumulate  and 
supplement  the  supply  derived  from  rainfall. 

Factors  in  Artesian  Flow. — The  controlling  factor  in  artesian 


534  A  TEXT-BOOK  OF  GEOLOGY. 

flow  is  hydraulic  pressure  or  head.     Among  the  many  modifying 
causes  are  : — 

Constant  Factors  :  (a)  Size  of  pores  or  openings   in  the   water- 
bearing stratum. 

(b)  Frictional  resistance,  which  is  dependent 

on  the  size   of   pores    and  distance  of 
flow  from  fountain-head. 

(c)  Rock-temperature,     which     affects     the 

viscosity  and  density  of  the  water. 

(d)  Kock-pressure. 

(e)  Conditions  of  leakage. 

Variable  Factors  :  (a)  Barometric       pressure,       diurnal       and 

seasonal. 

(b)  Surface  temperature  affecting  the  density 
of  water. 

Local  Artesian  Waters. — The  best  water-bearing  rock  in  England 
is  the  oolitic  limestone  in  Lincolnshire,  which  contains  many 
spouting  wells. 

The  Chalk  is  water-bearing  in  places,  but  irregular  and  often 
nearly  dry. 

The  Keuper  marl  yields  good  brewing  water  at  Newark,  Burton, 
and  Leicester,  but  the  amount  is  small  and  variable.  The  Keuper 
sandstone  gives  a  good  supply  of  soft  water. 

The  Permian  Sandstones  and  Magnesian  Limestone  in  some 
places  yield  a  copious  supply  of  good  water. 

The  Lower  Greensand  sometimes  furnishes  a  considerable  flow, 
and,  as  a  rule,  the  more  abundant  the  supply,  the  better  the  quality. 
With  a  small  flow  the  water  is  often  ferruginous. 

The  Lias  clays  are  liable  to  produce  saline  waters,  as  also  is  the 
London  Clay. 

The  Lower  Eocene  beds  in  the  London  Basin,  below  the  London 
Clay,  yield  a  fair  supply  which  is  often  slightly  alkaline. 

An  abundant  supply  of  artesian  water  is  obtained  in  the  United 
States  along  the  Atlantic  fringe  and  Mexican  Gulf  Plain  from  the 
Cretaceous  and  Tertiary  rocks,  at  depths  varying  from  50  feet 
along  the  inland  border  to  1000  feet  on  the  coast-line. 

In  the  Great  Plains  region  of  the  Western  Region  the  Dakota 
Sandstone  is  a  valuable  source  of  artesian  water,  which  is  extensively 
used  for  irrigation  in  the  arid  regions  of  South  Dakota,  Nebraska, 
and  Kansas. 

In  Western  Queensland  enormous  quantities  of  artesian  water 
are  obtained  from  the  Mesozoic  sandstones  underlying  the  Cretace- 
ous Rolling  Downs  Formation. 

Medicinal  Springs. — Many  mineralised  waters  possess  valuable 


ECONOMIC    GEOLOGY.  535 

therapeutic  properties.  Most  medicinal  springs  occur  in  volcanic 
regions  ;  among  the  best  known  being  those  of  the  Yellowstone 
National  Park  and  Rotorua,  New  Zealand. 

Medicinal  waters  according  to  their  composition  may  be  grouped 
as  follows  : — 

(1)  Alkaline,  containing  carbonate  of  soda  and  carbonic  acid — 

Vichy,  Saratoga  ;   and  Puriri,  Rotorua,  N.Z. 

(2)  Bitter,  containing  sulphate  of  magnesia  and  soda — Sedlitz  ; 

Rotorua. 

(3)  Muriated,  with   mainly  common   salt  —  Cheltenham,  Wies- 

baden, Hanmer,  N.Z. 

(4)  Calcareous,  with  sulphate  or  carbonate  of  lime  as  the  main 

constituent — Bath. 

(5)  Sulphurous   or   Hepatic,  with   alkaline   sulphides   and   sul- 

phuretted hydrogen,  and  frequently  free  sulphuric  acid — 
Harrogate,  Aix-la-Chapelle,  and  Rotorua. 

Many  mineral  waters  in  Europe,  America,  and  New  Zealand 
have  been  shown  to  possess  radio-active  properties. 

The  temperature  of  mineral  springs  varies  from  50°  or  60°  Fahr. 
to  212°  Fahr.  The  temperature  of  the  alkaline  waters,  as  their 
deep-seated  origin  would  suggest,  is  usually  high,  ranging  from  180° 
to  212°  Fahr.  ;  while  that  of  acid  waters,  which  usually  derive 
their  acid  constituents  from  contact  with  superficial  oxidising 
masses  of  pyrites,  is  generally  low,  as  a  rule  ranging  from  90°  to 
110°  Fahr. 

But  the  temperature  is  dependent  on  local  conditions  ;  hence 
that  of  some  alkaline  waters,  like  Puriri,  is  low,  while  that  of  some 
acid  waters  is  abnormally  high. 

Rock  Temperatures  in  Mining. 

During  the  driving  of  the  St  Gothard  railway  tunnel,  the 
temperature  of  the  rocks  was  found  to  increase  at  the  rate  of  1°  Fahr. 
for  every  60  feet  from  the  surface,  and  for  some  considerable  time 
this  rate  was  regarded  as  normal  for  all  parts  of  the  earth's  crust, 
and  for  all  depths.  These  beliefs  are  now  known  to  be  erroneous. 
Observations  taken  in  deep  mines  and  bore-holes  in  various  parts 
of  the  globe  have  shown  : — 

(a)  That  the  temperature-gradient  is  not  the  same  in  all  places. 

The  following  temperature-gradients  have  been  recorded  :— 

Comstock  lode          .  1°  Fahr.  for  every  30  feet  in  depth. 

Thames  lodes  .         .  1°     ,,  „  45   „ 

St  Gothard  tunnel  .  1°     „  „  60    „ 

Bendigo  mines         .  1°     ,,  ,,          75    ,,  ,, 


536  A  TEXT-BOOK  OF  GEOLOGY. 

Tamarack  Mine,  Lake 

Superior      .         .1°  Fahr.  for  every  100  feet  in  depth. 
Rand  Gold  mines  .     1°      „  „         200     „ 

Calumet  and  Hecla 

Mine,       Lake 

Superior  .     1°  „         223     „ 

From  the  above  it  will  be  seen  that  some  parts  of  the  crust  are 
abnormally  hot,  while  others  are  abnormally  cold. 

In  the  Wheeling  oil-well,  Western  Virginia,  4462  feet  deep,  both 
when  wet  and  dry,  the  increase  of  temperature  was  1°  Fahr.  for 
every  80  to  90  feet  in  the  upper  portion,  and  1°  Fahr.  for  every  60 
feet  in  the  lower. 

Observation  taken  in  bore-holes  near  Czuchow,  in  Rybnik, 
Silesia,  7280  feet  deep ;  near  Paruschovitz  in  the  same  coal-field, 
6510  feet  deep  ;  at  Schubin  in  Posen,  6988  feet  deep  ;  at  Schladen- 
bach,  near  Leipsic,  5630  feet  deep,  and  other  deep  bore-holes,  have 
confirmed  the  view — 

(6)  That  the  temperature-gradient  increases  with  the  depth. 


CHAPTER   XXXVI. 

ELEMENTS   OF   FIELD   GEOLOGY  AND   GEOLOGICAL 
SURVEYING. 

THE  essential  requirements  of  geological  surveying  is  the  ability  to 
run  natural  sections  accurately  and  methodically.  And  the  run- 
ning of  natural  sections  is  an  art  calling  for  the  open  mind,  the 
shrewd,  observant  eye,  sound  judgment,  a  good  knowledge  of  first 
principles,  and  a  large  measure  of  common  sense. 

The  chief  concern  of  the  field  geologist  is  to  observe  and  plot 
the  boundaries,  strikes,  and  dips  of  all  strata,  or  groups  of  strata, 
present  in  the  area  under  review  ;  to  map  the  position  of  dykes 
and  other  igneous  rocks,  of  faults,  lodes,  coal-seams,  and  mineral 
deposits.  His  report  or  thesis  deals  with  the  character,  thickness, 
arrangement,  age,  distribution,  and  relationships  of  the  stratified 
formations  ;  with  the  character,  composition,  mode  of  occurrence, 
tectonic  and  other  effects  of  intrusive  rocks  ;  and  with  the  clays, 
stones,  ores,  and  minerals  of  economic  importance.  Having 
mastered  the  geological  structure,  the  geologist  may,  with  some 
confidence,  review  the  character  and  genesis  of  the  topographical 
features. 

The  best  way  to  learn  the  methods  of  exact  observation  is  to 
attempt  the  geological  survey  of  some  area  of  simple  structure. 
But  before  making  this  attempt  it  will  be  necessary  to  acquire 
some  experience  in  field  observation,  and  a  good  way  to  gain  this 
is  to  go  over  some  area  that  has  been  already  mapped  and  de- 
scribed by  an  experienced  field  geologist.  Follow  the  clearest  lines 
of  section,  carefully  note  and  record  the  succession  and  arrange- 
ment of  the  strata,  and  verify  all  your  observations  by  comparing 
them  with  those  recorded  on  the  maps.  In  many  cases  you  will 
find  that  the  veteran  geologist,  aided  by  a  wide  experience  of  geo- 
logical structures  and  a  knowledge  of  the  succession  gained  by  his 
investigation  of  the  same  formations  elsewhere,  has  been  able  to 
read  a  meaning  into  isolated  facts  and  occurrences  that  to  you 
are  almost  meaningless.  Remember  that  all  the  facts  relating  to 
the  geological  structure  of  a  district  are  not  always  fully  disclosed 

537 


538  A  TEXT-BOOK  OF  GEOLOGY. 

in  any  one  section.  Some  important  point,  relating  to  the  geo- 
logical succession,  may  be  established  in  one  section,  and  another 
point  in  some  other  section.  Do  not  form  conclusions  based  on 
obscure  or  complicated  sections.  Sections  that  leave  room  for  two 
obvious  lines  of  interpretation  by  two  independent  observers  are 
frequently  the  cause  of  much  useless  contention,  and  ought  to  be 
avoided  when  possible.  When  all  the  sections  in  a  district  are 
obscure,  the  interpretation  will  sometimes  be  supplied  by  the 
clearer  sections  of  a  neighbouring  or  even  distant  area.  When 
the  stratigraphical  succession  is  involved,  the  problem  should  be 
assiduously  attacked  from  the  palseontological  standpoint, 
r?  Before  you  go  to  the  field  take  care  to  get  copies  of  the  best 
geological  and  topographical  maps  obtainable  of  the  area  you  have 
selected  for  your  preliminary  survey.  Bead  all  the  reports  dealing 
with  the  structure  of  the  district,  and  make  a  summary  of  the 
geology  for  your  guidance  in  the  field.  If  you  possess  a  fair  know- 
ledge of  first  principles,  a  good  eye  for  country,  and  the  persistency 
that  overcomes  all  difficulties,  you  will  soon  be  able  to  carry  out 
useful,  trustworthy  work.  Do  not  expect  to  unravel  all  the  in- 
tricacies of  the  geology  in  a  day  or  a  week.  You  will  usually  find 
that  as  the  mapping  progresses,  the  geological  structure  will  gradu- 
ally unfold  itself. 

Field  Equipment. — The  equipment  for  field  work  should  include 
a  3-inch  prismatic  compass  with  metallic  card  for  observing  strikes 
and  dips,  a  5-inch  Abney  level  for  measuring  angles  of  dip,  a  3-inch 
pocket  spirit-level,  a  field-book  with  a  stout  cover,  a  short  scale, 
a  4-inch  brass  protractor,  a  3-inch  aneroid  barometer,  a  66-feet 
tape,  a  geological  hammer,  geological  pick,  a  set  of  light  steel 
chisels  for  collecting  fossils,  and  a  stout  leather  bag. 

An  indispensable  part  of  the  equipment  is  a  large  scale  topo- 
graphical map  on  which  the  field  observations  are  plotted  as  the 
work  proceeds.  A  scale  of  twenty  chains  to  the  inch  will  be  found 
suitable  for  ordinary  surveys,  and  a  scale  of  ten  chains  to  the  inch 
for  more  detailed  work. 

Make  a  tracing  on  paper  of  the  portion  of  ground  to  be  examined 
during  one  or  two  days,  and  fix  it  with  paste  round  the  edges  in  a 
stiff  board  portfolio.  The  observations  are  marked  on  the  tracing 
as  they  are  made  in  the  field  and  afterwards  transferred  to  the 
topographical  map.  Each  tracing  should  show  the  cardinal  points 
of  the  compass,  to  enable  the  strikes  and  dips  to  be  plotted  with 
the  protractor,  either  in  the  field  or  on  your  return  to  your  head- 
quarters. 

The  collecting  of  fossils  may  be  carried  on  at  the  same  time  as 
the  field  survey,  but,  as  a  rule,  it  is  best  to  complete  the  field 
traverses  and  thereafter  devote  your  undivided  attention  to  the 


FIELD    GEOLOGY    AND    GEOLOGICAL    SURVEYING         539 

collecting  of  fossils.  When  the  mapping  and  collecting  are  carried 
on  at  the  same  time  there  is  always  a  danger  that  one  or  both 
may  suffer.  Besides,  after  the  district  is  examined  and  mapped, 
you  will  possess  a  better  knowledge  of  the  fossiliferous  beds  and 
of  the  places  where  they  are  likely  to  prove  the  most  productive. 
If  the  examination  were  of  the  nature  of  a  rapid  reconnaissance, 
it  is  the  duty  of  the  geologist,  while  running  the  traverses,  to 
supplement  his  field  observation  with  as  ample  collections  of 
fossils,  rocks,  and  mineral  specimens  as  the  time  and  circumstances 
will  permit. 

Eock  and  mineral  specimens  are  usually  collected  during  the 
progress  of  the  field  traverses,  marked  with  small  gummed  labels, 
and  then  wrapped  separately  in  pieces  of  paper  on  which  the  label 
numbed  is  also  marked.  The  number  and  locality  of  the  specimen 
are  carefully  recorded  in  the  field-book. 

A  day  or  even  a  few  days  spent  in  a  rapid  reconnaissance  of  the 
district  is  usually  time  well  spent.  By  this  procedure  you  will 
obtain  a  broad  view  of  the  topographical  features  and  general 
geological  structure,  which  will  enable  you  to  arrange  your  campaign 
and  mode  of  attack  on  a  systematic  basis.  Moreover,  before  you 
begin  the  detailed  survey  you  ought  to  have  the  lay  of  the  country 
clearly  impressed  on  your  mind. 

The  field  traverses  follow  all  the  main  streams  and  their  tribu- 
taries ;  also  all  the  salient  spurs,  ridges,  and  prominent  escarp- 
ments. 

General  Field  Procedure. — The  general  field  procedure  com- 
prises an  examination  of  all  cliffs,  rock-outcrops,  and  escarpments, 
the  position  of  which  should  be  carefully  marked  on  the  field-map. 

The  points  that  should  be  specially  recorded  in  the  field-book 
are  a  description  of  the  form  and  extent  of  the  outcrop  ;  char- 
acter, thickness,  strike,  and  dip  of  the  different  strata  ;  height  above 
sea-level  or  some  other  known  datum  ;  and  the  topographical 
features  formed  by  the  various  rocks. 

Make  profile  and  longitudinal  diagrams  in  your  field-book  of  all 
prominent  cliffs,  outcrops,  and  escarpments.  The  profile  is  neces- 
sary in  order  to  show  the  relationship  and  arrangement  of  the 
strata.  These  sketches  need  not  be  drawn  to  scale,  but  they  should 
show  the  direction,  height,  and  length  of  the  portion  of  the  section 
represented,  together  with  references  to  the  different  beds,  etc. 

The  position  and  extent  of  the  fossiliferous  beds  should  be  noted, 
and  a  provisional  list  made  of  the  more  abundant  fossils. 

Take  care  to  record  the  presence  of  all  igneous  masses,  dykes, 
rills,  or  lava-flows  ;  and  indicate  their  position  and  boundaries  on 
the  map.  Search  for  contacts  between  the  igneous  rock  and  the 
associated  sedimentary  rocks,  and  make  a  note  of  the  effects  due 


540  A  TEXT-BOOK  OF  GEOLOGY. 

to  thermal  metamorphism,  at  the  same  time  collecting  rock  speci- 
mens at  short  intervals  to  illustrate  the  progressive  alteration  of 
the  clastic  rocks.  It  is  also  important  to  select  a  series  of  speci- 
mens of  the  igneous  rock  from  the  selvedge  inwards,  in  order  to  be 
able  to  ascertain  by  analysis  and  laboratory  examination  what 
effect,  if  any,  the  clastic  rock  has  had  on  the  intruding  molten 
magma. 

Faults  are  features  of  special  interest  frequently  seen  in  the  face 
of  steep  sea-cliffs  or  walls  of  deep  ravines.  Their  vertical  and 
horizontal  displacement,  strike,  and  dip  should  be  recorded  in  the 
field-book,  and  their  course  marked  on  the  map.  Faults  of  large 
displacement  show  their  existence  by  repetitions  of  the  strata,  or 
by  bringing  one  rock-formation  up  against  another.  Such  faults 
are  disclosed  by  the  mapping. 

The  thickness  and  character  of  the  surface  soils  may  be  noted 
and  recorded,  but  no  attempt  should  be  made  to  show  the  soils  on 
the  map,  as  this  would  obscure  the  distribution  of  the  rock-forma- 
tions. Special  soil- maps  are  prepared  for  agricultural  purposes. 

A  careful  examination  should  be  made  of  all  accessible  mine 
workings  and  mine  plans,  from  which  much  valuable  informa- 
tion relating  to  the  geological  arrangement  of  the  strata  may  be 
frequently  gleaned. 

The  outcrop  of  coal-seams,  mineral  deposits,  and  lodes  should 
be  indicated  on  the  map,  and  a  full  description  of  the  strike,  dip, 
extent,  and  general  character  of  the  deposit  recorded  in  the  field- 
book.  Eepresentative  samples  of  the  coal  or  mineral  should  be 
collected  for  future  examination. 

Do  not  allow  yourself  to  be  hurried  in  making  your  observations. 
Undue  haste  may  lead  to  errors  in  observation  and  the  drawing  of 
crude,  ill-considered  conclusions.  Your  interpretation  of  the  geo- 
logical structure  may  be  altogether  wrong  and  little  harm  come  of 
it.  The  all-important  point  is  to  be  sure  of  your  facts.  Always 
remember  that  every  fact  correctly  recorded  advances  the  geology 
of  your  district  one  step  forward. 

Learn  to  rely  on  the  judgment  of  older  and  more  experienced 
observers  than  yourself,  and  in  your  writings  do  not  forget  to 
acknowledge  your  indebtedness  to  the  work  of  previous  workers 
in  the  same  field.  To  utilise  the  work  of  others  without  frank 
acknowledgment,  or  to  recognise  the  conclusions  of  others  only 
when  you  differ  from  them,  tends  to  lower  the  value  of  your  own 
work. 

Do  not  be  too  ready  to  challenge  the  views  of  the  veteran  geolo- 
gists who  have  preceded  you,  and  do  not  try  to  exalt  yourself  by 
holding  up  their  differences.  As  you  gain  more  experience  the 
more  will  you  respect  the  opinion  of  the  older  geologists. 


FIELD    GEOLOGY    AND    GEOLOGICAL   SURVEYING.        541 

When  you  come  to  construct  opinions  and  draw  conclusions, 
bear  in  mind  that  the  obvious  is  not  always  true.  Early  writers 
in  New  Zealand  found  broken  bones  of  the  gigantic  Dinornis  at 
some  old  native  camping-places  in  Otago,  and  hastily  concluded 
that  the  early  Maori  was  a  moa-hunter.  The  association  of  Maori 
and  moa  bones  led  at  once  to  the  conclusion  that  the  two  were 
contemporary.  But  closer  inquiry  failed  to  confirm  this  view. 
Moa  bones  were  scattered  plentifully  over  many  parts  of  Otago 
even  at  the  advent  of  the  first  white  settlers  fifty  years  ago,  and 
are  still  not  uncommon  in  places.  At  the  advent  of  the  Maori, 
moa  bones  must  have  been  even  more  abundant,  and  who  can 
doubt  that  a  native  so  highly  intelligent  and  so  observant  of  all 
natural  phenomena  would  fail  to  see  and  collect  them.  It  is 
significant  that  the  prolific  tradition  and  song  of  the  Maori,  rich 
enough  in  elaborate  detail  of  the  hunting  and  snaring  of  the  wood- 
pigeon,  kaka,  huia,  weka,  kiwi,  tui,  and  other  small  birds,  should 
be  silent  as  to  the  gigantic  moa.  If  the  Maori  had  ever  hunted 
and  killed  this  stately  bird,  it  is  certain  that  his  descendants  would 
have  preserved  the  fact  in  many  picturesque  traditions. 

Accidents  may  lead  to  curious  associations.  The  Yakubs  of  the 
frozen  taiga  of  Northern  Siberia  trade  in  mammoth  ivory.  They 
have  even  dined  off  the  frozen  flesh  of  this  extinct  elephant.  Per- 
haps early  man  did  the  same  in  Europe  long  after  the  retreat  of 
the  Pleistocene  ice-sheet.  The  intimate  association  of  the  North 
Siberian  and  the  mammoth  does  not  prove  that  they  are  now  or 
ever  were  coeval. 

The  Observation  of  Strike  and  Dip. — This  is  relatively  simple 
where  good  rock  outcrops  are  exposed  at  the  surface,  but  certain 
precautions  must  be  observed  to  ensure  accuracy.  The  strike  is 
the  horizontal  line  along  the  bedding-plane  of  the  rock  ;  and  the 
first  precaution  is  to  satisfy  yourself  that  the  plane  before  you  is 
a  true  bedding-plane  and  not  a  joint-plane.  In  most  cases  the 
bedding-plane  can  be  distinguished  by  some  difference  In  colour, 
texture,  or  composition  of  the  material. 

The  bedding-planes  of  shales,  flaggy  sandstones,  flaggy  lime- 
stones, and  of  all  thin-bedded  alternating  argillites,  sandstones,  and 
limestones  are  easily  distinguished.  In  most  cases  a  clastic  rock 
splits  more  or  less  readily  in  the  direction  parallel  with  the  original 
plane  of  deposition. 

The  bedding-planes  of  massive  beds  of  conglomerate  are  fre- 
quently indicated  by  intercalated  layers  of  sand  or  clay  ;  of  chalk, 
by  lines  of  flints  or  fossils ;  of  marine  clays,  by  lines  of  shells,  by 
layers  of  harder  material,  or  by  lines  of  hard  nodules ;  of  sand- 
stones, by  lines  of  material  of  different  texture  or  colour,  or  by 
layers  of  fossils. 


542 


A    TEXT-BOOK    OF    GEOLOGY. 


Many  massive  deposits  of  limestone,  claystone,  sandstone,  and 
conglomerate  exhibit  no  recognisable  bedding-planes.  When  such 
a  deposit  lies  between  two  stratified  beds  that  are  parallel  to  one 
another,  its  bedding-plane  is  usually  conformable  to  that  of  the 
enclosing  beds. 

But  it  is  not  safe  to  assume  on  the  mere  evidence  of  apparent 
parallelism  of  the  associated  strata  that  the  unstratified  rock  is 
invariablv  conformable  to  the  one  on  which  it  rests.  The  two 


FIG.  253. — Unstratified  rock  lying  between  two  stratified  beds. 
1  and  3,  Stratified  rock.  2,  Unstratified  rock. 

rocks  may,  after  all,  belong  to  different  formations,  separated  by 
a  wide  hiatus  notwithstanding  the  apparent  physical  conformity 
at  the  outcrop. 

Observing  the  Strike. — Expose  as  long  a  surface  of  the  bedding- 
plane  as  possible,  and  on  it,  with  the  aid  of  the  pocket  spirit-level, 
draw  a  horizontal  line  with  a  sharp  fragment  of  stone  ;  or  if  there 
is  a  long  exposure  of  rock,  mark  the  horizontal  line  along  the  out- 


FIG.  254" — Showing  unstratified  rock  unconformable  to  underlying  rock. 
1  and  3,  Stratified  rock.  2,  Unstratified  rock. 

crop  with  small  stones  or  stakes  set  at  intervals.  Observe  the 
bearing  or  course  of  this  line  with  a  pocket-compass,  or,  better 
still,  and  more  accurately,  with  the  prismatic  compass.  Record 
the  bearing,  which  is  the  strike  required. 

Highly  inclined  beds  frequently  follow  a  sinuous  course  along 
the  strike,  and  care  must  be  taken  to  obtain  the  general  strike  by 
taking  the  mean  of  a  number  of  observations,  or  by  setting  out  a 
long  line  along  the  outcrop. 

The  strike  may  be  recorded  as,  say,  N.E.-S.W.,  or  as  45°-225°, 
which  simply  means  that  when  you  are  looking  northward  along 


MELD    GEOLOGY   AKD    GEOLOGICAL   SURVEYING.       543 

the  outcrop  the  reading  is  45°,  and  when  looking  southward  225°. 
All  bearings  originate  from  the  north  point  as  zero,  and,  since  the 
two  ends  of  the  magnetic  needle  are  always  separated  by  180°,  it 
is  easy  to  supply  the  reverse  bearing  when  the  reading  has  been 
made  in  one  direction  only,  which  is  usually  the  case  when  using 
the  prismatic  compass.  For  example,  if  the  compass  reading  be 
30°,  the  reverse  reading  will  be  210°,  and  the  strike  may  be  recorded 
as  30°-210°  ;  if  the  bearing  be  165°,  the  reverse  bearing  will  be 
3450,  the  strike  being  165°-345° ;  or  if  the  reading  be  275°,  the 
reverse  reading  will  be  95°,  hence  the  strike  will  be  95°-275°. 

The  rule  to  find  the  reverse  bearing  is  as  follows  : — When  the 
observed  bearing  is  less  than  180°,  add  180°  to  obtain  the  reverse 
bearing,  and  when  more  than  180°,  subtract  180°. 

Instead  of  recording  the  strike  (i.e.  bearing)  as  45°-225°,  it 
may  be  recorded  as  45°,  or  as  225°,  following  the  practice  of  pro- 
fessional surveyors  and  engineers.  To  record  the  strike  as  N.E.— 
S.W.,  or  45°-225°,  is  a  redundancy ;  for,  obviously,  if  the  strike 
or  course  runs  N.E.,  it  must  also  run  S.W.,  and  if  45°,  also  225°. 

Moreover,  when  the  strike  is  plotted  on  the  map  with  the  pro- 
tractor only  one  direction  is  used  to  obtain  the  course,  that  is, 
either  45°  or  225°,  previously  corrected  for  the  magnetic  variation. 

It  will  therefore  fulfil  all  requirements  and  avoid  confusion  if 
you  simply  record  the  strike  as  45°,  62°,  186°,  or  347°,  as  the  case 
may  be. 

In  your  amateur  field  excursions  you  may  use  a  pocket-compass 
for  observing  the  strike  and  dip,  but  in  your  more  serious  work  it 
will  be  necessary  to  adopt  the  field  procedure  of  the  experienced 
geological  surveyor. 

Be  careful  to  check  all  your  observations  and  records  by  repeti- 
tion. It  is  never  safe  to  depend  on  a  single  observation.  Observe 
the  strike  and  record  the  reading  in  your  field-book.  Again,  observe 
the  bearing,  note  the  reading,  and  compare  it  with  the  recorded 
bearing.  By  this  procedure  both  the  observation  and  the  record 
are  checked. 

Take  special  care  to  satisfy  yourself  that  the  ledge  of  rock  or 
outcrop  where  you  have  made  your  observation  is  in  situ  and  not 
a  fallen  block.  In  deep  gorges  and  steep  sea-cliffs  weak  rocks,  such 
as  shales,  thin  bedded  clays,  and  soft  sandstones,  fissile  slates, 
mica-schists,  and  phyllite,  are  frequently  distorted  where  the  walls 
run  parallel  to  or  run  obliquely  across  the  strike.  In  such  cases 
the  most  trustworthy  observations  for  strike  and  dip  are  obtained 
from  the  water- worn  ledges  exposed  in  the  bed  of  the  streams,  or 
on  the  rocky  marine  platforms  at  the  foot  of  the  sea-cliffs. 

Observing  the  Dip. — The  direction  of  the  dip  is  always  at  right 
angles  to  the  strike,  and  may  incline  to  the  right  or  left  of  the 


544  A  TEXT-BOOK  OF  GEOLOGY. 

strike ;  that  is,  if  the  strike  were  N.-S.  the  dip  might  be  towards 
the  east  or  the  west. 

The  angle  of  dip  is  measured  with  the  swinging  pointer  or  bob 
in  the  compass-box,  or  more  accurately  with  the  Abney  level. 

Make  your  observations  for  dip  and  strike  at  points  where  the 
rocks  are  clearly  in  situ.  Avoid  large  tabular  masses  detached 
from  the  main  outcrop.  These  may  have  become  canted  by  the 
partial  undermining  of  an  underlying  softer  stratum  by  weathering 
or  underground  chemical  corrosion. 

False-bedding  will  seldom  be  misleading,  except  on  small  ex- 
posures. 

Be  specially  careful  concerning  the  direction  and  amount  of  dip 
in  the  walls  of  deep  gorges  and  steep  cliffs.  In  such  situations  the 
outcrops  of  the  strata  are  frequently  bent  and  warped  by  the  weight 


FIG.  255. — Showing  effects  of  outcrop  curvature. 
A,  Before  curvature.       B,  After  curvature. 

(a)  Beds  not  sagging  because  supported. 
(6)  Beds  sagging  on  steep  slope. 

of  the  superincumbent  rocks  ;  and  by  their  own  weight  where  they 
are  unsupported.  Outcrop  curvature  is  common  in  all  mountain 
regions  where  the  rocks  are  weak.  At  places  where  the  outcrop 
sag  is  considerable  the  direction  of  the  dip  may  be  reversed.  It  is 
always  difficult  to  obtain  trustworthy  observations  of  strike  and 
dip  in  gorges,  ravines,  and  steep  mountain  slopes  occupied  by  such 
weak  rocks  as  phyllite,  fissile  slates,  and  shales,  especially  in  recently 
glaciated  regions  where  the  weight  of  the  ice  has  shattered,  bent, 
and  distorted  the  strata.  Failure  to  recognise  the  difference  be- 
tween the  true  dip  and  the  distortion  caused  by  outcrop  sag  has 
led  to  the  construction  of  some  wonderful  examples  of  hypothetical 
folding. 

A  safe  rule  is  to  reject  all  doubtful  observations.  Or  if  recorded 
in  the  field-book  for  future  reference,  they  ought  to  be  marked  with 
a  note  of  interrogation.  On  no  account  should  they  be  used  as  a 
basis  for  the  interpretation  of  tectonic  structures. 

A  useful  point  to  remember  is  that  when  beds  have  been  tilted 


FIELD    GEOLOGY    AND    GEOLOGICAL    SURVEYING.        545 

at  high  angles  that  approach  the  vertical,  a  small  amount  of  push 
in  one  direction  or  the  other,  or  an  extra  amount  of  pressure,  will 
have  caused  them  to  incline  to  one  side  or  the  other.  Observe  the 
behaviour  of  highly  inclined  strata  in  the  core  of  a  steep  anticline. 
Although  you  are  dealing  with  a  simple  anticline,  the  strata  ex- 
posed in  the  core  as  exposed  by  denudation  along  a  river  course 
or  sea-cliff  may  be  seen  to  vary  from  75°  to  vertical,  then  incline 
in  the  opposite  direction  for  a  short  distance,  once  more  become 
vertical,  and  again  incline  a  little  in  one  direction  or  the  other. 
Such  rapid  variations  of  inclination  are  not  the  result  of  sharp 
anticlinal  folding,  but  merely  an  evidence  of  unequal  pressure  and 
packing  of  the  strata  in  the  zone  of  greatest  stress.  A  series  of 
true  anticlinal  folds  in  which  the  limbs  approach  the  vertical  posi- 
tion is  easily  distinguished  by  the  tracing  of  the  repetition  of  some 
distinctive  stratum. 

Measuring  the  Angle  of  Dip. — The  angle  of  dip  is  most  accurately 
measured  with  the  Abney  level.     The  longer  the  exposed  bedding- 


Fia.  256. — Showing  variations  of  dip  of  highly -inclined  strata  in 
the  centre  of  a  steep  anticline. 

plane  the  better.  Where  possible  it  is  advisable  to  place  a  light 
pine  lath  3  feet  long  along  the  direction  of  dip.  When  the  lath 
is  in  its  proper  place  the  Abney  level  is  laid  on  it,  the  arc  moved 
by  hand  until  the  bubble  is  central,  and  the  angle  of  inclination 
then  read  off  the  scale.  By  using  the  lath  the  minor  inequalities 
of  the  bedding-plane  are  avoided. 

Observations  made  in  deep  mines  and  in  profound  mountain 
gorges,  where  distinctive  beds  can  be  frequently  traced  by  the  eye 
through  a  vertical  height  of  many  thousand  feet,  have  shown  that 
the  strata  are  frequently  subject  to  great  variations  in  the  angle 
and  direction  of  the  dip  from  the  surface  downwards.  In  many 
cases  the  dip  will  repeatedly  change  from  one  direction  to  another 
in  a  depth  of  a  few  thousand  feet. 

As  a  rule,  strata  that  are  inclined  at  high  angles  at  the  surface 
flatten  with  increasing  depth. 

Measuring  Thickness  of  Strata. — This  is  a  simple  operation,  the 
computation  in  the  case  of  tilted  strata  being  based  on  the  angle 
of  dip,  the  angle  of  the  slope  of  the  ground,  and  the  measured 
distance  on  the  slope. 

35 


546 


A    TEXT-BOOK    OF    GEOLOGY. 


The  different  cases  that  may  arise,  together  with  worked-out 
examples  and  diagrams,  will  be  found  in  another  work  by  the 
author,1  and  need  not  be  repeated  here. 

When  measuring  the  thickness  of  strata  take  care  of  repetitions 
arising  from  faulting  or  isoclinal  folding.  In  the  case  of  lacustrine, 
fluviatile,  and  estuarine  beds,  beware  of  estimating  the  thickness 
across  the  tipping-plane,  which  is  a  pseudo  bedding-plane.  This 
precaution  also  applies  to  all  flysch  and  desert  sandstones. 


FIG.  257. — Showing  changing  dip  in  vertical  height. 

Locating  Positions  on  the  Map. — The  points  at  which  observa- 
tions are  made  in  the  field  must  be  fixed  on  the  map.  If  you  are 
provided  with  a  good  topographical  map  there  will  usually  be  little 
difficulty  in  doing  this.  As  a  rule  the  point  is  fixed  by  noting  its 
position  in  relation  to  some  known  point.  A  known  point  is  some 
spot  which  you  can  with  certainty  locate  on  the  map.  It  may  be 


FIG.  258. — Showing  pseudo  bedding-plane, 
(a)  Bed-rock.  (6)  Deltaic  sediments. 

a  stream,  junction,  house,  corner  of  some  field,  fence,  or  stone  wall, 
angle  or  bend  in  the  road,  quarry  reserve,  prominent  hill  or  peak, 
escarpment,  trigonometrical  station,  bay,  or  headland,  etc. 

If  you  take  care  to  start  your  traverse  at  some  known  point,  the 
points  of  observation  will  be  easily  fixed  on  the  map  in  orderly 
succession.  If  your  map  is  deficient  in  details,  it  may  be  necessary 
for  you  to  measure  the  distance  from  point  to  point  with  the 
measuring  tape.  Prominent  outcrops  on  a  distant  range  may  be 

1  James  Park,  Text- Book  of  Mining  Geology,  3rd  edit.,  chap.  iv.  p.  153. 
Charles  Griffin  &  Co.,  Limited,  London,  1911.  6s. 


FIELD    GEOLOGY    AND    GEOLOGICAL    SURVEYING.       547 

easily  and  accurately  fixed  by  what  is  called  intersections.  The 
procedure  is  as  follows : — Select  some  prominent  outcrop  that  you 
can  readily  distinguish  from  different  points  of  view.  If  there  is 
no  prominent  object,  erect  a  stake  with  a  piece  of  white  or  red 
cotton  material  tacked  to  it.  Observe  the  bearing  of  the  object 
or  flag  from  at  least  two  points  which  you  can  with  certainty  fix 
on  the  map.  Correct  these  bearings  for  magnetic  variation  so  as 
to  reduce  them  to  the  true  meridian,  and  carefully  plot  them  with 
the  protractor  on  your  map,  using  a  hard  pencil  drawn  to  a  fine 
point.  The  point  of  intersection  of  the  two  bearings  gives  the 
position  of  the  mark  or  object. 

With  increasing  experience  you  will  acquire  considerable  skill 
in  locating  your  field-points  on  the  map. 

The  strike  and  dip  are  shown  as  in  A  of  the  next  figure,  the  axis 
of  anticlines  as  in  B,  and  the  centre  of  synclines  as  in  C. 


A  /    B  /    c 

FIG.  259. — Conventional  marks  for  strike  and  dip. 

The  bearing  of  the  strike  is  marked  on  the  line  parallel  with  the 
strike,  and  the  angle  of  dip  on  the  line  indicating  the  direction  of 
the  dip,  as  shown  in  fig.  259. 

The  Geological  Map  and  Sections. — Your  first  business  is  to  draw 
up  a  table  of  the  geological  formations  present  in  the  area  you  have 
examined.  Each  formation  is  usually  distinguished  on  the  map 
by  a  distinctive  colour  or  conventional  sign.  But  a  formation  may 
comprise  two  or  more  distinct  beds  or  horizons  of  an  outstanding 
character,  each  covering  a  considerable  surface  area.  In  such 
cases  it  may  be  expedient  to  show  the  subdivisions  of  one  or  more 
of  the  formations  in  different  colours  ;  or  one  colour  may  be  used 
to  distinguish  the  formation,  its  subdivisions  being  shown  by 
various  hatching  or  other  conventional  signs.  The  point  to  aim 
at  is  clearness.  The  attempt  to  show  too  much  frequently  leads 
to  confusion. 

The  usual  practice  is  first  to  plot  the  stream  and  ridge  traverses, 
then  the  formation  boundaries,  and  afterwards  the  subdivisions  of 
the  formations. 


548  A  TEXT-BOOK  OF  GEOLOGY. 

When  the  map  is  finished  there  only  remain  the  sections  to  be 
plotted.  Select  the  lines  of  section  with  the  view  of  showing 
the  geological  structure,  and  the  relationship  of  the  different 
formations  to  one  another.  The  sections  are  simply  profiles  of  the 
upper  crust,  and  they  ought  to  be  drawn  as  if  you  were  looking 
northward. 

In  systematic  surveys  the  sections  are  always  drawn  to  natural 
scale  ;  that  is,  the  horizontal  and  vertical  scales  are  the  same. 
When  the  vertical  scale  is  a  half,  a  third,  or  a  fourth  of  the  hori- 
zontal, the  inclination  of  the  beds  is  exaggerated  and  the  folds 
are  distorted.  Sections  drawn  on  any  other  than  natural  scale 
cannot  claim  to  be  much  more  than  diagrammatic. 

Use  the  sea-level  datum  whenever  possible,  and  let  the  vertical 
scale  equal  the  horizontal  in  all  cases  except  where  the  surface 
features  are  very  low  and  flat. 

Select  the  section-lines,  and  mark  them  on  the  map  with  a  clear 
pencil  line.  Mark  the  ends  of  the  first  section  A-A,  of  the  second 
B-B,  and  so  on. 

Draw  the  datum  line  of  the  first  section,  scale  off  the  distance 
A-A,  and  at  the  ends  erect  perpendiculars.  Note  that  all  the  work 
is  plotted  in  pencil  before  it  is  coloured  and  inked  in. 

Next  draw  the  surface  lines,  the  heights  of  the  various  points 
being  obtained  from  the  contour  lines  on  the  map  or  from  aneroid 
or  other  data.  Mark  off  the  boundaries  of  the  formations,  as  shown 
along  the  section  line,  on  the  edge  of  a  strip  of  paper,  and  transfer 
the  marks  to  the  section.  Draw  lines  on  the  section  to  indicate 
the  boundaries  and  dips  of  the  formations  ;  apply  selected  colours 
for  the  different  formations,  ink  in  the  boundaries ;  and,  finally, 
put  on  the  conventional  marks  if  any  are  to  be  used.  The  fine 
maps  and  sections  published  by  Geological  Surveys  of  Great  Britain 
and  the  United  States  will  be  a  good  guide  as  to  what  your  map 
and  sections  ought  to  be  like. 

Preparation  of  Topographical  Maps. — No  geological  work  of  any 
moment,  either  stratigraphical  or  petrographical,  can  be  carried 
out  without  good  topographical  maps.  Of  some  regions  there  are 
no  maps,  and  sooner  or  later  you  will  be  called  on  to  make  your 
own  topographical  surveys. 

A  very  useful  and  fairly  accurate  topographical  survey  may  be 
made  with  the  prismatic  compass  and  a  5-chain  steel  tape,  J-inch 
wide.  A  compass  traverse  is  also  made  of  all  main  and  subsidiary 
streams  and  roads.  The  position  of  houses,  fences,  and  all  im- 
portant natural  features  are  fixed  by  offsets  from  the  traverse 
lines  when  within  a  distance  of  two  chains,  and  by  intersection 
bearings  when  further  off. 

The  stations  are  marked  by  stones  or  small  stakes,  and  numbered 


FIELD    GEOLOGY    AND    GEOLOGICAL    SURVEYING.       549 

in  consecutive  order  ;  and  the  usual  practice  is  to  post  up  the  day's 
work  at  night  so  as  to  note  the  gradual  development  of  the  survey 
and  prevent  the  undue  accumulation  of  field  notes. 

The  angles  of  elevation,  or  depression,  between  the  stations  are 
measured  with  the  Abney  level ;  and  since  all  maps  are  drawn  on 
the  horizontal  projection,  all  slope  measurements  must  be  reduced 
to  the  equivalent  horizontal  distance  for  .purpose  of  plotting. 

Rule. — -The  cosine  of  the  angle  of  elevation  (or  depression) 
multiplied  by  the  slope  measurement  equals  the  horizontal 
distance. 

The  natural  or  logarithmic  cosine  may  be  used  in  the  computa- 
tion. 

The  bearings  are  plotted  with  a  large  brass  protractor,  not  less 
than  6  inches  in  diameter,  to  a  scale  of  10  or  20  chains  to  the  inch, 
according  to  the  size  of  the  district  and  the  amount  of  geological 
detail  to  be  put  on  it.  Whenever  it  is  possible  contour  lines  should 
be  run  with  the  Abney  level.  The  contour  intervals  will  vary  with 
the  surface  relief  from  20  to  200  feet.  In  low  undulating  ground 
the  interval  may  be  20,  30,  or  more  feet,  and  in  mountain  regions 
100  or  200  feet.  The  point  to  be  observed  in  selecting  the  contour 
interval  is  to  see  that  it  is  not  so  great  as  to  miss  prominent  features. 
If  wide  intervals  were  used  in  low  undulating  ground  many  im- 
portant spurs  and  hills  might  be  passed  over.  Conversely,  the 
selection  of  too  close  intervals  in  a  mountainous  region  might 
involve  the  running  of  an  unnecessary  amount  of  lines. 

If  the  geological  work  you  are  called  upon  to  undertake  is  im- 
portant, your  topographical  map  should  be  made  by  theodolite 
survey  with  all  the  traverses  oriented  on  the  true  meridian.  You 
will  find  it  easier  to  use  a  theodolite  than  a  petrographical  micro- 
scope, and  after  a  little  practice  you  will  be  able  to  carry  on  the 
work  with  ease  and  precision,  while  the  greater  accuracy  of  your 
work  will  be  a  perpetual  source  of  pleasure. 

The  traverses  follow  the  main  streams  and  ridges,  the  prismatic 
compass  being  used  for  filling  in  minor  details.  The  compass 
bearings  are  reduced  to  true  bearings  by  applying  the  magnetic 
variation  in  the  manner  described  in  a  preceding  chapter. 

On  the  excellent  topographical  maps  provided  in  Europe  and 
many  States  in  America,  the  magnetic  variation  is  only  given  at 
the  major  trigonometrical  stations.  As  a  matter  of  fact  the  varia- 
tion is  liable  to  differ  widely  in  different  parts  of  the  same  district 
owing  to  the  proximity  of  igneous  dykes  and  bosses,  some  of  which 
may  not  be  exposed  at  the  surface.  A  serious  local  deflection  of 
the  needle  may  be  also  caused  by  iron  bridges,  tram  and  railway 
lines,  iron  houses,  iron  fences,  and  other  artificial  structures  in 
which  iron  is  present  in  considerable  quantity.  Hence,  you  will 


550  A  TEXT-BOOK  OF  GEOLOGY. 

find  it  advantageous  to  determine  the  variation  at  many  different 
points  during  the  progress  of  your  theodolite  survey.  This  is  quite 
a  simple  operation,  and  may  be  carried  out  as  follows  : — 

When  the  theodolite  is  set  over  a  station  observe  the  true  bear- 
ing of  some  prominent  distant  object,  such  as  a  tree  top,  church 
spire,  or  sharp  peak.  Record  the  bearing  and  the  number  of  the 
station. 

Now  unclamp  the  vernier  plate  and  set  it  at  zero.  Then  loosen 
the  long  box-needle,  and  swing  the  instrument  round  until  the  needle 
settles  in  the  N.-S.  line.  Clamp  the  bottom  plate,  and  with  the 
bottom  tangent-screw  orient  the  instrument  exactly  in  the  mag- 
netic meridian.  This  is  effected  by  bringing  the  engraved  line  at 
the  end  of  the  compass-box  exactly  opposite  the  north  end  of  the 
needle. 

Now  unclamp  the  vernier  plate,  direct  the  telescope  to  the  object 
previously  viewed,  and  read  the  bearing.  Repeat  the  operation, 
and  take  the  mean  of  the  two  readings.  The  difference  between 
this  mean  magnitude  bearing  and  the  true  bearing  is  the  magnetic 
variation  at  the  station  of  observation,  disregarding  the  small  cor- 
rection for  convergence  of  meridian. 

The  true  meridian  is  determined  at  the  initial  station  of  the 
theodolite  survey,  in  the  Northern  Hemisphere  by  observations  to 
Polaris,  and  in  the  Southern  Hemisphere  to  a  Crucis,  a  Centauri, 
or  other  conspicuous  circumpolar  star.  You  will  have  no  difficulty 
in  determining  the  meridian  within  half  a  minute  of  arc.  Detailed 
instructions  as  to  the  methods  and  procedure  to  be  pursued,  to- 
gether with  worked-out  examples  and  diagrams,  will  be  found  in  a 
little  work  by  the  author,1  in  which  also  the  methods  of  contour- 
ing with  the  Abney  level  are  fully  described. 

1  James  Park,  Text-BooTc\pf  Theodolite  Surveying,  2nd  edit.,  Charles  Griffin 
&  Co.,  Limited,  London,  1911. 


APPENDIX  A. 

To  Convert  Magnetic  Bearings  to  True  Bearings. 

ALL  geological  and  topographical  maps  are  projected  on  the  so- 
called  true  meridian  ;  hence,  when  the  strike  of  strata  is  determined 
with  a  magnetic  compass,  it  becomes  necessary  to  convert  the 
observed  magnetic  bearing  into  a  true  bearing  before  it  can  be 
plotted  on  the  map. 

Conversely,  if  the  strike  of  a  seam  or  stratum  is  taken  off  the  map 


FIG.  260. — Conversion  of  magnetic  bearings  to  true. 

with  the  view  of  setting  off  the  course  on  the  ground  with  a  compass, 
the  true  bearing  must  first  be  converted  into  a  magnetic  bearing. 

Only  in  a  few  places  does  the  true  meridian  coincide  with  the 
magnetic  meridian.  In  most  regions  the  magnetic  meridian  lies 
to  the  east  or  west  of  the  true  meridian.  The  difference  between 
the  two  meridians  is  called  the  magnetic  variation,  and  its  amount 
is  usually  marked  on  all  topographical  and  trigonometrical  district 
maps. 

In  Britain  the  magnetic  meridian  is  west  of  the  true  meridian, 
and  in  New  Zealand  east. 

551 


552 


A   TEXT-BOOK    OF    GEOLOGY. 


The  strike  is  best  determined  with  a  pocket  or  prismatic  compass 
graduated  into.  360°  where  360°  is  north  or  zero.  Obviously, 
east  will  be  90°,  south  =180°,  and  west  =270°. 

The  advantage  of  a  compass  graduated  in  this  way  is  that  all  the 
bearings  (i.e.  courses  or  strikes)  are  measured  from  the  north  point. 

To  Convert  a  Magnetic  Bearing  to  a  True  Bearing.— Only  two 
cases  are  likely  to  occur — namely,  the  variation  will  be  east  or  west. 

When  the  Variation  is  East. — Rule — To  the  observed  magnetic 
bearing  add  the  variation,  and  the  result  will  be  a  true  bearing. 

In  fig.  260  the  variation  is  15°  east  of  the  true  meridian,  and  the 
compass  bearing  of  line  a  b  along  the  course  of  a  stratum,  100°  50' ; 
find  the  true  meridian.  Obviously — 

100°  50'  +15°  =115°  50'  =true  bearing  ; 

that  is,  the  corrected  bearing  in  terms  of  the  true  meridian  is 
115°  50'. 

When  the  Variation  is  West. — Rule — From  the  observed  magnetic 
bearing  subtract  the  variation,  and  the  result  will  be  the  true  bearing. 


FIG.  261. — Conversion  of  magnetic  bearings  to  true. 

In  fig.  261  the  variation  is  12°  west  of  the  true  meridian,  and  the 
compass  course  of  a  lode  is  250° ;  find  the  bearing  or  strike  in  terms 
of  the  true  meridian.  Here — 

250°  -12°  =238°  -true  bearing. 

To  Convert  a  True  Bearing  to  a  Magnetic  Bearing. — This  is  the 
converse  of  the  above.  When  the  variation  is  easterly,  subtract 
the  variation  from  the  true  bearing  ;  and  when  westerly,  add  it  to 
obtain  the  corresponding  magnetic  bearing. 


APPENDIX   B. 


Determination  of  Strike  and  Dip  from  Contoured  Map. 

THE  exactitude  to  be  obtained  by  this  method  depends  on  the 
accuracy  of  the  survey  and  mapping.  The  results  are  trustworthy 
only  when  the  contours  have  been  run  with  the  spirit-level ;  the 
outcrops  and  contours  accurately  fixed  by  theodolite  traverse; 
and  the  positions  plotted  by  rectangular  co-ordinates  on  a  large 
scale. 


FIG.  262. — Showing  graphic  determination  of  strike  from 
contoured  map.     A,  B,  C,  Outcrop  of  bed. 

To  Determine  the  Strike. — Two  outcrops  on  the  same  level  must 
be  known. 

Let  A  and  B  be  two  outcrops  of  a  bed  or  vein  at  the  same  level. 
Join  points  A  and  B  with  a  straight  line. 

Then  the  line  A  B  is  the  direction  of  the  strike,  and  angle  x 
the  bearing  of  the  strike  in  terms  of  the  meridian  N-S,  which 
may  be  the  true  meridian  or  the  magnetic  meridian,  according  to 
the  orientation  of  the  map. 

At  all  other  levels  the  strike  will  be  parallel  to  A  B.  Thus  at 
point  C,  which  is  100  feet  below  B,  the  strike  is  C  E. 

553 


554  A  TEXT-BOOK  OF  GEOLOGY. 

To  Determine  the  Dip. — The  dip,  or  more  correctly  the  direction 
of  the  dip,  is  always  at  right  angles  to  the  strike.  If  we  assume 
that  the  bearing  of  the  strike  is  40°  (that  is,  angle  x  =40°),  then  the 
dip  being  south-easterly,  the  direction  of  the  dip  is  40°  +90°  =130°. 
If  the  dip  were  in  the  opposite  direction,  the  strike  being  the  same, 
then  its  bearing  or  direction  would  be — 

360°  +40°  -400° 
and  400° -90°  =310°. 

To  Determine  the  Angle  of  Dip. — Three  points  or  outcrops  must 
be  known,  namely,  two  at  the  same  level  and  one  at  a  lower  or 
higher  level. 

(1)  Let  A,  B,   and  C,  fig.  262,  be  the  known  outcrops,  A  and 

B  being  at  the  same  level,  and  C  at  a  lower  level,  100  feet 
below  B. 

(2)  Determine  the  strike  as  in  the  first  problem. 

(3)  Through  C  draw  C  E  parallel  to  A  B. 

(4)  At  B  draw  a  line  B  D  at  right  angles  to  A  B,  terminating 

at  line  C  E. 

(5)  Scale  as  accurately  as  you  can  the  length  of  B  D. 


250 
FIG.  263. — Showing  profile  along  B  D. 

Let  Bx  be  a  point  at  the  same  level  as  C  or  D,  immediately  below 
B,  and  let  BXD  =250  feet. 

In  the  right-angled  triangle  B  Bx  D  we  have  given  the  two  sides 
about  the  right  angle  to  find  the  angle  of  dip,  namely,  BjD  =  250 
feet,  and  B  Bx  =  100  feet. 

Let  0  =the  angle  of  dip. 

Then- 
Cot  0  =        ,  or  tan  0  =         - 


By  natural  tangents — 

Tan  <9  = 
By  logarithms — 


Tan  0  =^  =4000000  =21°  48'  =angle  of  dip. 
250 


Log  100  =2-0000000 

Log  250  =2-3979400 

Log  tan  21°  48'  =9-6020600 


BIBLIOGRAPHY. 

THE  following  list  contains  the  titles  of  most  of  the  works  to  which  reference 
has  been  made  in  the  preparation  of  this  volume.  Many  of  these  deal  with 
the  whole  or  a  portion  of  the  subject  in  a  comprehensive  manner.  Refer- 
ence has  also  been  made  to  special  points  in  the  Reports  and  Memoirs  of  the 
official  Geological  Surveys  of  Great  Britain,  United  States  of  America, 
India,  and  the  Oversea  Dominions ;  and  to  papers  scattered  throughout 
the  Quarterly  Journal  of  the  Geological  Society,  the  Philosophical  Trans- 
actions of  the  Royal  Society,  Comptes  Rendus,  Annales  des  Mines,  and  various 
American,  English,  and  Continental  scientific  and  technical  serials.  These 
papers  are  so  numerous  that  the  exigencies  of  space  make  it  impossible  to 
attempt  their  bibliographical  statement  here. 

History  of  Geology. 

Bonney,  Prof.  T.  G.,  Charles  Lyell  and  Modern  Geology.     London,  1901. 
Geikie,  Sir  A.,  Life  of  Sir  R.  1.  Murchison,  2  vols.     London,  1875. 

—  The  Founders  of  Geology,  2nd  ed.     London,  1905. 
Merrill,  G.  P.,  History  of  American  Geology.     Washington,  1906. 
Woodward,  H.  B.,  History  of  Geology.     London,  1911. 

History  of  the  Geological  Society  of  London.     London,  1907. 

Zittel,  Karl  von,   History  of  Geology  and  Palaeontology  (English  translation 
by  Maria  M.  Ogilvie-Gordon).     London,  1901. 

General  and  Physical. 

Arrhenius,  S.,  Worlds  in  the  Making.     London,  1908. 

Bischof,  Gustav,  Elements  of  Chemical  and  Physical  Geology  (English  trans- 
lation by  B.  H.  Paul  and  J.  Drummond),  2  vols.     London,  1854-55. 
Bonney,  Prof.  T.  G.,  The  Story  of  our  Planet.     London,  1902. 

The  Building  of  the  Alps.     Cambridge,  1912. 

The  Work  of  Rain  and  Rivers.     Cambridge,  1912. 

Chamberlin,  T.  C.,  and  Salisbury,  R.  B.,  Geology — Processes  and  their  Results, 

3  vols.,  2nd  ed.     London  and  New  York,  1906. 
Dana,  J.  D.,  Manual  of  Geology,  4th  ed.     New  York,  1895. 
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London,  1888. 


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London,  1910. 

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1906. 


INDEX. 


ABLATION  of  glaciers,  60. 

Abysmal  deposits,  110  ;   zone,  96. 

Abyssal  rocks,  237,  245,  247. 

Acadia  group,  321. 

Acicular  crystals,  182,  237,  245,  247. 

Acidic  rocks,  definition,  194,  242 ; 
typical  kinds,  241,  245. 

Acids  in  soil,  24. 

Acrodus,  376. 

Acrogens,  286. 

Actseonina,  423. 

Actinocamax,  408,  415. 

Actinoceras,  350. 

Actinocrinus,  350. 

Actinolite,  feel  of,  187  ;  in  metamor- 
phism,  260 ;  in  chlorite-schist, 
265  ;  in  actinolite-schist,  265  ;  in 
talc-schist,  265  ;  in  marble,  266. 

Actinolite-schist,  265. 

Actinozoa,  271,  273. 

Adirondack,  Cambrian,  319. 

Advancing  stage  of  Glacial  Period, 
463,  464,  474. 

^Eglina,  330. 

Affinity,  174,  175,  176. 

Afghanistan,  Cretaceous,  406. 

Africa,  coral  reefs  off  east  coast,  103  ; 
Eozoic  rocks  of  South,  306 ; 
Cambrian  glaciation  in  South,  324  ; 
Devonian,  344 ;  Carboniferous, 
349,  362  ;  Permian,  363,  367,  368, 
369,  370  ;  Mesozoic,  372  ;  Trias, 
378,  382,  383,  384  ;  Jurassic,  388  ; 
Chalk  absent,  405 ;  Cretaceous, 
406,  418,  421,  422,  423,  425; 
Cainozoic,  430,  433,  434,  438,  439, 
446,  447,  448,  449,  450,  454,  481  ; 
land  connection,  431  ;  rift- valleys, 
496  ;  contour  form  of,  502,  503  ; 
diamonds,  507. 

Agassiz,  61,  77,  106. 

Agassiz  lake,  77,  500. 

Age  of  faults,  158  ;  as  a  character  in 
igneous  rocks,  245. 

Ages,  303. 


Agglomerate-neck,  230. 

Agglomerates,  329,  334,  345. 

Agnostus,  328. 

Ailsa  Craig,  470. 

Air,  action  of,  15,  16,  17. 

Aix-la-Chapelle,  418. 

Alabama,  Cambrian,  319,  320  ;  Ordo- 
vician,  327  ;  Cretaceous,  420  ;  Cain- 
ozoic, 439. 

Alaska,  glaciers,  69  ;  coast  of,  85, 
134 ;  Carboniferous,  349  ;  Trias, 
378 ;  Jurassic,  388,  401  ;  Creta- 
ceous, 406,  420,  421  ;  Tertiary 
land  connection,  431  ;  Cainozoic, 
444,  448. 

Albian,  409,  412,  414,  415,  416, 
419. 

Albite,  193. 

Albs,  73,  74. 

Aldeburgh,  456. 

Alder,  443. 

Alethopteris,  351,  410. 

Aletsch  glaciers,  59. 

Aleutian  Islands,  232. 

Algse,  285,  286. 

Algeria,  raised  beaches,  132,  135 ; 
Cainozoic,  454. 

Algiers,  Cretaceous,  406,  418. 

Algonkian  Period,  304,  307  ;  rocks, 
309-316;  fossils,  310,  311. 

Alkali-granites,  243. 

Alkali-syenite,  251. 

Alkalic  igneous  rocks,  243,  244,  247. 

Alkaline  waters,  127,  535. 

Allegheny  Series,  359. 

Alligators,  442. 

AUodon,  402. 

Allotriomorphic  structure,  240. 

Alluvial  flats,  25. 

Alpine  region,  433,  438,  441. ' 

Alpine  Trias,  374,  378-380  ;  Jurassic, 
401. 

Alps,  glaciers,  59 ;  overthrusts,  143, 
149,  150  ;  comparative  youth  of, 
147  ;  a  mountain-building  fold, 


559 


560 


A   TEXT-BOOK    OF    GEOLOGY. 


233 ;  mica-schist,  265 ;  Eozoic 
rocks,  306  ;  Carboniferous,  349  ; 
Permian,  363 ;  Mesozoic,  372  ; 
Trias,  374,  376,  378  ;  Chalk  absent, 
405  ;  Cretaceous,  407,  418  ;  Cain- 
ozoic,  430,  431,  434,  438,  441,  444, 
445,  447,  448,  449,  450,  461,  462, 
471,  482. 

Altai,  147,  148,  472. 

Alteration  of  rocks,  4  ;  sediments,  5  ; 
igneous  rocks,  6. 

Alum,  392. 

Alumina,  189,  241. 

Aluminium,  its  percentage  in  the 
earth's  crust,  176. 

Alveolina,  439. 

Amazon,  32,  116;  plains  near  the, 
61  ;  swampy  forests,  213. 

Amber,  443. 

America,  faults  in  Great  Basin  of 
Western  States  of,  169,  170; 
volcanoes,  232,  233 ;  Jurassic, 
388  ;  land  connections  in  Tertiary 
time,  431  ;  Cainozoic,  434 ;  con- 
tour form  of,  502. 

America,  Central,  corals,  103. 

America,  North,  ice  in,  54  ;  raised 
beaches  on  Pacific  side,  132 ;  Eozoic 
rocks  and  fossils  of,  306,  307  ; 
Algonkian  rocks  of,  309-311  ; 
Cambrian,  319,  320,  321  ;  Calci- 
ferous  Series,  322 ;  Ordovician 
rocks,  326,  327,  335 ;  Silurian, 
333,  335,  338,  339,  340;  Devo- 
nian, 344,  345,  347,  348  ;  Carboni- 
ferous, 353,  359  ;  Trias,  372,  374, 
383,  384 ;  Jurassic,  388,  389 ; 
Chalk,  405  ;  Cretaceous  beds,  406, 
407,  419,  420,  421,  425  ;  Cainozoic, 
439,  440,  441,  445,  448,  449,  451, 
452,  454,  459,  462,  463,  472,  473, 
474,  480,  481,  482  ;  contour  form 
of,  502. 

America,  South,  ice  in,  54  ;  flints  in 
Upper  Chalk,  130  ;  raised  beaches 
on  Pacific  side,  132 ;  Permian, 
363,  367  ;  Tertiary  land  bridges, 
431  ;  Cainozoic,  454,  473,  474, 
481  ;  contour  form  of,  502. 

Amethyst,  192. 

Ammonites,  271,  279,  298,  366,  373, 
376,  379,  380,  381,  389,  392,  393, 
394,  395,  396,  397,  398,  402,  403, 
404,  408,  411,  413,  414,  415,  417, 
419,  421,  427,  428,  429,  481. 

Amorphous  minerals,  188. 


Amphibia,  271,  281,  282,  283,  350S 
365,  366,  374,  376,  380. 

Amph'ibole,  195,  241. 

Amphibolite,  264. 

Amphidromus,  442. 

Amphigraptus,  330. 

Amphilestes,  394. 

Amphistegina,  451. 

Amuri  limestone,  424. 

Amygdaloidal  structure,  229. 

Amygdaloids,  242. 

Analcime,  242. 

Ananchytes,  422. 

Ancillaria,  448. 

Andalusite,  266. 

Andalusite -slate,  266. 

Andaman  Islands,  450. 

Andes,  snow-line  in  the,  54 ;  com- 
parative youth  of,  147  ;  volcanoes 
in,  232  ;  a  mountain- building  fold, 
233 ;  Eozoic  rocks,  306 ;  Cam- 
brian, 320  ;  Mesozoic,  373  ;  Creta- 
ceous, 421  ;  Cainozoic,  430,  474. 

Andesine,  193. 

Andesite,  joints  in,  152  ;  spheroidal 
weathering,  228 ;  lavas,  234 ; 
zeolites  in,  242. 

Andesites,  241,  243,  245,  257,  258, 
264,  361,  470,  523. 

Angiosperms,  285,  407,  427,  429,  436, 
481. 

Angle  of  dip,  136. 

Angles,  measurement  of,  183. 

Anglesey,  pre-Cambrian,  311. 

Anglo-Gallic  region,  433,  434,  438, 
441. 

Anhedral  structure,  240. 

Anhydride,  458. 

Anhydrous,  187. 

Animal  trails,  120,  122. 

Animals,  classification,  270-285. 

Annelida,  271,  310. 

Annelids,  276 ;  trails  of,  122,  310 ; 
Algonkian,  310  ;  Cambrian,  320. 

Annularia,  351. 

Annulata,  271,  276. 

Anodonta,  346. 

Anomodontia,  383. 

Anoplotherium,  442,  443. 

Anorthite,  193,  194. 

Antarctic,  ice  in,  59  ;  Carboniferous, 
349  ;  Gondwana  System,  367  ; 
Cretaceous,  406  ;  land  connection 
during  the  Cainozoic  era,  431,  473, 
475  ;  Recent,  476. 

Antelopes,  455. 


INDEX. 


561 


Anthracite,  211,  212,  214,  260,  328, 

335,  421,  508. 
Anthropoid  apes,  429. 
Anticlinal  axis,  139. 
Anticlinal  folds,  153,  154. 
Anticline,  138,  139,  149. 
Anticlinoria,  141. 
Antigorite,  253. 
Antrim,  257  ;   Jurassic,   391  ;    Cain- 

ozoic,  437. 
Apatite,  in  scale  of  hardness,  184  ; 

rock  former,   190 ;     properties  of, 

196  ;    in  plutonic  rocks,  248  ;    in 

andesites,  257  ;    in  chlorite-schist, 

265. 
Apennines,  372,  378,  430,  431,  434, 

438,  441,  458. 
Apes,  285,  450,  455,  459. 
Aplite,  254. 
Apophyses,  261. 
Aporrhais,  407,  413,  435. 
Appalachian   Mountains,    Cambrian, 

319. 

Applied  Geology,  13. 
Aptian,  409,  411,  416,  419. 
Aqueous  sediments,  199. 
Aquitanian,  443. 

Arabia,  Carboniferous,  349  ;    Creta- 
ceous, 406,  422  ;    Cainozoic,  430, 

431,  438,  441,  448,  481. 
Aracaurites,  394,  422. 
Arachnoidea,  273,  279. 
Aragonite,   rock  former,    190 ;     tri- 

metric  carbonate  of  lime,  183,  196, 

407. 

Area,  395,  411,  413,  448,  456. 
Arcestes,  379. 
Archaean,    304,   307,   308,   313 ;     no 

evidence  of  life,  308. 
Archaeopteryx,  284,  390,  426. 
Arch-limbs,  139. 
Arctic,     ice     in,     59 ;      Ordovician, 

326. 

Arenaceous  limestone,  209. 
Arenaceous  rocks,  200. 
Arenaceous  shale,  206. 
Arenig  beds,  329,  330,  331. 
Argentina,  Cambrian  in  North- West, 

320  ;      Permian    glaciation,    370  ; 

Trias,  382,  383  ;   Jurassic,  403. 
Argillaceous  limestone,  208,  209,  217. 
Argillaceous  rocks,  200,  206-208. 
Argillaceous  sandstone,  204. 
Argillites,  260. 

Arid  type  of  erosion,  485,  486. 
Arizona,  Ordovician,  326. 


Arkansas,  Ordovician,  327 ;  Cain- 
ozoic, 439. 

Armadillos,  474. 

Armenia,  Permian,  363,  366,  368. 

Arkansas,  Silurian,  333. 

Armorican,  pre-Permian  mountain 
chain,  147,  353,  365. 

Arnusian,  458. 

Arrhenius,  476. 

Artesian  wells,  530-534;  waters, 
534,  535. 

Arthropoda,  271,  279,  281. 

Articulata,  286. 

Asaphus,  328,  330,  331. 

Asbestos,  185. 

Ascensional  theory  of  vein -formation, 
528. 

Asch  lode,  518. 

Ash  in  coal,  212  ;  from  volcanoes, 
229,  230. 

Ashdown  sand,  411. 

Ashgill  beds,  329. 

Asia,  Silurian,  334  ;  Permian,  366  ; 
Trias,  374 ;  Cretaceous,  406 ; 
Tertiary  land  connections,  431  ; 
Cainozoic,  438,  447,  472  ;  contour 
form  of,  502. 

Asia  Minor,  Devonian,  344 ;  Trias, 
374,  378;  Jurassic,  388,  389; 
Cretaceous,  406,  418,  419;  Cain- 
ozoic, 430,  431,  438,  441,  448. 

Assam,  Cainozoic,  439,  451,  459. 

Astarte,  393,  394,  395,  396,  397,  423, 
457,  465. 

Asteroidea,  271,  274,  275. 

Astian,  458. 

Atherfield  beds,  411. 

Athyris,  350,  354,  361,  375,  376,  379, 
381. 

Atlantic  Ocean,  461. 

Atlantic  type  of  igneous  rocks,  243, 

244,  247. 
Atlantosaurus,  401. 

Atlas  Mountains,  422,  430,  438. 

Atmosphere,  1. 

Atolls,  105-107,  110,  133,  134. 

Atrypa,  336,  338,  345. 

Aturia,  429,  435. 

Aucella,  404. 

Auckland,  volcanoes,  220,  223,  224. 

Augite,  190,  195,  203,  204,  241,  242, 

245,  247,  248,  251,  252,  254,  255, 
257. 

Augite -andesite,  257. 
Augite-lamprophyre,  255. 
Augite-peridotite,  252. 

36 


562 


A  TEXT-BOOK   OF   GEOLOGY. 


Augite-syenite,  251. 

Austmannatjern,  65. 

Australasia,  Carboniferous  System, 
360-362 ;  Trias,  381  ;  Jurassic, 
402 ;  Chalk  absent,  405 ;  Cretaceous 
deposits,  406,  423,  424,  425; 
Cainozoic,  440. 

Australia,  plateau,  46  ;  ice  in,  54 ; 
marine  erosion  in  Western,  87 ; 
Great  Barrier  Reef  of,  88,  107  ; 
effect  of  subsidence  of,  98  ;  corals 
off  East  coast  of,  103  ;  barrier 
reef,  107 ;  raised  beaches,  132 ; 
rock  salt  in  Central,  216  ;  mica- 
schist  in  Western,  265 ;  Eozoic 
rocks,  306,  313  ;  Cambrian,  320, 
324 ;  Ordovician,  326,  330,  332  ; 
Silurian,  340;  Devonian,  304; 
Carboniferous,  349,  361  ;  Permian, 
363,  364,  367,  369,  370  ;n  Mesozoic, 
372  ;  Trias,  378,  379,  382 ;  Jurassic, 
388,  402  ;  Cretaceous,  406,  417, 
423,  424;  Cainozoic,  429,  440, 
446,  449,  452,  453,  454,  459,  460, 
474,  481  ;  land  connection  during 
Cainozoic  era,  431  ;  contour  form 
of,  503. 

Australites,  131. 

Auvergne,  volcanoes,  219,  220 ; 
craters,  223. 

Avalanche  slides,  55. 

Avalanches,  55. 

Aves.     See  Birds. 

Avicula,  366,  379,  380,  423. 

Axes,  178. 

Axis  of  anticline,  137. 

Aymestry  limestone,  336,  338. 

Ayrshire,  Permian,  367. 

Azoic,  307. 

Azores,  232. 

Azurite,  187. 

BACCHUS  Marsh  conglomerate,  369. 

Back-water,  118. 

Baculite,  416,  417,  423,  428,  481. 

Badger,  440. 

Baggy  beds,  346. 

Bagshot  Sands,  434,  436. 

Bajocian,  391,  393. 

Baku,  gushers  at,  27. 

Bala  beds,  329,  331,  332. 

Balsenoptera,  442. 

Balearic  Isles,  Trias,  378. 

Balkans,  Trias,  378  ;   Cainozoic,  438, 

441. 
Ballantrae  basalt,  228. 


Baltic,  Cretaceous,  406  ;  Cainozoic, 
462,  471. 

Baltic  Sea,  446. 

Baluchistan,  Jurassic,  401  ;  Creta- 
ceous, 406,  419  ;  Cainozoic,  430, 
439,  441,  444,  448,  459. 

Banket,  202,  255,  511,  513. 

Banwell  Cave,  480. 

Bar,  91. 

Barium  in  earth's  crust,  176. 

Barrier  coral  reef,  105,  107,  110. 

Barry  glacier,  61. 

Barton  beds,  434,  436. 

Barysphere,  1,  6,  7,  11,  12. 

Basal  planes,  180. 

Basalt,  242,  245,  257-258,  264,  415  ; 
valley  protected  by,  48 ;  table- 
topped  outliers  of,  146  ;  joints  in, 
152,  172;  nepheline  in,  196;  at 
Malvern,  212  ;  in  Victoria,  Aus- 
tralia, and  the  Deccan,  220  ; 
pillow-structure,  228  ;  zeolites  in, 
242 ;  Carboniferous,  358,  361  ; 
Jurassic,  401  ;  Cainozoic,  436,  437, 
451,  452,  453,  454,  459. 

Base-level,  30,  46,  53. 

Bases,  189,  241. 

Basic  group  of  rocks,  242,  245,  246, 
247. 

Basic  rock,  194. 

Basic  secretions,  251. 

Bat,  436,  457. 

Batholith,  238. 

Bathonian,  391,  394. 

Batrachians,  282. 

Bavaria,  graphite,  214  ;  Dyas,  368  ; 
Trias,  379;  Archaeopteryx,  390; 
lodes,  518. 

Bayeux,  393. 

Beacon  sandstone,  205,  382. 

Beans,  285. 

Bear,  455,  459,  471,  480. 

Beardmore  glacier,  59,  60. 

Beaufort  Series,  382,  383. 

Beavers,  450,  455. 

Beckmantown  limestone,  330. 

Bedding  planes,  151. 

Bedfordshire,  Cretaceous  beds,  410, 
504,  508  et  seq. 

Beech,  421,  427,  436,  448,  452. 

Beetles,  390,  394. 

Belemnitella,  408,  414,  415,  416,  417. 

Belemnites,  279,  298,  373,  389,  393, 
394,  396,  402,  408,  411,  412,  417, 
423,  424,  427,  428,  429,  477,  481. 

Belfast,  conformity  at,  294. 


INDEX. 


563 


Belgium,  faults  in,  168,  169  ;  Petit 
Granit,  208 ;  limestones,  217  ; 
Cambrian,  319,  320;  Devonian, 
342,  344  ;  Carboniferous,  349,  365  ; 
Cretaceous,  405,  406,  415,  416 ; 
Cainozoic,  438,  443,  454,  457. 

Bellerophon,  366. 

Belodon,  376. 

Bembridge  beds,  442. 

Bendigo  Series,  331,  332. 

Ben  Lomond,  New  Zealand,  56. 

Ben  More,  New  Zealand,  rock-cut 
terrace  at,  70. 

Betula,  457. 

Biafo  glacier,  59. 

Bibliography,  555-558. 

Bicarbonate  of  lime,  175. 

Big  Blue  Series,  369. 

Biological  provinces,  Jurassic,  402. 

Biotite,  194,  241,  248,  249,  250,  251, 
252,  254,  255,  257,  261,  264. 

Biotite-granite,  249. 

Birch,  452,  457. 

Birds,  271,  281,  283,  284,  373  ;  tracks 
of,  122  ;  absent  in  Palaeozoic,  317  ; 
Jurassic,  390  ;  Cretaceous,  409  ; 
Cainozoic,  429,  436,  457,  477. 

Bison,  457. 

Bithinia,  416. 

Bituminous  coal,  211,  212,  508. 

Bituminous  limestone,  210. 

Bivalves,  271. 

Black  band  ore,  355. 

Black  Forest,  471. 

Black  Head,  New  Zealand,  89. 

Black  River  limestone,  330. 

Black  sands,  203. 

Black  Sea,  447,  462. 

Blackheath  beds,  435. 

Blastoidea,  271. 

Blende,  186. 

Block- mountains,  170,  491. 

Blocks,  volcanic,  229. 

Bloodstone  192. 

Blow-holes,  83,  84. 

Blue,  colour,  187. 

Blue  ground,  22. 

Blue  Mountains,  37. 

Boar,  457. 

Bog-iron  ore,  26,  197,  507. 

Bohemia,  Variscan  mountain  chains 
in,  147  ;  graphite,  214 ;  Eozoic 
rocks,  306  ;  Cambrian,  319  ;  Dyas, 
368  ;  Cretaceous,  406,  418  ;  Cain- 
ozoic, 449. 

Bohemian  Pfahl,  518. 


Bolivia,  Silurian,  333 ;  Carboni- 
ferous, 349  ;  Jurassic,  388,  403  ; 
Cretaceous,  406  ;  Cainozoic,  474. 

Bone  bed,  Ludlow,  335  ;  Pvhsetic,  378. 

Bonne viUe,  473. 

Bonney,  Prof.,  74. 

Borax,  511,  525. 

Boric  acid,  511. 

Borneo,  Jurassic,  388,  402;  Cain- 
ozoic, 439. 

Borrowdale,  Cumberland,  graphite, 
214  ;  volcanic  series,  329,  331. 

Bosses,  236,  238,  245,  246,  248, 
260. 

Boticino  stone,  210. 

Boulder  Bank,  New  Zealand,  91. 

Boulder-clay,  65,  72,  78,  307,  463, 
465,  466,  469,  470,  471,  474. 

Boulders,  32,  33,  203,  362,  368,  369. 
See  also  Erratics. 

Bovey  Tracey  beds,  436. 

Bowen  Series,  361,  383. 

Box-stones,  456. 

Brachiopoda,  271,  276,  277  ;  Palae- 
ozoic, 317  ;  Cambrian,  320  ;  Ordo- 
vician,  328;  Silurian,  334,  336, 
338,  340;  Devonian,  342,  345, 
347 ;  Carboniferous,  350,  361  ; 
Permian,  366  ;  Mesozoic,  373  ; 
Trias,  379,  381,  384;  Jurassic, 
389,  392,  393,  394,  395,  396,  397, 
398,  404  ;  Cretaceous,  407  ;  Cain- 
ozoic, 429,  440,  460,  477. 

Bracklesham  beds,  434,  436. 

Brassy  lustre,  186. 

Brazil,  Silurian,  333  ;  glacial  con- 
glomerate, 370 ;  Trias,  383;  Creta- 
ceous, 421  ;  rubies,  507. 

Break,  292,  304,  336,  352,  360,  363, 
401,  428,  429,  430,  451. 

Breccia,  200,  201,  202,  217,  363,  364, 
367,  368. 

Breccia-conglomerate,  202. 

Brecciated  faults,  157,  162,  164. 

Brick  clays,  207. 

Brine  springs,  26,  27. 

British  Columbia,  coast,  134  ;  Algon- 
kian  of  South,  309 ;  Cambrian  in 
319,  322  ;  coalfields,  356  ;  Trias, 
378  ;  Cretaceous,  406,  420. 

British  Isles,  Eozoic  rocks,  306,  311- 
313;  pre-Cambrian,  311-313  ;  Or- 
dovician,  335  ;  Silurian,  335 ; 
Devonian,  342,  343  ;  Carbonifer- 
ous, 349,  350,  351,  352,  353,  354  ; 
Permian,  363,  366,  367,  368  ;  Ju- 


564 


A   TEXT-BOOK   OF   GEOLOGY. 


rassic,  391  ;  Cainozoic,  434-437, 
441-442,  447,  455-457,  466-470. 

Brittle,  185. 

Brittle-stars,  275. 

Broken  Hill  lode,  523. 

Broken  Hill  mine,  22,  23. 

Brontosaurus,  401. 

Bronze  age,  483. 

Bronzite,  196,  252. 

Brown  coal,  211,  212,  214,  437,  449, 
450,  453,  508. 

Brownian  motion,  8. 

Brunswick  group,  383. 

Brunton  sandstone,  205. 

Bryozoa,  276;  Silurian,  334,  340; 
Carboniferous,  359;  none  in  New 
Zealand  Trias,  381 ;  Jurassic,  392, 
395,  396 ;  Cainozoic,  449,  450,  453. 

Buccinum,  443. 

Buchiceras,  418,  419. 

Building  stone,  210,  411,  437,  509. 

Bunches,  520. 

Bunter,  375,  377,  378. 

Burma,  333  ;  Trias,  378 ;  Creta- 
ceous, 419 ;  Cainozoic,  441,  446, 
450,  451,  459  ;  rubies,  507. 

Butt  cleats,  152,  172. 

Butterflies,  390,  394. 

Buttes,  425. 


CACTUS,  429,  436,  437. 

Caen  Stone,  210. 

Cainozoic,  304,  428-484  ;  fauna  and 
flora,  428-429;  rocks,  429-430; 
distribution,  430 ;  distribution  of 
land  and  water,  430-431 ;  climate, 
431;  subdivision,  43 1-433;  Eocene 
System,  433-440  ;  Oligocene,  440- 
446;  Miocene,  447-453;  Pliocene, 
453-460 ;  Pleistocene,  461-476  ; 
Recent,  476-481 ;  summary,  481- 
484. 

Cairngorm,  192. 

Caithness,  Old  Red  Sandstone,  344. 

Calamites,  351,  355,  366,  368,  373. 

Calcareous  ooze,  95,  99,  100,  101, 
103. 

Calcareous  organic  rocks,  208-210, 
217. 

Calcareous  rocks,  200. 

Calcareous  sandstone,  128,  204. 

Calcareous  sinter,  26,  215. 

Calcareous  springs,  25,  26,  535. 

Calceola  beds,  347. 

Calcic  igneous  rock,  243,  244,  247. 


Calcite,  a  solid,  174 ;  stability  of, 
175;  pseudomorphosed  by  quartz, 
182  ;  hexagonal  carbonate  of  lime, 
183 ;  cleavage  in,  184 ;  in  scale 
of  hardness,  184  ;  lustre,  186  ;  as 
marble,  188,  266  ;  constituents  of, 
189  ;  rock  former,  190,  196,  216  ; 
as  alteration  product,  242  ;  as  a 
gangue,  517. 

Calcium,  its  percentage  in  the  earth's 
crust,  176  ;  a  constituent  of  calcite, 
189. 

Caledonian,  a  pre-Devonian  moun- 
tain chain,  147  ;  folding,  343. 

California,  drowned  valleys,  134; 
lava-flow,  220  ;  Ordovician,  326  ; 
Trias,  378;  Jurassic,  401,  402; 
Cretaceous,  420  ;  Cainozoic,  434, 
451  ;  gold,  507  ;  Mother  Lode, 
518. 

Callipteris,  366. 

Callovian,  391,  395,  396,  400,  401. 

Calymene,  328,  334,  338. 

Cambrian  Period,  304,  321-324. 

Cambrian  System,  317,  319-324,  335, 
481  ;  rocks,  320  ;  fauna,  320-321  ; 
flora,  320  ;  subdivisions,  321-323  ; 
economic  products,  323  ;  deposi- 
tion, 323-324  ;  glaciation,  324. 

Cambridgeshire,  Cretaceous,  410 ; 
Glacial  Period,  467. 

Camptonite,  255. 

Camptosaurus,  397. 

Canada,  coast  of  North-East,  134  ; 
graphite,  214;  gneiss,  264;  mica- 
schist,  265 ;  Cambrian  in  East, 
319 ;  Calciferous  Series,  322 ; 
Ordovician,  326, 327  ;  Silurian,  333. 

Canadian  beds,  330. 

Canary  Islands,  232. 

Canis,  271.     See  also  Dog. 

Cannel,  212,  213,  508. 

Canons,  37,  496. 

Canterbury  Plains,  New  Zealand,  51, 
53,  86. 

Cap  of  lodes,  197. 

Cape  Colony,  Devonian,  344 ;  Juras- 
sic, 403. 

Cape  Farewell,  New  Zealand,  90. 

Cape  Oamaru,  basalt,  228. 

Cape  sandstone,  205. 

Cape  System,  344,  362. 

Caradoc  beds,  329,  330. 

Carbon,  free,  as  solid,  174  ;  reducing 
power  of,  175  ;  in  earth's  crust, 
176;  as  diamonds,  188,  189;  a 


INDEX. 


565 


constituent  of  calcite,  189;  fixed, 
in  coal,  212. 

Carbon  dioxide,  denuding  action  of, 
16,  80  ;  a  gas,  174  ;  feeble  affinity 
in  bicarbonate  of  lime,  175. 

Carbonaceous  limestone,  210. 

Carbonaceous  rocks,  200,  211-214. 

Carbonaceous  sandstone,  204. 

Carbonaceous  shale,  206. 

Carbonate  of  iron,  127,  215. 

Carbonate  of  lime,  25,  26,  127,  189, 
190,  208,  209,  210,  215,  216,  217. 

Carbonate  of  magnesia,  128,  189, 
190,  209,  216,  217. 

Carbonates,  176,  189,  200,  215. 

Carbonic  acid,  action  of,  16,  21,  30, 
31,  108,  127,  175,  242 ;  in  sea- 
water,  79. 

Carboniferous  age,  coals  of,  214. 

Carboniferous  limestone,  354. 

Carboniferous  Period,  304,  317. 

Carboniferous  System,  317,  346, 
347  ;  349-362  ;  distribution,  349  ; 
rocks,  349,  350  ;  fauna  and  flora, 
350,  351  ;  condition  of  deposition, 
351-353  ;  subdivision,  353-357  ; 
Carboniferous  limestone,  354,  355  ; 
Yoredale  beds,  Millstone  grit,  355  ; 
Coal  -  Measures,  355-358  ;  con- 
temporaneous volcanic  rocks,  358, 
359  ;  North  America,  359  ;  India, 
359,  360;  Australasia,  360-362; 
South  Africa,  economic  products, 
362. 

Cardioceras,  404. 

Cardita,  423,  436. 

Cardium,  395,  450,  456,  465. 

Carinthian,  378,  379. 

Carlsbad  twins,  193. 

Carnelian,  192. 

Carnivores,  448,  457,  459. 

Carolina,  South,  glauconitic  sands  off, 
204. 

Caroline  Islands,  107 

Carpathians,  147  ;  Mesozoic,  372, 
373  ;  Trias,  378  ;  Cretaceous,  406, 
418;  Cainozoic,  430,  431,  434, 
438,  441,  444,  447,  448,  458,  461, 
482. 

Carrara  marble,  209. 

Cascades,  37,  38. 

Caspian,  447. 

Cassis,  448. 

Cassiterite,  pseudomorph  after  ortho- 
clase,  182  ;  fracture  of,  184. 

Castlemaine  Series,  331. 


Casts,  268. 

Cat,  440,  445. 

Catfish,  389,  390. 

Catskill  beds,  348. 

Caucasus,   147,   148,  233,  430,  444, 

462,  471. 

Cavan,  anthracite,  214. 
Caves,  formation  of,  21 ;  in  limestone, 

26,  340  ;   Cainozoic,  480. 
Cayugan  Series,  338. 
Cefn,  480. 
Cement,  425. 
Cement-stone,  436. 
Cementing  medium  of  rock,  4,  126- 

131,  204,  217. 
Cenomanian,    409,    412,    413,    414, 

415,  416,  419,  420,  422,  423,  426, 

427. 

Central  Sea.     See  Tethys. 
Centre  of  crystal,  178. 
Cephalaspis,  345. 
Cephalopoda,  271,  279  ;    Cambrian, 

321  ;     Ordovician,  328 ;     Silurian, 

334,    338 ;     Devonian,    345,    347  ; 

Carboniferous,     350,     351,     354 ; 

Permian,  366  ;   Jurassic,  389,  393, 

394,   395,   397,  398;     Cretaceous, 

407,  408  ;   Cainozoic,  477,  481. 
Ceratites,  379. 
Ceratodus,  376,  383,  394. 
Cereals,  285,  380. 
Cerithium,  393,  396,  398,  407,  413, 

437,  438,  442,  443,  450,  456,  460. 
Cetaceans,  440.     See  also  Whales. 
Cetiosaurus,  394,  397,  408. 
Ceylon,  graphite,  214. 
Chagos  Bank,  107. 
Chalcedony,  192. 
Chalk,  95,  103,  407,  409,  411,  412, 

413,  414,  416,  420,  422,  424,  425, 

427  ;    flints  in  Upper,  130  ;    Fora- 

minifera  in,  208,  405. 
Chalky  boulder  clay,  469. 
Chalky  clays,  89. 
Chalky  marls,  95. 
Chalybeate  springs,  26. 
Chamberlin,  planetismal  hypothesis, 

6. 

Chara,  442. 
Charcoal,  235. 
Chase  stage,  369. 
Chautauquan,  347. 
Chazy  limestone,  330. 
Cheirotherium,  375. 
Cheirurus,  329. 
Cheltenham,  393. 


566 


A   TEXT-BOOK    OF    GEOLOGY. 


Chemical   classification   of   minerals, 

187. 

Chemical  denudation,  17,  80. 
Chemical  sediments,  199,  200,  215- 

218. 

Chemical  work  of  streams,  30,  31. 
Chemically  formed  rocks,  215-218. 
Chert,  192,  200,  209,  210,  211,  217, 

218,  340,  398. 
Chesapeake  Series,  451. 
Cheshire,     brine     springs     in,     27  ; 
Jurassic,     391  ;      Glacial     Period, 
467,  470  ;   rock  salt,  509. 
Chesil  Bank,  91. 
Cheviot  Hills,  344,  345,  467. 
Chiastolite,  260,  266. 
Chiastolite-slate,  266. 
Chile,  coast  of,  85 ;     Jurassic,  388, 
403  ;    Cretaceous,  406,  421  ;    Cain- 
ozoic,  431. 

ChiUesford  clays,  455,  456,  457. 
Chilterns,  434. 

China,  loess  in  Northern,  207  ;  coal 
in,  214 ;  Eozoic  rocks,  306  ; 
Cambrian  glaciation  in,  324 ; 
Ordovician,  326  ;  Silurian,  333  ; 
Devonian,  344 ;  Carboniferous, 
349,  360  ;  Trias,  380  ;  Cretaceous, 
406,  419. 

China-clay,  206,  207. 
Chloride  of  iron,  231. 
Chloride  of  magnesium,  79. 
Chloride  of  sodium,  27,  79,  80,  231  ; 

salt  deposits,  99. 
Chlorides,  200,  215,  216. 
Chlorine,  free  as  gas,  174  ;  affinity  of, 
175,  176  ;    in  earth's  crust,  176  ; 
in  salt,  189. 

Chlorite,  187,  190,  195,  242,  265. 
Chlorite-schist,  265. 
Chceropotamus,  442,  449. 
Chonetes,  345,  347. 
Chromite,  525. 

Cidaris,  389,  393,  397,  407,  438. 
Cimarron  Series,  369. 
Cincinnati,  Ordovician,  327. 
Cincinnatian  beds,  330. 
Cinder  beds,  399. 
Cinnabar,  187. 
Cinnamon,  436,  443. 
Cirque,  74,  75. 
Cirripedes,  477. 
Cladophlebis,  381,  382,  422. 
Classes,  270,  271. 

Classification  of  marine  deposits,  96  ; 
of  faults,  159  ;  of  minerals,  187  ; 


chemical  and  economic,  187  ;  of 
igneous  rocks,  241,  242-247  ;  of 
animals,  270-285  ;  of  plants,  285, 
286  ;  of  mineral  deposits,  505. 

Clastic  sediments,  199. 

Clay- band  ore,  356. 

Clay-pans,  208. 

Clays,  100,  115,  126,  129,  155,  188, 
198,  200,  206,  207,  215,  217,  228, 
234,  235,  260,  347,  376,  379,  381  ; 
Jurassic,  387,  391,  392,  394,  396, 
397,  398,  399,  400;  Cretaceous, 
405,  409,  410,  411,  412,  413,  414, 
422,  423,  424,;  Cainozoic,  429, 
433,  435,  436,  437,  441,  443,  449 
450,  451,  453,  455,  457,  458,  459, 
464,  466,  468,  469,  470,  471,  483  ; 
for  building,  510. 

Clay-slate,  266.    • 

Claystone,  126. 

Cleats  in  coal,  152. 

Cleavage  of  rocks,  170,  171,  173,  260  ; 
of  crystals,  183,  184. 

Cleavage-plane,  173. 

Cleveland,  oolitic  limestones,  210, 
215,  392. 

Climacograptus,  332. 

Climate,  Cambrian,  323 ;  Silurian, 
339;  Carboniferous,  355;  of 
Gondwana  Land,  367  ;  Triassic, 
382,  426  ;  Jurassic,  382,  402,  403, 
426  ;  Cretaceous,  415,  417  ;  changes 
due  to,  428  ;  Tertiary,  431,  436, 
440,  448,  452,  464,  476. 

Clinometer,  136. 

Club-mosses,  Palaeozoic,  317. 

Clunian  beds,  336. 

Clutha  River,  New  Zealand,  45. 

Clyde  basin  coalfield,  357. 

Clymenia,  347. 

Coal,  101,  110,  200,  211-214;  sub- 
merged, 133  ;  cleats  in,  152  ; 
analysis  and  composition,  212  ; 
conditions  of  formation  of,  213  ; 
quality  of,  213  ;  age  of,  213  ; 
metamorphism,  260 ;  Carboni- 
ferous, 350,  352,  353,  354,  355, 
356,  357,  359,  360,  361,  362  ;  Trias, 
380,  381,  382,  385  ;  Jurassic,  386, 
388,  401  ;  Cretaceous,  405,  420, 
421,424,425;  Cainozoic,  439,  440, 
452  ;  stages  in  formation,  508. 

Coal- Measures,  355-358. 

Coastal  recession,  effect  on  river 
grading,  85. 

Coastal  sag,  88. 


INDEX. 


567 


Coccosteus,  345. 

Cockfield  dyke,  359. 

Cockle,  278. 

Cod,  457. 

Coelenterata,  271,  286. 

Ccenograptus,  328,  330. 

Coke  in  coal,  212. 

Colley  sandstone,  205. 

Collingwood,  New  Zealand,  171. 

Colorado  plateau,  43 ;  Jurassic, 
401  ;  Cretaceous,  420,  421,  425  ; 
Cainozoic,  434. 

Colorado  Series,  421. 

Colour,  187. 

Columbia,  451. 

Columnar  structure,  172,  226-228. 

Comanchean  System,  420. 

Comb-structures,  517. 

Compass,  136. 

Composition  of  sea-water,  79 ;  of 
earth's  crust,  174-187  ;  some 
chemical  principles,  174-176  ;  some 
physical  properties  of  minerals, 
176-187  ;  a  mineral  defined,  176  ; 
crystalline  form,  177-182;  dimetric 
system,  179-180 ;  trimetric  system, 
186 ;  monoclinic  system,  181  ; 
triclinic  system.  181,  182  ;  hexa- 
gonal system,  182  ;  pseudomorphs, 
182,  183 ;  dimorphism,  183  ; 
macles,  183  ;  geodes,  183  ;  mea- 
surement of  angles  of  crystals, 
183  ;  cleavage,  183,  184  ;  fracture, 
184;  hardness,  184, 185  ;  tenacity, 
185 ;  specific  gravity,  185,  186 ; 
lustre,  186,  187  ;  colours  and 
streak,  187  ;  classification  of 
minerals,  187  ;  of  rocks,  241-242, 
245,  246,  247. 

Compounds,  174,  175,  176. 

Compressed  air,  erosive  effect  of,  83. 

Compression.  154. 

Comstock  Lode,  518,  523. 

Conchoidal,'l84. 

Conchothyra,  424. 

Concretions,  129,  130,  356,  424. 

Cone-in-cone  structure,  210. 

Conemaugh  Series,  359. 

Conformity,  291,  294-296,  299. 

Congeria,  450,  458. 

Congerian  stage,  458. 

Conglomerates,  15,  100,  101,  113, 
118,  120,  127,  128,  202,  203,  213, 
217,  293 ;  cleavage  in,  171  ; 
schistose,  266  ;  Keewatin,  307  ; 
Algonkian,  309 ;  Torridonian, 


312,  313  ;  Palaeozoic,  317  ;  Ordo- 
vician,  326 ;  Silurian,  335,  336, 
338  ;  Carboniferous,  350,  360,  361  ; 
Permian,  363,  364,  367,  368,  369, 
370  ;  Mesozoic,  372  ;  Trias,  374, 
375,  377,  381,  382,  383  ;  Cretace- 
ous, 422,  424  ;  Cainozoic,  458. 

Congo,  plains  near  the,  51. 

Coniferse,  285,  286  ;  Palaeozoic,  317  ; 
Carboniferous,  351  ;  Jurassic,  390  ; 
Cretaceous,  407,  422,  427  ;  Cain- 
ozoic, 442,  443,  452. 

Coniston  limestone,  329. 

Conites,  422. 

Connaught,  323. 

Consequent  rivers,  46. 

Consolidated  rock,  125. 

Constructive  work  of  rivers,49-51,  53. 

Contact  action,  260. 

Contact  deposits,  511,  513. 

Contact-metamorphism,  259  -  262, 
526,  527. 

Contact- minerals,  260. 

Continental  deposits,  388,  400,  401, 
402. 

Continental  earth-movements,  132. 

Continents,  298,  299,  323,  405,  426, 
453  ;  contour  form  of,  501-503. 

Contorted  drift,  469. 

Contorted  rocks,  149. 

Contraction,  heat  caused  by,  7. 

Conybeare,  390. 

Cook  Island,  volcanic  rocks,  244." 

Cook  Strait,  mollusca  of,  96. 

Copper,  32i> ;  free,  or  solid,  174 ; 
fracture  of,  184 ;  malleability, 
185 ;  ore  of,  364  ;  at  Mansfeld,  368, 
371,  511  ;  in  serpentine,  525. 

Coprolite  bed,  412. 

Copt  Point,  413. 

Coral  reefs,  88,  103,  110,  128,  134  ; 
distribution,  103,  107,  108  ;  growth 
of,  104,  105-107  ;  builders,  271  ; 
Jurassic,  387,  396,  404. 

Corallian,  391,  395,  396,  397. 

Coralline  Crag,  455,  456. 

Coralline  limestone,  208,  217. 

Corals  as  fossils,  268 ;  Palaeozoic, 
317  ;  Cambrian,  320  ;  Ordovician, 
328;  Silurian,  334,  336,  340; 
Devonian,  342,  345  ;  Carboniferous, 
350,  354  ;  Permian,  366  ;  Jurassic, 
389,  393,  395,  396,  397,  398; 
Cretaceous,  407  ;  Cainozoic,  429, 
440,  446,  449,  450,  453,  455,  470, 
481. 


568 


A    TEXT-BOOK    OF    GEOLOGY. 


Cormorant,  457. 

Cornbrash,  392,  394,  395. 

Cornwall,  Ordovician,  326 ;  Cain- 
ozoic,  456  ;  tin  lodes,  617  ;  lodes, 
518. 

Corrasion,  34. 

Corrie  lakes,  74. 

Corrosion,  16,  34. 

Corrugations,  125. 

Corundum,  on  scale  of  hardness, 
184  ;  crystallised  alumina,  189. 

Cotopaxi,  219,  220. 

Country-rock,  influence  of,  520. 

County  Down,  333. 

County  Dublin,  333. 

Course  of  lode,  136,  137. 

Crabs,  271;  trails  of,  122  ;  Cainozoic, 
477. 

Cracks,  125. 

Crag  and  Tail,  70,  71. 

Craigleith  sandstone,  205. 

Crania,  395,  407,  477. 

Crassatella,  436. 

Crater  lakes,  500. 

Craters,  223. 

Crayfish,  477. 

Cretaceous  Period,  304,  372  ;  vol- 
canic activity  at  close  of,  233 ; 
sea-urchins,  389. 

Cretaceous  System,  405-427  ;  rocks, 
405  ;  distribution,  406 ;  flora  and 
fauna,  407-409  ;  subdivisions,  409 
-424;  climate,  415;  conditions 
of  deposition,  414,  415  ;  landscape 
and  physical  features,  424,  425 ; 
economic  minerals,  425  ;  summary, 
425-427. 

Crinoidea,  271,  274,  275;  Palaeozoic, 
317 ;  Cambrian,  320 ;  Ordo- 
vician, 328  ;  Silurian,  334  ;  Devo- 
nian, 345 ;  Carboniferous,  350, 
354  ;  Jurassic,  389  ;  Recent,  477. 

Crioceras,  408,  423. 

Cristellaria,  272. 

Crocodiles,  401,  408,  409,  421,  429, 
436,  442,  450. 

Cromer  Forest  bed.  455,  457. 

Cromer  Till,  469. 

Cromwell  Basin,  45. 

Crush-breccias,  202. 

Crush-conglomerates,  203. 

Crustacea,  271,  279-281,  310  ;  Cam- 
brian, 320  ;  Permian,  366  ;  Juras- 
sic, 387,  394,  396  ;  Cainozoic,  429, 
435,  443,  477. 

Crustal  folds,  146-148,  150. 


Cryptogamia,  285,  286. 

Crystalline  limestone,  208,  209. 

Crystalline  rocks,  5  ;   minerals,  188. 

Crystallisation  of  plutonic  rocks,  248, 
258. 

Crystallites,  240. 

Crystallographic  systems,  177-182. 

Crystals,  formation  of,  177  ;  cubic 
system,  178 ;  dimetric  system, 
179  ;  trimetric  system,  180  ;  mono- 
clinic  system,  181  ;  triclinic  sys- 
tem, 181,  182  ;  hexagonal  system, 

182  ;      pseudomorphs,    182,     183  ; 
dimorphism,  183  ;    macles  or  twin 
crystals,  183  ;    geodes,  183  ;    mea- 
surements   of   angles    of    crystals, 

183  ;    cleavage,  183,  184;   in  rock, 
239,  240. 

Cubic  system,  178,  182. 

Cucullsea,  393,  413,  423. 

Culm,  254. 

Culm  measures,  354. 

Cumberland,  Jurassic,  391  ;    Glacial 

Period,  467. 

Current  bedding,  117,  123. 
Current-laid  stones,  119. 
Curve  of  erosion,  42. 
Cutch,  401. 
Cuttle-fish,  279,  477. 
Cuvier,  443. 
Cuzco  Valley,  480. 
Cyathophyllum,  350. 
Cybele,  329. 
Cycadites,  394. 
Cycads,  285,  286  ;    Palseozoic,  317  ; 

Jurassic,    390,    399 ;     Cretaceous, 

407,  410,  422,  427. 
Cycle  of  deposition,  100,  110. 
Cypresses,  452. 
Cyprina,  413,  465,  471. 
Cyrena,  393,  410,  442,  443. 
Cyrthograptus,  336,  338. 
Cystoidea,  271. 
Cytherea,  442. 

DACHSTEIN  caves,  Austria,  26. 

Dacite,  257. 

Dakota,  Ordovician,  326  ;    Jurassic, 

401. 

Dakota  Series,  420. 
Damuda  Series,  368,  380. 
Danacopteris,  380. 
Danian,  409,  416,  417,  419. 
Daonella,  379. 
Darling  Downs,  37. 
Darriwell  Series,  331. 


INDEX. 


569 


Darwin,  106,  107,  134,  474. 

Dasornis,  284. 

Daubree,  experiments  on  rocks,  261, 
262. 

David,  324. 

David,  Prof.,  on  floating  ice,  58. 

Dayia,  334,  338. 

Dead  Sea,  216. 

Decapoda,  281,  429. 

Deccan,  laterite,  207. 

Deep-leads,  453,  459,  506. 

Deep-seated  rocks,  247. 

Deer,  440,  448,  450,  455. 

Definition  of  fault,  155 ;  of  rock, 
125,  188. 

Deinosaurians,  283,  383,  390,  401, 
408,  409,  411,  421,  422,  428,  481. 

Deinotherium,  448,  450,  454. 

Deltas,  formation  of,  93. 

Dempsey  Series,  361. 

Denmark,  Cretaceous,  416,  417. 

Dentalina,  272. 

Dentalium,  375,  413. 

Denudation,  3,  4,  5,  10,  11,  15-29, 
30-49,  51-53,  79-88;  100,  108, 
109,  144-146,  150,  297,  298,  299, 
300,  301,  353,  358,  364,  436,  444  ; 
pre-Cambrian,  316 ;  Devonian, 
342  ;  of  Cretaceous  beds,  424,  425  ; 
after  the  Pliocene,  461  et  seq.,  475, 
486  et  seq. 

Deposition,  49-51,  53,  80  ;  in  estu- 
aries, 93,  94  ;  time-plane  of,  95  ; 
of  sediments,  98  ;  effect  of,  on 
rising  sea-floor,  99  ;  during  uplift, 
99,  100  ;  during  subsidence,  100  ; 
cycle  of,  100, 110;  on  sea-floor,  113- 
115;  break  in,  292,  296,  297; 
continuous,  296,  297,  298;  pre- 
Cambrian,  316  ;  Cambrian,  323, 
324 ;  Silurian,  339 ;  Devonian, 
343,  346,  347  ;  Carboniferous, 
351-353,  355,  356  ;  Permian,  364, 
365,  366  ;  Cainozoic,  430,  445, 
464  et  seq.  ;  Recent,  476,  479,  480. 

Derbyshire,  Carboniferous,  354,  358. 

Derived  fossils,  203,  269,  270. 

Desert,  Permian,  365  ;  Triassic,  374, 
375 ;  Jurassic,  387  ;  colour  of, 
486. 

Desert  sandstone,  North  Australia, 
124,  145,  205,  422,  423,  424,  425. 

Deshayes,  432. 

Desor,  60. 

Detrital  benches,  69.     < 

Detrital  mineral  deposits,  506,  507. 


Detrital    sediments,    199,    200-208, 

217. 

Detritus,  sorting  of,  111-113. 
Development    of    surface    features, 

485-503. 

Devitrified  glass,  240. 
Devon,  Devonian  System,  342,  343, 

344,  346,  347  ;  Carboniferous,  352, 

353,  354,  358  ;  Permian,  367. 
Devonian  Period,  304. 
Devonian    System,    317,    342-348 ; 

marine  and  lacustrine  types,  342  ; 

conditions     of     deposition,     343 ; 

distribution,  343,  344  ;  rocks,  344, 

345  ;  fauna,    345,    346  ;     subdivi- 
sion, 346-348  ;  economic  products, 

348. 

Diabase,  255,  320. 
Diabase-tuff,  320. 
Diallage,  195,  241,  252. 
Diamond,  on  scale  of  hardness,  184  ; 

crystalline   carbon,    188,    189 ;     in 

deposits,  506,  507. 
Diatoms,    101,    103,   210,   211,   218, 

285,  286,  451,  528. 
Dicellograptus,  329. 
Dicotyledons,  285,  286,  407,  418,  424, 

427,  429,  443,  481. 
Dicranograptus,  330. 
Dicynodon,  383. 
Didymograptus,  330,  331. 
Differential  uplift,  133. 
Dimetric  system,  179,  182. 
Dimorphism,  183. 
Dinornis,  284. 
Diopside,  260. 

Diorite,  238,  245,  252,  258,  320. 
Diorites,  241,  252,  261  ;     Lewisian, 

312. 

Dip,  135,  136,  537  et  seq. 
Dip-fault,  160,  173  ;    effect  on  syn- 

cline,   164,   167  ;    effect  on  lodes, 

524. 

Diplograptus,  328,  329,  334. 
Diprotodon,  460. 
Direction  of  dip,  136. 
Dirt  beds,  399. 
Discina,  477. 
Disintegration,  17. 
Dislocation,  125. 
Displacements  caused  by  faults,  159- 

170,  172,  173. 

Distribution  of  glaciers,  59,  60. 
Docodon,  402. 
Dog,  440,  457. 
Dogger,  394,  400. 


570 


A    TEXT-BOOK    OF    GEOLOGY. 


Dolerite,  242,  245,  246,  253,  255,  257, 
382. 

Dolomite,  189,  190,  196,  208,  209, 
216,  217  ;  Cambrian,  319  ;  Ordo- 
vician,  328;  Silurian,  334,  338; 
Permian,  363,  364,  366,  367,  368  ; 
Mesozoic,  372 ;  Trias,  372,  374, 
375,  379,  380,  384,  385. 

Dolphins,  440,  448. 

Dormant  volcanoes,  235. 

Dorocidaris,  417. 

Dorsal  valve,  276. 

Dorsetshire,  building  stone,  210 ; 
Lower  Oolite,  392,  393 ;  Middle 
Oolite,  396. 

Dosinia,  448. 

Doubly  oblique  system,  181,  182. 

Douglas  Island,  264. 

Downthrow,  159,  161. 

Downton  sandstones,  336. 

Downtonian  beds,  336. 

Dragon-flies,  394. 

Dream  Cave,  480. 

Drift,  65,  443,  454,  458,  459,  460, 
463,  465,  466,  468,  469,  471,  472, 
473,  474,  475,  478,  479,  480,  483. 

Dromornis,  460. 

Drowned  valleys,  84,  133,  134,  498. 

Drumlins,  65,  78. 

Drums,  65,  78. 

Dudley  Port  Mine,  168. 

Dufton  shales,  329. 

Dumfriesshire,  Permian,  367. 

Dunes,  17,  117. 

Dunite,  188,  252,  253. 

Dunstan  Gorge,  45. 

Durham,  Coal-Measures,  357  ;  Per- 
mian, 367. 

Durness  limestone,  322. 

Dust.  116,  117,  229. 

Dwyka  Series,  368,  369,  370,  382. 

Dyas,  363,  368. 

Dykes,  236,  238,  239,  245,  246,  253, 
255,  256,  257,  258,  260. 

Dynamical  geology,  13. 

Dynamo-metamorphism,  263. 

EAGLE,  457. 

Earth,  origin  of  the,  2-10  ;  crust 
mostly  sedimentary,  4  ;  state  of 
interior,  6  ;  age  of,  7  ;  history  of 
the,  by  eras  and  systems,  300- 
484  ;  gaps  in  the  record,  301,  302  ; 
the  record,  302-305. 

Earth-movements,  132-150, 155-170, 
172,  173. 


Earth-pillars,  23. 

Earthquakes,  134,  135,  156,  158; 
Yukutat,  62,  135. 

East  Falkland  Island,  Permian,  370. 

East  Maitland  Series,  361. 

East  Nelson,  453. 

Ecca  Series,  382. 

Echinobrissus,  389,  395,  407. 

Echinodermata,  271,  273,  274,  341, 
350,  366,  392,  395,  396,  397. 

Echinoidea,  271,  274,  275,  389. 

Echinos  patagus,  412. 

Economic  classification  of  minerals, 
187. 

Economic  geology,  13,  504-536 ; 
definitions,  504  ;  mineral  deposits, 
505  et  seq. 

Economic  products,  Cambrian,  323  ; 
Ordovician,  332  ;  Silurian,  340  ; 
Devonian,  348 ;  Carboniferous, 
362  ;  Permian,  370,  371. 

Edge,  177. 

Effusive  magma,  236,  245,  247. 

Egypt,  raised  beaches,  132 ;  con- 
formity in  Lower,  294  ;  Carboni- 
ferous, 349  ;  Cretaceous,  406,  418, 
422 ;  Cainozoic,  438,  439,  448, 
481. 

Eifel,  volcanoes,  219 ;  Devonian, 
347. 

Elseolite,  196. 

Elastic,  185. 

Elements,  174,  175,  176. 

Elephant,  448,  456,  457,  459,  469. 

Elevation,  4 ;  effect  of  uplift  on 
river  erosion,  42,  43  ;  on  sea-level, 

98  ;    geographical  effect  of  uplift, 

99  ;    deposition  during  uplift,  100, 
109 ;      earth-movements     during, 
132-150  ;    of  west  coast  of  Italy, 
135 ;       of     Tunis,     Algeria,     and 
Alaska,  135  ;    during  formation  of 
coal,  213  ;   ridges  of,  233  ;   relation 
of  breaks,  292,  293  ;   at  Worcester, 
South  Africa,  294 ;  relation  to  land, 
298,     299,    300,    301,    302,    316; 
Silurian,  339  ;  Devonian,  343,  346, 
347;  Carboniferous,  351,  352,  353, 
359,    360 ;     Permian,    365,    370  ; 
Jurassic,  388,  389,  401 ;  Cretaceous, 
406  ;  Cainozoic,  441,  447,  448,  450, 
451,  452,  453,  454,  455,  458,  460, 
461,  476,  482. 

Elgin,  Belodon  at,  376. 
Elina,  378. 
Elk,  471,  480. 


INDEX. 


571 


Elm,  436,  448,  452. 

Elvan,  253,  254. 

Embotyi,  423. 

Emeralds,  506. 

Enaliornis,  409. 

Encrinus,  376,  379. 

End-cleats,  152. 

Endogens,  286. 

Ends,  138. 

Energy,  7. 

England,  rivers  of  Pennine  Chain, 
45,  46  ;  denudation  of,  53  ;  flints 
in  Upper  Chalk,  130 ;  raised 
beaches,  132  ;  outliers  in  Central, 
146  ;  conformity  in  South-East, 
294  ;  Cambrian,  320  ;  Tremadoc 
slates,  322  ;  Ordovician,  327,  329, 
331  ;  Silurian,  340 ;  Caledonian 
folding,  343 ;  Devonian,  344 ; 
Carboniferous,  352 ;  coalfields, 
356-357  ;  Permian,  363 ;  Trias, 
372,  380  ;  Jurassic,  388,  390,  391, 
392,  393,  396,  397,  398,  399,  400, 
401,  403 ;  Chalk,  405  et  seq.  ; 
Cretaceous  deposits,  406,  409,  410, 
411,  412,  413,  414,  415,  416,  417, 
425  ;  Cainozoic,  429,  430,  433,  434, 
436,  440,  441,  454,  455,  457,  462, 
467,  469,  470,  478,  480,  481. 

Enon  beds,  423. 

Enstatite,  196,  241,  248,  252,  255. 

Enstatite-dolerite,  255. 

Enstatite-peridotite,  252. 

Entomostraca,  281. 

Eocene  Period,  304,  428,  429,  432, 
433-440  ;  distribution,  433  ;  rocks, 
433,  434 ;  British  Isles,  434 ; 
Thanet  sands,  Woolwich  and 
Reading  beds,  435  ;  London  clay, 
435-436 ;  Bagshot  beds,  Bovey 
Tracey  beds,  436  ;  contemporan- 
eous volcanic  activity,  436-437  ; 
France,  437,  438  ;  Belgium,  438  ; 
South  Europe,  438-439;  India, 
439 ;  North  America,  439-440  ; 
Australasia,  440. 

Eozoic  era,  304,  306-316  ;  deriva- 
tion, 306. 

Eozoon,  310,  311. 

Epeirogenic  action,  132. 

Epidote,  190,  261,  265. 

Epochs,  303,  304. 

Equisetums,  351,  375,  376. 

Equus,  455. 

Eras,  303. 

Erian,  347. 


Erosion,  15,  30-49,  66,  79,  84,  87, 
108,  109 ;  arid  type,  485,  486 ; 
pluvial  type,  486-488. 

Erratics,  32,  33,  68,  469,  470,  471, 
475. 

Escarpments,  487. 

Eskers,  72,  468. 

Essential  minerals,  191. 

Essex,  455. 

Estheria,  375,  376,  378. 

Estuaries,  Jurassic,  386,  387,  390, 
393,  394,  396,  397,  398. 

Estuarine  deposits,  thickness  of,  96, 
97. 

Estuarine  Series,  393. 

Etna,  219,  225,  243,  458. 

Euomphalus,  354. 

Eurite,  254. 

Europe,  erratics,  33 ;  ice  in,  54 ; 
flints  in  Upper  Chalk  of  North- 
West,  130  ;  extinct  birds  of  Lower 
Tertiary  in,  283,  284;  pre-Cambrian 
of,  311  ;  Cambrian  of  Central,  319  ; 
Cambrian  of  West,  321  ;  Ordovi- 
cian, 326,  332,  335 ;  Silurian, 
334,  335,  339 ;  Devonian,  342, 
343,  344,  348  ;  Carboniferous,  349, 
350,  352,  353,  359,  360,  362  ;  Per- 
mian, 363,  364,  365,  366;  Trias, 
373,  374,  377,  378,  379,  380,  425, 
426  ;  Jurassic,  388,  389,  401,  402  ; 
Cretaceous,  405,  406,  414,  416,  418, 
419,  427 ;  Cainozoic  (Tethys), 
430 ;  land  connections,  431  ; 
Eocene,  433,  438,  440  ;  Oligocene, 
440,  441,  442-446  ;  Miocene,  447, 
448,  449,  450  ;  Pliocene,  454,  455, 
458  ;  Pleistocene,  461,  462,  471, 
472  ;  Post-Glacial,  476,  479,  480, 
482  ;  contour  form  of,  502. 

Eurypteridse,  339,  340,  342. 

Eurypterus,  335,  338,  345. 

Even  fractures,  184. 

Exogens,  286. 

Exogyra,  397,  407,  410,  411,  423. 

External  casts,  268. 

Extinct  volcanoes,  235. 

FACE  cleats,  152,  172. 

Faces,  177. 

Fahlbands,  511,  512,  513. 

Falkland    Islands,    Cainozoic,    473, 

475. 
False  bedding,    117,    118,    123,    124 

323. 
False-bottom,  506. 


572 


A   TEXT-BOOK    OF    GEOLOGY. 


Faluns,  449. 

Families,  270,  271. 

Fan,  149. 

Fans,  formed  by  rivers,  61. 

Faroe  Islands,  431,  434,  437. 

Fault-breccia,  201. 

Faults,  125, 155-170/172, 173  ;  effect 
on  course  of  streams,  36  ;  defini- 
tion, 155 ;  origin  of,  155 ;  rela- 
tion to  joints,  156 ;  relation  to 
folding,  156,  157  ;  linear  extent, 
157  ;  evidences  of  circulation  of 
water  in  ore  veins  in,  157,  158  ; 
age  of,  158 ;  structure  of,  158 ; 
hade  of,  159 ;  classification  of, 
159 ;  displacements  caused  by, 
159,  160  ;  effects  of,  on  horizontal 
strata,  161,  162  ;  effects  of  strike-, 
162;  thrust-planes,  162,  164; 
effect  of  dip-,  164-167  ;  step-,  167  ; 
trough-,  167,  168 ;  evidences  of, 
168-170 ;  in  Carboniferous  beds, 
350  ;  Jurassic,  389-390  ;  effect  on 
lodes,  524  ;  surveying,  540  et  seq. 

Fault-valleys,  496. 

Fauna,  difference  of,  in  the  same 
plane,  95,  96  ;  relation  of  breaks 
to,  292,  293,  294 ;  Ordovician, 
327-332  ;  Silurian,  334-339  ;  De- 
vonian, 345-347  ;  Carboniferous, 

350,  354,  355,  359,  361,  362  ;   Per- 
mian,   365,   366,   368 ;     Mesozoic, 
373  ;   changes  of,  428-429  ;   Cain- 
ozoic,  428,  429,  435,  436,  437,  438, 
439,  440,  442,  443,  448,  449,  450, 
451,  452,  453,  454,  455,  456,  457, 
458,  459,  460,  468,  469,  470,  471, 
472,    473,    474,    481,    482,    483  ; 
Recent,  477,  478,  479,  480. 

Favosites,  336. 

Feel,  186. 

Felis,  270,  271. 

Felsites,  241,  253,  254. 

Felsitic,  254. 

Felspar,  21,  188,  190,  192,  194,  204, 
205,  241,  242,  244,  249,  250,  252, 
254,  256,  257,  261,  264,  265,  266. 

Felspathic  sandstones,  205. 

Felspathoids,  241. 

Fenestella,  359,  361,  366. 

Fenestella  beds,  359,  360. 

Fermanagh,  Old  Red  Sandstone,  344. 

Ferns,  285,  286  ;  Palaeozoic,  317  ; 
Devonian,  346  ;  Carboniferous, 

351,  355  ;    Jurassic,  390,  407,  410, 
422,  427. 


Ferro-magnesian  minerals,  241. 
Ferruginous  conglomerate,  202. 
Ferruginous  quartzose  conglomerate, 

202. 

Ferruginous  quartz,  192. 
Ferruginous  rock,  200,  215,  217. 
Ferruginous  sandstone,  128,  204. 
Ferruginous  springs,  26. 
Field  geology,  elements  of,  537-541  ; 
equipment,     538,     539 ;      general 
procedure,  539-541. 
Fig,  421,  436,  443. 
Fiji,  coral  reefs,  105. 
Finland,   pre-Cambrian   rocks,    313 ; 
Ordovician  plants,  328,  329  ;   Silu- 
rian, 333  ;   Cainozoic,  471,  481. 
Fiords,  498. 

Fire-clay,  207,  213,  355,  356. 
Firs,  452. 

Fishes,  271,  281,  286,  328,  335,  338, 
340,  342,  345,  346,  350,  366,  368, 
373,  380,  383,  387,  389,  390,  392, 
394,  395,  396,  399,  408,  410,  411, 
416,  426,  429,  440,  442,  443,  450, 
456,  457,  459,  477. 
Fissure- veins,  516,  517,  519. 
Fissured  rocks,  125. 
Flagstone,  204,  331. 
Flagstones,  510. 
Flamborough  Head,  410. 
Flexible,  185. 
Flies,  390. 
Flints,  130,  184,  192,  200,  210,  211, 

217,  218,  414,  417,  435,  456. 
Flood-plain,  50. 
Floods,  effects  of,  33,  34. 
Flora.     See  Plants. 
Florida  limestone,  451. 
Flow  structure,  225,  226,  240. 
Fluoride  of  calcium,  196. 
Fluorine,  affinity  of,  174,  175. 
Fluorite,  190,  196',  517. 
Fluor-spar,  on  scale  of  hardness,  184. 

See  Fluorite. 
Fluviatile  beds,  111. 
Flysch,  402,  418,  419,  434,  438,  441, 

443,  444,  445,  446,  451,  459,  481. 
Folded  mountains,  488,  489. 
Folding    of    sediments,    5,    125 ;     of 

strata,  137-150,  335,  340. 
Folds,   different  forms  of,   139-143, 
350,  353,  365,  430-431,  447,  448, 
451,  453,  488,  489. 
Folia,  264. 
Foliated  rocks,  264. 
Folkestone  beds,  411. 


INDEX. 


573 


Footwall,  159,  520. 

Foramen,  276,  277. 

Foraminifera,  101,  102,  208,  268,  271, 
272,  354,  389,  407,  414,  429,  432, 
434,  438,  439,  444,  446,  449,  450, 
451,  453,  455,  477,  481. 

Foreland  sandstone,  346. 

Forest  marble,  394,  395. 

Forest  sandstone,  205. 

Forests,  436,  468 ;  submerged,  133, 
134. 

Formation  of  crystals,  177. 

Formations,  difficulty  of  correlating 
old,  317,  318  ;  extent  not  related 
to  outcrop,  318,  319. 

Formby  Point,  submerged  forest  at, 
134. 

Fossils  in  sediments,  97,  98,  109,  268- 
290 ;  preservation,  268,  269 ; 
classification  of  animals,  270-285  ; 
Protozoa,  272  ;  Porifera,  272  ; 
Ccelenterata,  272,  273  ;  Echinoder- 
mata,  273-275  ;  Annulata,  276  ; 
Molluscoidea,  276,  277  ;  Mollusca, 
277-279 ;  Arthropoda,  279-281  ; 
Vertebrata,  281-285;  vegetables, 
285-286 ;  uses,  286,  287  ;  as 
time  registers,  287-288;  Homo- 
taxis,  288-289. 

Fox,  455,  457. 

Fox  glacier,  57. 

Fracture  of  minerals,  184. 

Fractures  of  rocks,  125,  155. 

Fragmentary  ejecta,  229. 

Fragmentary  sediments,  199. 

France,  Armorican  and  Variscan 
mountain  chains  in,  147  ;  faults 
in  Northern,  168,  169 ;  pre- 
Cambrian  rocks,  313  ;  Cambrian, 
319,320;  Ordovician,  326 ;  Silu- 
rian, 333  ;  Devonian,  342,  344  ; 
(Carboniferous,  349  ;  Permian,  363; 
Jurassic,  388,  391,  393,  399,  400, 
403;  Cretaceous,  405,  406,  411, 
415,  416,  417,  418;  Cainozoic, 
433,  441,  442,  443,  447,  449,  454, 
458,  480,  481. 

Franz  Josef  glacier,  57,  468. 

Free  elements,  174,  176. 

Freestones,  204. 

Freshwater  West,  submerged  forest 
at,  134. 

Friction  breccia,  163,  164,  168,  201, 
202. 

Fringing  beach,  90. 

Fringing  coral  reefs,  110. 


Frogs,  271,  281. 

Frost,  action  of,  15,  27,  28. 

Fucoids,  321. 

Fugi,  285. 

Fulgurites,  130,  131. 

Fuller's  Earth,  207,  391,  393,  394. 

Fumaroles,  177,  511,  525. 

Funafuti,  coral  reef  at,  103,  104,  134, 

209. 
Fusus,  407,  413,  416,  435,  437. 

GABBRO,  238,  242,  243,  252,  258, 419  ; 
Lewisian,  312. 

Gaisa  beds,  324. 

Gaj  Series,  444,  450. 

Galena,  186,  189. 

Galesaurus,  383. 

Galway,  Ordovician,  326. 

Gangamopteris,  361,  367,  368,  370. 

Ganges,  plains  near  the,  51. 

Gangue,  177,  517. 

Gannister,  207,  356. 

Ganodus,  394. 

Ganoidei,  281,  282  ;    Palaeozoic,  317. 

Gaps  in  the  geological  record,  301, 
302. 

Gardonia,  378. 

Garnet,  189,  203,  248,  261,  264,  265, 
266. 

Gar-pikes,  389,  411. 

Gas,  332. 

GaseB  from  volcanoes,  231. 

Gash-veins,  515,  516. 

Gasteropoda,  271,  278,  279  ;  Cam- 
brian, 321  ;  Ordovician,  328  ; 
Silurian,  334;  Devonian,  345, 
347;  Carboniferous,  351,  354; 
Permian,  366,  379,  381,  384; 
Jurassic,  389,  393,  394,  395,  396, 
398  ;  Cretaceous,  407  ;  Cainozoic, 
429,  440,  442,  427,  480. 

Gastrioceras,  351. 

Gault,  409,  413,  414,  415. 

Geanticlinals,  141,  149. 

Genera,  270. 

Geodes,  183. 

Geological  record,  302-305. 

Geological  time,  beginning  of,  3  ; 
age  of  the  earth,  7,  8  ;  succession 
of  life,  8 ;  periods  distinguished 
by  kinds  of  life,  9  ;  changes  of  life 
during,  297,  298 ;  duration  of, 
in  years,  304,  305  ;  duration  of 
Glacial  Period,  461. 

Geology,  principles,  1-12  ;  scope  of, 
13-14;  purpose,  1;  history  of 


574 


A   TEXT-BOOK   OF   GEOLOGY. 


the  science,  14 ;  surveying,  541-^ 
550 ;  observation  of  strike  and 
dip,  541-545 ;  measuring  thick- 
ness of  strata,  545-547  ;  maps 
and  sections,  547-550. 

Georgia,  Cambrian  in,  319 ;  Cre- 
taceous, 420. 

Georgia  group,  321. 

Geosynclinals,  141,  149. 

Germany,  brown  coals  of  North,  214  ; 
Ordovician,  326  ;  Devonian,  342, 
345  ;  Permian,  363,  364,  365,  366, 
368,  370,  371  ;  Trias,  374,  376, 
389  ;  Jurassic,  388,  391,  400,  403  ; 
Chalk  absent  in  Central,  405 ; 
Cretaceous  deposits,  417,  418,  422  ; 
Cainozoic,  440,  441,  443,  447,  454, 
461,  471,  480. 

Gervillia,  375,  376,  379,  407,  411,  423. 

Geysers,  27,  232,  525. 

Giant's  Causeway,  437. 

Gigantosaurus,  397. 

Gilbert  group  of  islands,  107. 

Gippsland,  Carboniferous,  361  ;  Cain- 
ozoic, 452. 

Giraffes,  455. 

Glacial  benches,  69. 

Glacial  conglomerate,  Permian,  368. 

Glacial  drift,  65,  71. 

Glacial  lakes,  76-78. 

Glacial  Period,  461-476. 

Glacial  rock-terraces,  69,  70. 

Glacial  striae,  67,  68. 

Glacial  terraces  in  New  Zealand,  77, 
78. 

Glaciation,  Cambrian,  324 ;  Per- 
mian, 369,  370;  Post-Pliocene, 
462-475. 

Glacier-rivers,  60,  61,  71. 

Glacier  tongues,  58. 

Glaciers,  action  of,  16,  54-79  ;  dis- 
tribution, 54,  59 ;  motion,  55 ; 
size,  57,  58 ;  rate  of  flow,  58  ; 
ablation,  60  ;  rivers  of,  60,  61  ; 
retreat,  61  ;  as  transporters,  62- 
65  ;  as  erosive  agents,  66,  72,  73, 
75-78 ;  thickness  of,  66 ;  fluvio- 
glacial  work  of,  71  ;  as  dams  for 
lakes,  76-78  ;  Cainozoic,  431,  461- 
476,  482,  483. 

Glarus,  443. 

Glasgow,  shale,  357. 

Glassy  lustre,  186. 

Glassy  matrix  of  magma,  239,  240. 

Glauconite,  187,  197,  198,  204,  405, 
411,  412,  413,  414,  424,  435,  443. 


Glauconitic  sandstone,  204. 

Glenroy,  parallel  roads  of,  77,  500. 

Globigerina,  102,  272,  407,  414. 

Globigerina  ooze,  101. 

Glossopteris,  360,  361,  367,  368,  370, 
380,  382,  383. 

Gloucestershire,  Stonesfield  slate,  394. 

Glyders,  332. 

Gneiss,  171,  264,  265  ;  Laurentian, 
307,  309 ;  Algonkian,  309  ;  pre- 
Cambrian,  311,  315 ;  Lewisian, 
312,  313. 

Gneissic  texture,  249. 

Gold  in  sea-water,  29  ;  free,  as  solid, 
174 ;  native,  189  ;  Palaeozoic, 
317 ;  Cambrian,  323 ;  Ordo- 
vician, 332  ;  Cainozoic,  451,  453, 
460  ;  in  placer  deposits,  506,  507  ; 
in  reefs,  511  ;  in  stockwork  de- 
posits, 511  ;  in  veins,  519  ;  in 
sinters,  525  ;  in  sands  and  gravels, 
527. 

Golden  Bay,  New  Zealand,  84. 

Gondwana  Land,  367,  401,  402,  427, 
445,  446. 

Gondwana  System,  366,  367,  368, 
370,  380. 

Goniaster,  393. 

Goniatites,  345,  347,  351,  354. 

Goniometer,  183. 

Gorge,  36,  37. 

Gossan,  197. 

Graben,  168. 

Grade-level,  42. 

Graham's  Land,  475. 

Grampian  sandstone,  205. 

Grand  Canon,  Colorado,  37  ;  fossils, 
310. 

Granite,  188,  191,  205,  231,  238,  241, 
243,  245,  246,  249-251,  254,  258, 
308  ;  crumbling  down  of,  17,  32  ; 
decomposition  of,  21  ;  joints  in, 
152  ;  spheroidal  weathering,  228  ; 
metamorphism,  260,  261  ;  Laur- 
entian, 307,  308,  309,  456  ;  Algon- 
kian, 309;  pre-Cambrian,  311- 
315;  erratics,  470  ;  building  stone, 
509. 

Granite-conglomerate,  202. 

Granite-porphyry,  250. 

Granitoid  texture,  248. 

Granulitic  texture,  248. 

Graphic  texture,  249. 

Graphite,  209,  214,  260,  266,  273. 

Graphite -schist,  312. 

Graphite-slate,  266. 


INDEX. 


575 


Graptolites,  271,  273,  373;  Palae- 
ozoic, 317  ;  Cambrian,  320  ;  Ordo- 
vician,  326, 327, 328,  329,  330,  331  ; 
Silurian,  334,  336,  338,  345. 

Grasses,  285. 

Gravel,  transportation  of,  88,  89 ; 
sorted  by  action  of  the  sea,  91-93  ; 
distribution  in  estuaries,  93,  94 ; 
deposition  of,  113  ;  cementing 
medium  of,  126,  127. 

Gravels,  32,  34,  100,  101,  113,  117, 
118,  119,  188,  207,  471,  476. 

Great  Barrier  reef,  107. 

Great  Britain,  erratics,  33  ;  faults, 
168,  169  ;  coals  and  anthracites, 
214;  Cambrian,  320;  Carboni- 
ferous, 350,  352. 

Great  Ice  Age,  54,  60. 

Great  Ice  Barrier,  59,  475. 

Great  Pfahl,  518. 

Great  Salt  Lake,  216,  473. 

Greece,  Cretaceous,  406,  418  ;  Cain- 
ozoic,  454. 

Green,  colour,  187. 

Green,  L.,  9. 

Greenland,  coast  of,  134 ;  Silurian, 
333  ;  Cretaceous,  406,  418  ;  Cain- 
ozoic,  434,  448,  452,  476. 

Greensand,  204,  409,  424. 

Greenstones,  264. 

Greenstone-schist,  307. 

Gregory,  J.  W.,  9. 

Greta  Coal-Measures,  361. 

Grey  marl,  424. 

Grey  wethers,  436. 

Greywackes,  205,  206,  228,  229; 
Cambrian,  320;  Ordovician,  326, 
330  ;  Devonian,  342. 

Grindstones,  510. 

Grits,  205  ;  Torridonian,  312  ;  Palae- 
ozoic, 317  ;  Cambrian,  320,  322  ; 
Ordovician,  326,  329  ;  Silurian, 
338 ;  Devonian,  342,  344,  346 ; 
Carboniferous,  350,  353,  355,  361. 

Gritstones,  205,  355. 

Grossularite,  260. 

Ground- moraines,  65. 

Groups,  303. 

Gryphsea,  392,  396,  397. 

Gushers,  27. 

Gymnosperms,  285,  351 

Gympie  Series,  361. 

Gypsum,  on  scale  of  hardness,  184  ; 
crystalline  form,  selenite,  189 ; 
occurrence,  216,  218,  340 ;  Per- 
mian, 368,  370,  371  ;  Mesozoic, 


372  ;  Trias,  374,  375,  376,  378/382, 
384 ;     Jurassic,    387  ;     Cainozoic, 
439,  458. 
Gyrodus,  398. 

HACKLY  fracture,  184. 

Hade  of  faults,  159. 

Haematite,  29,  197,  204,  241,  248. 

Hail,  prints  of,  120,  121. 

Haliotis,  460. 

Halobia,  379. 

Halysites,  336. 

Ham  Hill  stone,  210. 

Hamites,  408,  413,  422,  428,  481. 

Hampshire    Basin,    434,    435,    436, 

441,  442. 

Hamstead  beds,  442. 
Hanging  valleys,  73. 
Hanging  wall,  520. 
Hangman  grits,  346. 
Hanover,  Cretaceous,  406,  418. 
Hardened  rock,  125-130. 
Hardness  of  water,  25  ;   of  minerals, 

184. 

Hares,  450. 
Harlech,  322. 
Harlech  Series,  321,  322. 
Harpoceras,  404. 
Harsh  feel,  187. 
Harz     Mountains,    364,     368,     471, 

518. 

Hastings  sand,  409,  410,  411. 
Hauraki    Peninsula,    New    Zealand, 

volcanic  activity,  234,  235 ;  igneoas 

rocks,  244  ;    Siliceous  sinter,  424. 
Hawaii,  volcanic  rocks,  244. 
Hawkesbury    sandstone,    205,    381, 

383. 

Hazel,  452. 
Headon  beds,  442. 
Heat  of  contraction,  7  ;    loss  of,  by 

the  earth,   10,   1,1  ;    in   metamor- 

phism,  259,  260,  261,  262,  263,  267. 
Heave,  164. 

Heave,  apparent,  160,  164,  165,  524. 
Heavy  soils,  24. 
Hebrides,  Jurassic,  391. 
Heim,  149,  150. 
Hekla,  219,  220. 
Helderbergian,  347. 
Heliolites,  336. 
Heliotrope,  192. 
Helix,  442,  443,  472. 
HelveUyn,  331. 
Helvetian,  450. 
Hemiaster,  407. 


576 


A   TEXT-BOOK   OP   GEOLOGY. 


Hemicidaris,  389. 

Herbivores,  429. 

Herculaneum,  221. 

Hercynian  fold,  353,  365. 

Heterocercal,  282. 

Hexagonal  system,  182. 

Hickory,  421. 

Highlands,  New  South  Wales,  37. 

Highlands,  Scotland,  -gneiss,  264 ; 
mica-schist,  265  ;  Ordovician,  326. 

High- water  mark,  81. 

Hills,  486. 

Himalayas,  snow-line  in  the,  54 ; 
glaciers,  59  ;  comparative  youth, 
147  ;  included  in  the  Variscan  fold, 
147, 148  ;  effect  on  specific  gravity, 
233 ;  Eozoic  rocks,  306 ;  Ordo- 
vician, 326  ;  Mesozoic,  372,  373  ; 
Jurassic,  389,  401  ;  Cretaceous, 
406,  419  ;  Cainozoic,  430,  431,  434, 
438,  441,  445,  447,  448,  458,  461, 
472,  482. 

Hindu  Kusch,  147,  148. 

Hinnites,  395. 

Hippopodium,  392. 

Hippopotamus,  448,  449,  457,  469, 
472,  473. 

Hippotherium,  455. 

Hippurites,  407,  416,  417,  418,  419, 
422,  427,  428. 

Hokonui  System,  381. 

Holaster,  407,  417. 

Holland,  Cretaceous,  416. 

Hollow  casts,  268. 

Holocrystalline,  240,  248,  252,  253, 

254,  255,  257,  258. 
Homalonotus,  334. 
Homo,  478. 
Homocercal,  282. 

Hooker  glacier,  New  Zealand,  707. 
Horizontal  shaft,  166. 
Hornblende,  190,  195,  203,  241,  242, 
245,  247,  248,  249,  251,  252,  254, 

255,  257,  261,  264,  265. 
Hornblende -andesite,  257. 
Hornblende-diabase,  255. 
Hornblende-dolerite,  255. 
Hornblende-granite,  249. 
Hornblende-lamprophyre,  255. 
Hornblende-peridotite,  252. 
Hornblende-porphyrite,  254. 
Hornblende-schist,  265,  307. 
Hornblende-syenite,  251. 
Hornstone,  206. 

Horse,  440,  450,  455,  456,  457,  459, 
472. 


Horsetails,    285  ;     Palaeozoic,    317  ; 

Carboniferous,  351,  355. 
Horsts,  491. 
Howchin,  324. 
Humboldt  glacier,  59. 
Humic  acid,  24. 
Hungary,    brown    coals    of    South, 

214. 

Hunstanton,  414. 
Huronian,  307,  309. 
Hutton,  J.,  14. 
Hysena,  457,  473  ;   dens,  480. 
Hybodus,  376,  394,  424. 
Hydration,  23,  242. 
Hydraulic  cement,  209,  217,  510. 
Hydraulic  limestone,  209. 
Hydrocarbons  in  coal,  212. 
Hydrosphere,  1. 
Hydrous,  187. 
Hydrozoa,  271,  272,  273. 
Hyopota'mus,  442. 
Hypabyssal  rocks,  237,  238,  245,  247, 

253-255,  258. 
Hyperodapedon,  378. 
Hypersthene,  196,  241,  248,  252,  257. 
Hypersthene  andesites,  257. 
Hypidiomorphic  structure,  240. 
Hypogenic  action,  155. 
Hythe  beds,  411. 

ICE,  viscosity  of,  56  ;  expansion  of, 
58 ;  distribution  of,  59 ;  melting- 
point,  61 ;  erosive  effects  of  float- 
ing, 84,  108 ;  a  solid,  176  ;  boulders 
scratched  by,  361  ;  Post-Pliocene, 
461-476. 

Icebergs,  59,  84. 

Ice-cascade,  59. 

Iceland,  lava-flow,  220 ;  Tertiary 
land  connections,  430  ;  Cainozoic, 
434,  437,  448. 

Icelandic  type  of  volcanic  eruption, 
220,  235. 

Ice-tables,  60. 

Ichthyornis,  409. 

Ichthyosaurians,  281,  283,  428. 

Ichthyosaurus,  390,  392,  396,  397, 
408,  481. 

Idaho,  220  ;  Algonkian  of,  309. 

Idiomorphic  structure,  240. 

Idocrase,  261. 

Igneous  rocks,  origin  of,  5 ;  part 
played  by,  5,  6,  11  ;  alteration  of, 
6,  11  ;  classification,  99,  241,  242, 
244-246,  247  ;  occurrence  and 
properties,  236-247  ;  mode  of 


INDEX. 


577 


occurrence,  236-239 ;  texture, 
239-241  ;  composition,  241-242  ; 
alteration,  242 ;  petrographical 
provinces,  242,  243 ;  magmatic 
differentiation,  243  ;  Atlantic  and 
Pacific  types,  243-244. 

Iguanodon,  397,  408,  411. 

Ilfracombe  beds,  346. 

Illaenus,  328,  334,  338. 

Illinois  ores,  332  ;  Silurian,  333. 

Ilmenite,  252,  257. 

Impervious  rocks,  25. 

Impregnations,  512,  513. 

Inclined  rocks,  125. 

India,  raised  beaches,  132  ;  specific 
gravity  observations,  233  ;  gneisses 
of  North,  313  ;  Cambrian,  319  ; 
Ordovician,  325  ;  Silurian,  333  ; 
no  Devonian,  344  ;  Carboniferous, 
349,  359,  360  ;  Permian,  363,  366, 
367,  368,  370;  Mesozoic,  372; 
Trias,  374,  378,  380,  382,  383; 
Jurassic,  388,  389,  400,  401,  403  ; 
Cretaceous,  417,  419,  423,  427  ; 
Cainozoic,  430,  433,  438,  439,  440, 
444,  446,  448,  450,  451,  454,  458, 
459,  481. 

Indian  Ocean,  448,  451  ;  coral  reefs, 
103. 

Indiana,  Silurian,  333. 

Induration  of  rocks,  4. 

Indies,  flood  caused  by  landslide  in 
valley,  33,  34. 

Influence  of  rapid  flow  of  water,  31, 
35. 

Infusorial  earth,  211,  528. 

Inliers,  145,  146,  150. 

Inoceramus,  407,  413,  428. 

Insecta,  271,  335,  366,  394. 

Insectivores,  448. 

Insects,  271,  392,  396,  426,  443. 

Interglacial  drifts,  471. 

Interglacial  loads,  63. 

Interglacial  Period,  463. 

Intermediate  group  of  rocks,  241, 
242,  245,  247. 

Internal  casts,  268,  269. 

Intrusive  magma,  236. 

Inverted  folds,  140. 

Iowa,  ores,  332. 

Irawadi  beds,  459. 

Ireland,  valley  train  in  Central  Plain, 
72 ;  fauna  of,  96 ;  Armorican 
mountain  chain  in,  447  ;  marbles, 
209  ;  lignites,  214  ;  pre- Cambrian, 
311;  Ordovician,  326 ;  Silurian, 


333  ;  Devonian  elevation,  343  ; 
Old  Red  Sandstone,  344  ;  Carboni- 
ferous, 354  ;  coalfields,  358  ;  Car- 
boniferous volcanic  rocks,  358 ; 
Permian,  367 ;  Cainozoic,  434, 
436,  437,  462,  466,  467. 

Irish  Sea,  466,  467,  470. 

Iron,  free,  as  solid,  174 ;  affinity  of, 
175  ;  its  percentage  in  the  earth's 
crust,  176  ;  a  mineral,  189  ;  native, 
197. 

Iron  age,  483. 

Iron  ores,  190,  196,  197  ;  Keewatin, 
307  ;  Palaeozoic,  317  ;  Cleveland, 
392  ;  in  deposits,  506. 

Iron  oxides,  197. 

Iron  peroxide,  197. 

Iron  protoxide,  197. 

Ironstones,  200,  210,  350,  355,  357, 
362,  396,  411. 

Isastraea,  398. 

Isle  of  Arran,  Permian,  367. 

Isle  of  Man,  Ordovician,  326  ;  Car- 
boniferous volcanic  rocks,  358 ; 
Glacial  Period,  467. 

Isle  of  Mull,  415,  437. 

Isle  of  Skye,  437. 

Isle  of  Staffa,  437. 

Isle  of  Wight,  monoclinal  fold  in,  140, 
141  ;  Cretaceous,  409  ;  Cainozoic, 
434,  441,  442,  448. 

Isoclinal  fold,  140,  149. 

Isometric  System,  178,  179. 

Isopods,  477. 

Italy,  raised  beaches  in  South,  132  ; 
Carrara  marble,  209 ;  Boticino 
stone,  210  ;  Cretaceous,  406,  418  ; 
Cainozoic,  443,  454,  458,  481  ; 
minerals,  511. 

JADE,  185. 

Jakutat  earthquakes,  62. 

Jakutat  Bay  glaciers,  61. 

Jamieson,  77. 

Japan,  mineral  springs,  27  ;  vol- 
canoes, 232  ;  Carboniferous,  349  ; 
Trias,  378,  379 ;  Jurassic,  388  ; 
Cretaceous,  406,  417,  423  ;  Cain- 
ozoic, 448. 

Jasper,  192  ;  Keewatin,  307. 

Java,  232,  439. 

Jenolan  caves,  New  South  Wales, 
26,  340. 

Jet,  213. 

Joints,  125,  151-155,  171,  172; 
structure,  151 ;  causes  of,  153-155. 

37 


578 


A   TEXT-BOOK   OF   GEOLOGY. 


Jolly,  age  of  the  earth,  8. 

Jubbulpore  Series,  380. 

Jupiter  Serapis,  pillars  at  Temple  of, 
Bay  of  Baise,  134,  135. 

Jura,  391,  400. 

Jurassic  Period,  304,  372,  426  ; 
mammals  of  Lower,  285,  386-404  ; 
rocks,  386 ;  different  facies  of 
deposits,  386-388  ;  distribution, 
388-389;  fauna  and  flora,  389- 
390  ;  subdivisions,  390-404  ;  Lias, 
391-392  ; .  Lower  Oolite,  392-395  ; 
Middle  Oolite,  395-397  ;  Corallian, 
396-397  ;  Upper  Oolite,  397-399  ; 
Kimeridgian,  397-398;  Portland- 
ian,  398  ;  Purbeckian,  398-399  ; 
Jurassic  in  other  countries,  399- 
404;  France,  399-400  ;  Germany, 
400 ;  Russia,  400  ;  India,  400-401 ; 
North  America,  401-402  ;  Austra- 
lasia, 402 ;  zonal  distribution  of 
faunas,  402,  403 ;  Jurassic  bio- 
logical zones,  403-404. 

Juvavian  province,  379. 

KAIBAB  structure  of  mountain,  491. 

Kaipara,  New  Zealand,  flints  at,  130. 

Kaitanga,  New  Zealand,  coals  at,  213. 

Kalgoorlie,  46,  264. 

Kames,  72. 

Kamthi  stage,  380. 

Kamtschatka,  232,  448. 

Kansas,  Permian,  366,  369. 

Kaolin,  21,  206,  207,  242,  510. 

Karoo  System,  382,  383. 

Kaskaskia  Series,  359. 

Kawarau  Gorge,  45. 

Keewatin  Series,  307,  308  ;    volcanic 

lavas  and  tuffs,  308. 
Keweenawan  Series,  307,  309. 
Kellaways,  395. 
Kellaways  rock,  395. 
Kelvin,  age  of  the  earth,  7. 
Kent,  Cretaceous,   410,   411  ;    Cain- 

ozoic,  435,  455,  456. 
Kentish  Rag,  411. 
Kent's  Hole,  480. 
Kentucky,  White  Stone,  210. 
Keuper,  374,  376,  377,  378. 
Khirthar  Series,  439. 
Kiger  stage,  369. 
Kilauea,  219,  220. 

Kimberley,  decomposed  rock  at,  22. 
Kimeridge  clay,  397. 
Kimeridgian,  391,  397,  398. 
Kinderhook  Series,  359. 


King  County,  New  Zealand,  con- 
glomerate, 203. 

King-crabs,  280,  281. 

Kirkdale  Cave,  480. 

Knob  and  Basin  scenery,  75,  76. 

Knorria,  346. 

Kojak  shales,  444. 

Korea,  Cambrian  in,  320. 

Kotchurla  river,  472. 

Krakatoa,  97,  221,  222. 

Krakatoan  type  of  volcanic  eruption, 
220-223,  235. 

Kurile  islands,  232. 

LABRADOR,  fauna  of,  96. 

Labradorite,  194,  252. 

Labyrinthodonts,  283,  350,  366,  368. 

Laccadive  Islands,  107. 

Laccolith,  238,  246,  255. 

Lacustrine  beds,  111,  117,  215,  217, 
218. 

Lafayette  Series,  459. 

Lahontan  lake,  473. 

Lake  basins  overdeepened,  74. 

Lake  District,  Ordovician,  326,  331, 
332  ;  scenery,  331  ;  Silurian,  333, 
336. 

Lake  Superior,  copper  and  iron  ores, 
314. 

Lakes,  early,  3  ;  filling  up  of,  49-51  ; 
Jurassic,  386,  387  ;  *  Cainozoic, 
431,  436,  443,  447,  449,  450,  454, 
458,  473  ;  kinds  of,  498-501. 

Laki  Series,  439. 

Lamellse,  193. 

Lamellibranchiata,  278  ;  Cambrian, 
320  ;  Ordovician,  328  ;  Silurian, 
334 ;  Devonian,  347  ;  Carboni- 
ferous, 351  ;  Permian,  366  ;  Trias, 
379,  381,  384;  Jurassic,  389,  392, 
393,  394,  395,  396,  397,  398,  404 ; 
Cretaceous,  407,  421,  423  ;  Cain- 
ozoic, 429,  435,  437,  440,  442,  455, 
460,  477. 

Laminae,  115,  116,  117,  123. 

Lamination,  115,  117,  123. 

Lamna,  424,  436. 

Lamplugh,  412. 

Lamprophyre,  253,  254,  255. 

Lamp-shells,  271. 

Lanarkshire  coal  beds,  357. 

Lancashire,  Carboniferous,  354,  357  ; 
Glacial  Period,  467,  470. 

Lancefield  Series,  331. 

Land,  connection  of,  between  Mozam- 
bique and  Malabar,  107 ;  Per- 


INDEX. 


579 


mian,  364,  365,  367  ;  oscillations 
of,  in  Mesozoic  era,  372  ;  in  Trias, 
374;  in  Jurassic,  386,  388,  389, 
398,  400,  401,  402;  Cretaceous, 
405,  406,  410,  426  ;  connections 
in  Cainozoic,  431  ;  Cainozoic,  441, 
445,  446,  447,  448,  450,  452,  453, 
454. 

Landenian,  416. 

Landslides,  33,  34. 

Land  surface,  1,  101,  235,  364  ;  pre- 
Cambrian,  315,  316 ;  Carboni- 
ferous, 352. 

Lapilli,  219,  229. 

Lapworth,  330. 

Laramie  formation,  417,  420,  421, 
425. 

Lateral  axes,  179,  180. 

Lateral  cones,  224,  225. 

Lateral  secretion  theory  of  vein 
formation,  528. 

Lateral  thrust,  138. 

Laterite,  208,  507. 

Latrobe,  452. 

Laurel,  429,  436,  437,  442. 

Laurentian  Period,  304,  307  ;  rocks, 
309. 

Laurvikite,  469. 

Lava,  flow  of  basaltic,  in  Victoria, 
47,  48  ;  joints  in,  153  ;  Keewatin, 
307  Palaeozoic,  317  ;  Ordovician, 
331  Carboniferous,  350,  354,  358, 
361  Cretaceous,  419  ;  Cainozoic, 
436,  451,  453. 

Lavas,  188,  225,  226,  228,  230,  234, 
236,  238,  239,  240,  242,  243,  246, 
247,  255,  256,  257,  258,  259,  327, 
329,  331,  345,  358,  361. 

Law  of  liquation,  7. 

Layers,  504. 

Lead,  a  mineral,  189  ;  in  sandstone, 
511. 

Lead  ores,  332,  517. 

Leasowe,  submerged  forest  at,  134. 

Lebanon,  422. 

Leda,  443,  457,  471. 

Ledbury  shales,  336. 

Leicestershire,  pre-Cambrian,  311. 

Leithakalk  limestone,  449. 

Lemuroids,  448. 

Lenham  beds,  455,  456. 

Lenticular  form  of  marine  deposits, 
93. 

Lepidodendron,  351,  360,  361,  373. 

Lepidolite,  194. 

Lepidotus,  410. 


Leptaena,  328. 

Leucite,  241. 

Lewis,  Isle  of,  Lewisian  rocks  of,  312. 

Lewisian  Period,  304  ;   rocks  of,  312. 

Lias,  391,  392,  400,  401. 

Libya,  422,  438. 

Life,  3  ;  succession  of,  8  ;  origin  of, 
8,  10 ;  Cambrian  and  pre-Cam- 
brian, 8 ;  distinction  of  periods, 
9  ;  sea  as  a  source  of,  97.;  sea  as 
a  highway  for,  97  ;  changes  during 
geological  time,  297,  298,  481  ;  no 
evidence  of,  in  Archaean  rocks,  308. 

Light  soils,  24. 

Lignite,  211,  213,  214,  235,  421,  425 
436,  437,  439,  442,  443,  459,  508. 

Lignitic  Series,  417. 

Lima,  376,  392,  393,  394,  395,  396, 
397,  402,  416,  423. 

Limbs,  139. 

Lime,  241,  425,  452. 

Limestones,  5,  100,  103,  128,  188, 
198,  200,  209,  215,  216,  217,  228, 
231,  260,  284';  action  of  rain  on, 
21  ;  of  spring  water  on,  25; 
crumbling  down,  32  ;  at  Funafuti, 
103,  104,  134;  lacustrine,  108; 
formed  of  calcite,  196  ;  Algonkian, 
309  ;  Dalradian,  312  ;  Palaeozoic 
317;  Cambrian,  319,  320,  322; 
Ordovician,  326,  327,  328,  329, 
331,  332  ;  Silurian,  334,  335,  336, 
338,  340  ;  Devonian,  342,  344,  346, 
347,  348  ;  Carboniferous,  350,  351, 
352,  353,  354,  355,  356,  359,  360, 
362  ;  Permian,  363,  364,  365,  366, 
367,  368  ;  Mesozoic,  372  ;  Trias 
374,  375,  379,  380,  381,  384; 
Jurassic,  386,  387,  389,  391,  392, 
393,  394,  396,  397,  398,  399,  400, 
403;  Cretaceous,  405,  411,  416, 
417,  418,  419,  420,  422,  423,  424, 
425  (see  also  Chalk) ;  Cainozoic, 
429,  430,  433,  434,  435,  437,  438, 
439,  441,  442,  444,  449,  451,  453; 
for  building,  509,  510. 

Limnsea,  438,  442,  443. 

Limonite,  17,  22,  26,  127,  197. 

Limonitic  conglomerate,  128,  129. 

Limonitic  sandstone,  128,  129. 

Lincolnshire,  Cretaceous,  410 ;  Glacial 
Period,  467,  469. 

Lingula,  375,  477. 

Lingula  flags,  321,  322. 

Lingulella,  320. 

Lipak  Series,  359. 


580 


A   TEXT-BOOK   OF   GEOLOGY. 


Lipari  Islands,  257. 

Liquation,  law  of,  7. 

Lithia-mica,  194. 

Lithosphere,  1,  7,  11,  12. 

Lithostrotion,  350. 

Littoral  zone,  96. 

Lizards,  382,  401. 

Llanberis  Series,  321. 

Llandeilo  beds,  329,  330,  331. 

Llandovery  (Lower)  beds,  330. 

Llandovery  Series,  336. 

Llandovery  (Upper)  beds,  336. 

Loam,  207. 

Lobsters,  trails  of,  122  ;    Cainozoic, 

477. 
Loch  Maree,   Dalradian  rocks,   312, 

313. 

Local  earth-movements,  132. 
Lockatong  group,  383. 
Lodes,  518,  524. 
Loess,  207,  472. 
London  Basin,  434-436. 
London  clay,  434,  435. 
Longitudinal  rivers,  46. 
Longmynd,  pre- Cambrian,  311. 
Lorraine  beds,  330. 
Low  Archipelago,  107. 
Low-water  mark,  81. 
Lower     Marine     Series,     Australian 

Carboniferous,  361. 
Lowville  limestone,  330. 
Lucina,  396,  457. 
Ludlow,  338. 

Ludlow  bone  bed,  335,  338. 
Ludlow  (Lower)  shales,  336,  338. 
Ludlow  Series,  336,  338. 
Ludlow  (Upper)  beds,  336,  338. 
Lustre,  186. 

Lycopods,  346,  351,  355. 
Lydian  stone,  206. 
Lyell,  C.,  14,  432. 
Lynton  slates,  346. 
Lyons  glacial  conglomerate,  369. 
Lytoceras,  403,  404. 

MAASTRICHT  chalk,  416. 

Macclesfield,  470. 

Macles,  183. 

Mactra,  448. 

Magas,  407. 

Magellania,  394,  395,  460,  477. 

Magma,  5,  233,  236,  239,  241,  243, 
245,  246,  256,  261,  262  ;  connec- 
tion of  ores  with  eruption  of,  524 
et  seq. 

Magmatic  differentiation,  243. 


Magmatic  segregation,  524,  525. 

Magmatic  water,  233. 

Magnesia,  241. 

Magnesian  limestone.     See  Dolomite. 

Magnesite,  265. 

Magnesium,  its  percentage  in  the 
earth's  crust,  176. 

Magnesium  salts,  370. 

Magnetic  bearings,  conversion  to 
true  bearings,  551,  552. 

Magnetite,  29,  197,  203,  241,  242, 
248,  252,  257,  264,  265,  525,  526. 

Magnolia,  429,  452. 

Mahadeva  Series,  380. 

Mainz  basin,  447. 

Maikop  oilfield,  gushers  in,  27. 

Maitland,  G.,  369. 

Malabar  land  connection  with  Mozam- 
bique, 107. 

Malachite,  187. 

Malaspina  glaciers,  59,  60,  468. 

Malaysia,  Cainozoic,  446,  481. 

Maldive  Islands,  107. 

Maleri  stage,  380. 

Malleable,  185. 

Malm,  400. 

Malvern,  anthracite,  212. 

Malvern  Hills,  pre -Cambrian,  311. 

Mammalia,  271,  281,  284,  285,  286, 
373,  399,  402,  442,  448,  449,  450, 
454,  455,  456,  457,  458,  459,  460, 
468,  471,  472,  474,  477,  479,  480, 
481,  482,  483. 

Mammals,  281,  284,  285,  407,  409  ; 
tracks  of,  122  ;  Jurassic,  390,  402  ; 
Triassic,  426  ;  Cretaceous,  409  ; 
Cainozoic,  428,  429,  436,  440,  442, 
443,  448,  449,  450,  454,  455,  456, 
457,  458,  459,  460,  468,  469,  471, 
472,  474,  477,  478,  479,  480,  482. 

Mammoth,  271,  471,  472,  473. 

Mammoth  caves,  26. 

Man,  285,  286,  429,  478-480,  483, 
484. 

Manganese  in  earth's  crust,  176. 

Manuherikia  basin,  45. 

Map,  preparing,  537  et  seq, 

Maple,  421,  436,  452. 

Marble,  188,  208,  266. 

Marcasite,  197,  198,  392. 

Marine  denudation,  15;  plain  of,  86, 
87;  109. 

Marine  deposits,  lenticular  form  of, 
93,  388,  400  ;  classification  of,  96  ; 
thickness  of,  96,  97  ;  structure  of 
strata  of,  113-115. 


INDEX. 


581 


Marine  organic  deposits,  95. 

Marine  tuffs,  230,  231. 

Marjelen  See,  76. 

Marlborough,    New    Zealand,    flints 

at,  130  ;   moraine,  475. 
Marls,  206,  207,  363,  364,  367,  372, 

374,  375,  379,  380,  386,  399,  400, 

429,  441,  449,  451. 
Marlstones,  206. 
Marsh,  401. 
Marshall  Islands,  107. 
Marsupial  hyaena,  460. 
Marsupial  lion,  460. 
Marsupials,  426,  429,  452,  459,  460, 

474. 

Marsupites,  407,  414,  415. 
Marten,  457. 
Martin,  L.,  62. 
Maryland,  Permian,  366  ;   Cainozoic, 

449. 

Massive  limestone,  208. 
Massive  mineral  deposits,  507,  508. 
Master- joints,  151,  152,  153,  172. 
Mastodon,  448,  449,  450,  454,  456, 

459,  473,  482. 
Mastodonsaurus,  376. 
Matavanu  volcano,  228. 
Matrix  of  rocks,  126,  127. 
Matukituki  river,  avalanche  and  ice 

in  bed  of,  55. 
Mauiosaurus,  424. 
Mauna  Loa,  220. 

Mauritius,  coral  reefs  off,  107,  108. 
Maximum   stage   of    Glacial  Period, 

463,  464,  466,  467,  468  et  seq. 
Mayencian,  450. 
Mayo,  Ordovician,  326. 
Mechanically-formed  sediments,  199. 
Mediterranean,  233,  431,  448,  449- 

451  ;   basin,  Cainozoic,  431. 
Mediterranean    province     (Triassic), 

379. 

Mediterranean  region,  433. 
Mediterranean  seas,  99,  109. 
Mediterranean  Series,  449. 
Medlicottia,  366. 
Medusse,  Cambrian,  320. 
Megalodon,  347,  380. 
Megalosaurus,  394,  397,  408. 
Megascopic    examination    of    rocks, 

188,  244. 

Mekran  Series,  444,  450. 
Melania,  442,  450. 
Melanopsis,  450. 
Menevian  Series,  321. 
Mercury,  174;   a  mineral,  189. 


Mer-de-Glace,  59. 

Meristella,  336. 

Merostomata,  280,  281. 

Mesas,  425. 

Mesozoic  era,  304,  372-427  ;  vol- 
canic quiescence,  233. 

MetaUic  lustre,  186. 

Metamorphic  rocks,  199, 264-266, 267. 

Metamorphism,  259-263 ;  genesis, 
259  ;  contact-metamorphism,  259- 
262  ;  regional,  262-263. 

Metamorphism  of  rocks,  4,  5. 

Metasomatic  replacement,  521-524. 

Mexico,  Trias,  374,  378  ;  Cretaceous, 
419,  420,  421. 

Mica,  171,  185,  188,  189,  190,  191, 
194,  203,  206,  241,  242,  249,  264, 
265,  266. 

Mica-andesite,  257. 

Mica-diorite,  252. 

Mica-dolerite,  255. 

Mica-lamprophyre,  255. 

Mica-porphyrite,  254. 

Mica-schist-breccia,  201. 

Mica-schist,  crumbling  down,  32 ; 
cleavage  in,  171  ;  change  of  shale 
into,  260;  non-folded,  263  ;  occur- 
rence, 265  ;  Keewatin,  307  ;  Dal- 
radian,  312  ;  Cambrian,  320. 

Mica-slate,  171,  206. 

Mica-syenite,  251. 

Micaceous  quartz-schist,  265. 

Micaceous  sandstone,  204. 
!   Micaceous  shale,  207. 
I  Michigan,  Silurian,  333. 

Micrabacia,  407. 

Micraster,  407,  415,  416,  417. 

Microcline,  248. 

Microgranite,  254. 

Microgranulite,  254. 

Micrographic  texture,  249. 

Microlestes,  378. 

Microscopic  examination  of  rocks, 
188,  244. 

Middle  life,  372. 

Millepora,  394. 

Millepore  limestone,  394. 

Millstone  Grit,  352,  355. 

Millstones,  510. 

Mineral  character  of  igneous  rocks, 
244. 

Mineral,  definition  of,  176. 

Mineral  deposits,  classification,  505. 

Mineral  springs,  27. 

Minerals,  physical  properties  of, 
176-187 ;  formation  of  crystals, 


582 


A    TEXT-BOOK    OF    GEOLOGY. 


177-184;  cleavage,  183,  184; 
fracture,  184 ;  hardness,  184 ; 
tenacity,  185 ;  specific  gravity, 
185, 186  ;  lustre  and  feel,  186,  187  ; 
colour  and  streak,  187  ;  classifica- 
tion, 187. 

Mining  geology,  13. 

Miocene  Period,  304,  432,  433,  447- 
453,  481  ;  volcanic  activity,  234  ; 
fauna  and  flora,  448-449  ;  France, 
449 ;  Vienna  basin,  449-450 ; 
Switzerland,  450  ;  India,  450-451  ; 
North  America,  451-452  ;  Green- 
land, 452;  Australia,  452-453; 
New  Zealand,  Antarctic  region,  453. 

Mississippi,  32  ;  plains  near  the,  51  ; 
delta  swamp,  213. 

Mississippian  System,  359. 

Missouri,  Silurian,  333. 

Mitra,  448. 

Mitre  Peak,  New  Zealand,  75. 

Moa,  284. 

Moanataiara  fault,  157,  168. 

Mode  of  occurrence,  as  a  character  of 
igneous  rocks,  245,  246. 

Modiola,  394,  402,  423. 

Moffat,  336. 

Mohawkian  beds,  330. 

Moh's  scale  of  hardness,  184. 

Moine  thrust-plane,  313. 

Mojsisovics,  Trias,  378. 

Molasse,  441,  443,  450. 

Mollusca,  271,  277-279,  286  ;  in  the 
same  time-planes,  95  ;  in  the  sea, 
97 ;  trails  of  shellfish,  122 ;  in 
shelly  limestone,  209 ;  marine, 
the  most  abundant  fossils,  268 ; 
Ordovician,  331  ;  Silurian,  340  ; 
Devonian,  342,  345 ;  Carboni- 
ferous, 350  ;  Permian,  366  ;  Juras- 
sic, 387,  389,  402  ;  Cretaceous,  407, 
411,  412,  413,  414,  415,  416,  418, 
421,  422,  423,  424,  426, 427  ;  Cain- 
ozoic,  431,  432,  433,  435,  436,  437, 
438,  440,  442,  443,  446,  448,  449, 
450,  452,  453,  454,  455,  456,  457, 
458,  459,  460,  469,  471,  472,  477, 
482. 

Molluscoidea,  271,  276. 

Molybdenite,  266. 

Mon  Pelee,  222,  223. 

Monchique  type  of  lamprophyre,  255. 

Monoclinal  fold,  140. 

Monoclinic  system,  181,  182. 

Monocotyledons,  285,  286,  407,  418, 
427,  429,  443,  481. 


Monograptidse,  334. 

Monograptus,  336,  338. 

Monongahela  Series,  359. 

Monotis,  378,  380. 

Monotremes,  474. 

Mons,  416. 

Montana,  Algonkian  of  North,  309, 
310  ;  Cretaceouj^20,  421  ;  Cain- 
ozoic,  434, 

Montana  stage! 

Monte  Nuova,< 

Monte  Somma, 

Monterey  Series,  461. 

Montian  substage,  416. 

Monzonite,  251. 

Moraine-breccia,  201. 

Moraines,  lateral,  medial,  terminal, 
62-65,  78,  465,  468,  470,  472,  475, 
483;  ground,  65-67,  69,  75,  78, 
465,  466,  468. 

Moravia,  449. 

Morocco,  raised  beaches,  132  ;  Car- 
boniferous, 349  ;  Cretaceous,  406, 
418 ;  Cainozoic,  438. 

Morte  beds,  346. 

Morvern  Peninsula,  415. 

Mosasaurus,  408,  416. 

Mosses,  285. 

Mother  Lode,  518. 

Moths,  390,  394. 

Motion  of  glaciers,  55-58,  61. 

Mount  Aspiring,  avalanche  on,  55. 

Mount  Bischoff,  513. 

Mount  Egmont,  46  ;  rivers  on,  46  ; 
graphite,  214  ;  eruption,  220. 

Mount  Morgan  rivers,  23. 

Mount  Rosa,  New  Zealand,  68. 

Mount  Wapta,  British  Columbia,  322. 

Mountain  building,  146-148,  150, 
243,  296,  430,  448,  476. 

Mountain  chains,  Palaeozoic,  147  ; 
Cambrian,  324  ;  formed  by  Cale- 
donian folding,  343  ;  Armorican, 
365  ;  Permian,  370  ;  most  are  of 
Post-Miocene  age,  373. 

Mountains,  486,  488  et  seq. 

Mozambique,  land  connection  with 
Malabar,  107. 

Mud,  distribution  in  estuaries,  94  ; 
on  rising  sea-floor,  99  ;  deposition 
of  silt,  111,  112,  113,  115,  116,  123  ; 
sun-cracks  on,  120 ;  rain-  and 
hail-prints  on,  121  ;  hardening  of, 
126  ;  as  also  of  silt,  126. 
i  Muds,  100,  113,  116,  206;  Jurassic, 
387,  388,  390,  391. 


INDEX. 


583 


Mudstones,  5,  113,  228,  229. 
Mull,  Isle  of,  land  surface,  235. 
Munster,  333  ;    Old  Red  Sandstone, 

344. 

Murchison,  325,  333,  335,  342,  363. 
Murchisonia,  321,  347,  381. 
Murex,  435,  448,  450. 
Murray,  80.  ^^^ 
Muscovite,  JB       »241,  248,  249, 

260,  2<U,fld 
MuschelkaWT37jJ^,  376,  377,  379, 

384. 

Mushtuaire  River,  472. 
Mussel,  278. 
Myliobates,  436,  442. 
Myophoria,  375,  376,  379. 
Myrtle,  429. 
Mytilus,  380,  393,  410,  413,  423. 

N^GGERATHIOPSIS,  361,  382. 

Nari  Series,  444. 

Narrabeen  shales,  381. 

Nassa,  456. 

Nassau,  Devonian,  344. 

Natal,  Devonian,  344. 

Natica,    375,    394,    413,    423,    435, 

456. 

Naticella,  379. 
Naticopsis,  366. 
Natrolite,  242. 
Nautilus,   271,   279,   298,   317,   354, 

376,  393,  408,  416,  417,  419,  427, 

429,  477. 

Neanderthal,  480. 
Nebraska,  Permian,  366. 
Nebula?,  2. 

Nebular  hypothesis,  2,  6,  11. 
Neithea,  416. 

Neocomian,  409,  410,  411,  416,  419. 
Neogene,  432,  433. 
Neolithic,  483,  484. 
Nepheline,  190,  196,  241,  251,  257. 
Nephelirie -syenites,  243. 
Nephelinitoid  phonolites,  257. 
Nephrite,  184,  185. 
Neptunists,  14. 
Nerinaea,  395. 
Nests,  520. 
Neuropteris,  351. 
Neutral  axis,  154. 
Nevada,  257  ;    Cambrian  in  Central, 

319,  322  ;   Ordovician,  326  ;  Trias, 

378 ;     Jurassic,    402 ;     Cainozoic, 

451. 
Neve,    55,    56 ;     stratification,    57  ; 

foliation,  64. 


New  Caledonia,  reefs  off,  107. 

New  England,  37. 

New  Guinea,  Trias,  378 ;  Jurassic, 
388,  402. 

New  Jersey,  383  ;  Cretaceous,  420. 

New  Mexico,  Ordovician,  326. 

New  Red  Sandstone,  363. 

New  South  Wales,  Highland,  37; 
submerged  coals,  133  ;  faults  in, 
168,  169 ;  sandstone,  205 ;  age 
of  coals,  214 ;  graphite,  214 ; 
mica-schist,  265  ;  Ordovician,  326, 
330  ;  Silurian,  333,  340  ;  Carboni- 
ferous, 360,  361  ;  Trias,  381,  383  ; 
Jurassic,  402  ;  Cretaceous,  423  ; 
Cainozoic,  452,  473,  474;  saddle 
reefs,  515. 

New  York  State,  Silurian,  333,  339  ; 
oil  and  gas  in  Devonian,  348. 

New  Zealand,  erratics,  33  ;  plains, 
43,  51,  53,  86  ;  rivers,  44,  45,  55  ; 
ice  in,  54  ;  glaciers  of,  56,  57,  58  ; 
coast  of,  85  ;  currents  off,  89,  90  ; 
deep  sea  near  shore,  95  ;  differ- 
ence in  fauna  off  coast,  95,  96  ; 
flints  in  Upper  Chalk,  130  ;  raised 
beaches,  132 ;  drowned  valleys, 
134  ;  outliers  and  inliers  in,  146  ; 
faults  in,  168,  169,  170;  black 
sand,  203;  Oamaru  stone,  208; 
anthracite,  212 ;  coal  in,  214 ; 
craters,  223  ;  volcanoes,  232 ; 
igneous  rocks  in,  244  ;  rhyolites 
in,  257  ;  andesites,  257  ;  gneiss, 
264  ;  mica-schist,  265  ;  Dinornis 
of,  284  ;  conformity  in,  294,  296  ; 
Ordovician,  326 ;  Silurian,  333, 
340 ;  Carboniferous,  362 ;  Per- 
mian, 367  ;  Mesozoic  in  Alps  of, 
373  ;  Trias,  374,  378,  379,  381  ; 
Jurassic,  388,  402,  403,  404; 
Cretaceous,  406,  417,  424;  Ter- 
tiary land  connections,  431  ;  Cain- 
ozoic, 434,  436,  440,  444,  446,  448, 
453,  454,  460,  462,  468,  473,  474, 
475,  479,  481  ;  fiords,  498  ;  gold, 
507. 

Newark  System,  383,  384. 

Newcastle  Coal- Measures,  Australia, 
361. 

Ngauruhoe,  219. 

Niagara  Falls,  38,  39. 

Niagara  Series,  338,  339. 

Nickel-iron  in  serpentine,  525. 

Nile  valley  soil,  25  ;  plains  in  the 
valley,  51. 


584 


A    TEXT-BOOK    OF    GEOLOGY. 


Nitrogen  in  sea-water,  79  ;    free   as 

gas,  174  ;  affinity  of,  175. 
Noble  opal,  192. 
Nodosaria,  272. 
Norfolk,  Cretaceous,  410,  417  ;  Cain- 

ozoic,  455,  457  ;     Glacial  Period, 

467,  469. 
Norian,  378,  379. 
Norite,  243,  252,  258. 
Normal  folds,  140  ;  faults,  159,  173. 
Normandy,  Caen  Stone,  210. 
North  Auckland,  mollusca  of,  96. 
North  Island,  mineral  springs,  27. 
North  Sea,  Post-Pliocene,  462,  466, 

475. 

Northampton,  oolitic  ironstones,  210. 
Northumberland,  Carboniferous,  354, 

357. 
Norway,  84  ;   deep  sea  in  fiords,  95  ; 

raised  beaches,  132,  133  ;  drowned 

valleys,    134 ;     Cambrian    glacia- 

tion  in,  324  ;    Caledonian  folding, 

343  ;   Cainozoic,  481  ;   fiords,  498  ; 

silver,  511,  512. 
Norwich  Crag,  455,  456. 
Nothosaurus,  377. 
Nototherium,  460. 
Nottingham  coalfield,  357  ;    Glacial 

Period,  468. 

Nova  Scotia,  coals,  213. 
Nubia,  422. 

Nubian  sandstone,  422. 
Nucula,  351,  456,  457. 
Nuculana,  351. 
Nukumaru,  pebbles  at,  20. 
Nummulites,  272,  433,  437,  438,  439, 

451,  477. 
Nummulitic  limestone,  430,  434,  441, 

444. 
Nunatak,  65,  464,  474. 

OAK,  285,  421,  427,  436,  437,  442, 

443,  448,  452. 
Oamaru  Series,  453. 
Oblique  system,  181. 
Obsidian,  226,   240,   245,   246,   256, 

257. 
Ocean,    basins    not    permanent,    4 ; 

denuding  action  of,  15.     See  also 

Sea. 

Ochills,  346. 
Octahedron,  179. 
Octopus,  279,  477. 
Ogygia,  328,  330,  331. 
Ohio,  Silurian,  333  ;  Permian,  366. 
Oil,  451,  508,  509. 


Oil  shale,  206,  508. 

Oil  springs,  27  ;  Ordovician,  332  ; 
Devonian,  348. 

Old  Red  Sandstone,  205,  335,  340, 
342,  343,  344,  345,  346,  351,  352, 
363. 

Oldhamia,  321. 

Oldhaven  beds,  435. 

Olenellus,  322. 

Olenellus  beds,  321,  322. 

Olenus  beds,  321,  322,  323,  432. 

Oligocene  Period,  304,  432,  440-446  ; 
distribution,  441  ;  British  Isles, 
441-442 ;  Continental  Europe, 
442-443  ;  India,  444 ;  origin  of 
the  Flysch  facies  of  deposits,  444- 
445  ;  Central  Sea  or  Tethys,  445- 
446 ;  Baltic  Sea,  446. 
Oligoclase,  252. 

Olivine,  188,  190,  194,  195,  203,  204, 
242,  245,  248,  252,  253,  255,  257. 

Olivine-basalt,  258. 

Olivine-diabase,  255. 

Olivine-dolerite,  255,  358. 

Ontario,  333  ;  oil  in,  348. 

Onyx,  192. 

Oolite,  210,  391-400. 

Oolite,  Great,  391,  394,  395. 

Oolite,  Inferior,  391,  393,  394. 

Oolite,  Lower,  392-395. 

Oolite,  Middle,  395-397. 

Oolite,  Upper,  397-399. 

Oolitic  ironstones,  210. 

Oolitic  limestone,  210. 

Oomaru  stone,  208. 

Ooze,  95,  99,  100,  101,  414. 

Opal,  192. 

Ophiuroidea,  275. 

Opossum,  436,  450. 

Oppelia,  404. 

Orange  States,  sandstone,  205. 

Orbitolina,  407. 

Orders,  270,  271. 

Ordovician  Period,  304. 

Ordovician  System,  317,  325-332, 
335  ;  relationships,  325-326  ;  dis- 
tribution, 326;  rocks,  326,  327  j 
fauna,  327-328 ;  Arenig  group, 
331  ;  Llandeilo  group,  331  ;  Bala 
group,  332 ;  conditions  of  de- 
position, 332  ;  economic  products, 
332. 

Oregon,  lava- flow,  220  ;  Cretaceous, 
420  ;  Cainozoic,  434,  439. 

Ores,  189  ;  connection  with  eruption 
of  magmas,  524  et  seq. 


INDEX. 


685 


Orford,  456. 

Organic  deposits,   pelagic,    101-103, 

528. 
Organic  sediments,    199,   200,   208- 

215,  217. 
Origin  of  the  earth,  2  ;  igneous  rocks, 

5  ;  faults,  155,  156. 
Orinoco,  swampy  forests,  213. 
Oriskanian,  347. 
Orleans  conglomerate,  370. 
Ornamental  stones,  510. 
Ornithorhynchus,  474. 
Orogenic  action,  132,  155,  296. 
Orthis,  320,  328,  336,  338,  361. 
Orthoceras,  321,  328,  334,  338,  347, 

350,  354,  379. 
Orthoceratites,  366. 
Orthoclase,  244,  245,  247,  248,  249, 

251,  254,  255,  257  ;  pseudomorphs 

of,  182  ;    general  properties,  193, 

194 ;     in   sands,   203 ;     source   of 

kaolin,  207. 

Orthoclase-porphyry,  254. 
Osage  Series,  359. 
Osborne  beds,  442. 
Osmanda,  436. 
Ostrea,  381,  393,  394,  395,  396,  397, 

399,  402,  407,  410,  411,  413,  416, 

417,  422,  442,  450. 
Oswegan  Series,  338. 
Otago,  currents  off,  89 ;  igneous 

rocks  in,  244  ;  gold,  460. 
Otodus,  436. 
Otozamites,  382,  394. 
Otter,  440,  457. 
Outcrop,  143,  144,  150;    extent  of 

formation  not  indicated  by,  318, 

319. 

Outcrop  sag  or  curvature,  144,  150. 
Outliers,  145,  146,  150. 
Overdeepened  lake  basins,  73,  74. 
Overfold,  140. 
Overlap,  351. 
Overlap  faults,  159,  173. 
Overlap,  landward,  1 10, 1 14, 1 15,  335. 
Overlap,  seaward,  99,  109,  114. 
Overthrust,  143,  149,  150. 
Overthrust  folding,  157. 
Overturned  folds,  149. 
Owl,  457. 
Ox,  455,  471. 

Oxford  clay,  391,  395,  396. 
Oxfordian,  391,  395. 
Oxfordshire,  Stonesfield  slate,  394. 
Oxide  (sesqui-)  of  aluminium.     See 

Alumina. 


Oxide  of  iron,  formation  of,  21,  22, 
26,  29 ;  as  cementing  medium, 
127,  217  ;  as  base,  241  ;  in  clay- 
slate,  266.  See  Rust  and  Limonite. 

Oxides  of  iron,  175,  241. 

Oxide  of  magnesium.     See  Magnesia. 

Oxide  of  potassium.     See  Potash. 

Oxide  of  sodium.     See  Soda. 

Oxides  of  manganese,  128. 

Oxidised  zone,  520. 

Oxygen,  decomposing  action  of,  17, 
21,  22,  30,  31,  108,  242;  in  sea- 
water,  79,  108  ;  a  gas,  174  ;  affin- 
ity of,  175,  176  ;  its  percentage 
in  the  earth's  crust,  176  ;  a  con- 
stituent of  calcite,  189. 

Oyster,  271,  278. 

Ozone,  81. 

PACIFIC  Islands,  coral  reefs,  105,  107. 
Pacific  type  of  igneous  rocks,  243, 

244,  247. 

Palaeogene,  432,  433. 
Palaeohatteria,  366. 
Paleolithic,  483. 
Palaeontology,  14,  270. 
Palseopteris,  346. 
Palaeosaurians,  283. 
Palseotherium,  442,  443. 
Palaeozoic   era,    304,   317-324;    vol- 
canoes,   233,    234 ;     flora,    fauna, 

rocks,  minerals,  317. 
Palestine,    Cretaceous,    406 ;     Cain- 

ozoic,  438. 
Palissya,  381. 
Palms,  285,  429,  436,  437,  442,  443, 

448,  450. 

Panchet  stage,  380,  383. 
Panidiomorphic  structure,  240. 
Panopsea,  448. 
Paradoxides,  322. 
Paradoxides  beds,  321,  322. 
Paragenesis,  520. 

Parallel  Roads  of  Glenroy,  77,  500. 
Parasitic  cones,  225. 
Parasmilia,  407. 
Paris  basin,  442,  443. 
Particles  falling  in  water,  111,  112, 

113,  116. 
Passage  beds,  336,  378,  386,  413,  416, 

417. 
Patagonia,    244,    247  ;     Cretaceous, 

406,  421. 
Patches,  520. 

Patella,  278,  394,  395,  423. 
Paterina,  320. 


586 


A    TEXT-BOOK    OF    GEOLOGY. 


Pay-shoot,  520. 

Pay- wash,  506. 

Peach,  149,  150. 

Peak  caves,  Derbyshire,  26. 

Peas,  286. 

Peat,  211,  387,  388,  437,  508. 

Pebbles,  32,  200,  202,  203,  209,  433, 
441. 

Pebbly  limestone,  209. 

Pecopteris,  351,  368,  394,  402. 

Pecten,  366,  375,  376,  379,  384,  393, 
394,  395,  396,  397,  402,  413,  414, 
415,  422,  423,  451. 

Pectunculus,  448. 

Pegmatite,  249,  250. 

Pegmatitic  texture,  249. 

Pegu  System,  461. 

Pelagic  organic  deposits,  101-103. 

Pelagic  zone,  96. 

Peltoceras,  404. 

Pembrokeshire,  pre-Cambrian,  311  ; 
Glacial  Period,  467,  470. 

Peneplain,  46,  53. 

Pennine  chain,  45,  46,  353,  367. 

Pennsylvania,  faults  in,  168,  169 ; 
anthracites,  212 ;  Cambrian  in, 
319  ;  Silurian,  333  ;  oil  and  gas 
in  Devonian,  348  ;  Permian,  366. 

Pennsylvanian  sub-system,  359. 

Pentacrinus,  393. 

Pentamerus,  334,  336,  338. 

Perch,  457. 

Perched  blocks,  68,  69,  78. 

Peridotite,  194,  195,  241,  252,  253, 
258,  525. 

Periods,  303,  304 ;  distinguished  by 
kinds  of  life,  9,  303. 

Perlitic  structure,  240. 

Perm  Province,  Permian,  368. 

Permian  Period,  304,  317,  363-371, 
386,  387  ;  distribution  of  system, 
363 ;  rocks,  363-364 ;  relation- 
ship to  Carboniferous,  364  ;  con- 
dition of  deposition,  364,  365 ; 
fauna,  365-367  ;  British  Isles,  367- 
368  ;  ^Germany,  Russia,  India,  368  ; 
Africa,  368,  369  ;  North  America, 
369 ;  glaciation,  369-370 ;  economic 
products,  370,  371. 

Perna,  411,  423,  443. 

Persia,  Cretaceous,  406,  417,  419; 
Cainozoic,  430,  431,  438,  441,  444, 
448,  450,  482. 

Peru,  Silurian,  333  ;  Carboniferous, 
349;  Trias,  374,  378;  Jurassic, 
388,  403  ;  Cretaceous,  406,  421. 


Pervious  rocks,  25. 

Petit  Granit,  208. 

Petrified  plants,  26. 

Petrifying  springs,  215. 

Petrographical  provinces  of  igneous 
rocks,  242-244,  247. 

Petroleum.     See  Oil  Springs. 

Petrology,  13,  188. 

Pfahl,  518. 

Phacops,  330,  334,  338. 

Phanerogamia,  286,  286. 

Phascolotherium,  394. 

Phenocrysts,  240,  254,  257. 

Philippines,  232,  439. 

Phillips,  390. 

Phillipsia,  350,  354,  359. 

Pholadomya,  393. 

Pholas,  413. 

Phonolite,  196,  228,  242,  257. 

Phosphate,  332,  348,  412,  413,  451, 
456,  508. 

Phyllite,  171,  206,  260,  265  ;  Cam- 
brian, 320. 

Phylloceras,  403,  404. 

Phyllograptus,  330. 

Phyllotheca,  382. 

Physa,  399,  416. 

Pickwell  Down  Sandstone,  346. 

Piedmont  glacier,  59. 

Pierre  a  Bot,  69. 

Pike,  457. 

Pillow-structure,  228. 

Piltdown  Common,  480. 

Pilton  beds,  346. 

Pinacoceras,  379. 

Pinacoid  faces,  179,  180. 

Pines,  286,  452. 

Pinna,  393,  402,  413,  423. 

Pinnipeds,  440. 

Pinus,  443. 

Pipeclay,  207. 

Pipes,  520. 

Pisces,  271,  281.     See  also  Fishes. 

Pisolitic  limestone,  416. 

Pitch  of  folds,  153. 

Placentals,  440. 

Placer,  506. 

Placodus,  383. 

Placoparia,  330. 

Plagioclase,  241,  245,  247,  248,  252, 
254,  255,  267  ;  general  properties, 
193,  194  ;  in  granites,  249. 

Plain  of  erosion,  46. 

Plain  of  fluviatite  erosion,  46. 

Plain  of  marine  denudation,  86,  109. 

Plains.  488. 


INDEX. 


587 


Plain-tract,  497. 

Plaisancian,  458. 

Planes,  177,  421,  427,  436,  452. 

Planetismal  hypothesis,  6,  12. 

Planetismals,  7. 

Planorbis,  442,  443. 

Plants,  classification,  285,  286  ; 
Ordovician,  328,  329,  335;  Silu- 
rian, 335;  Old  Red  Sandstone, 
340  ;  Devonian,  342,  345,  346 ; 
Carboniferous,  350,  351,  355,  356, 
369,  360,  361  ;  Permian,  365,  366, 
367,  368,  370;  Mesozoic,  373; 
Trias,  375,  376,  377,  380,  381,  382, 
383,  384 ;  Jurassic,  387,  390,  394, 
396,  399,  401,  402;  Cretaceous, 

407,  410,  415,  418,  421,  422,  424, 
426,  427  ;  Cainozoic,  429,  440,  442, 
443,  448,  449,  450,  452,  457,  459, 
466,  468,  477,  478,  482. 

Plasma,  192. 

Plateau,  Colorado,  43. 

Plateau  gravels,  72. 

Plateau-Mountains,  490,  491. 

Plateaux,  43,  485,  493,  494. 

Platinum,  free,  as  solid,  174  ;  native, 
189,  506,  507  ;  in  peridotite,  525. 

Pleistocene  Period,  304,  432,  433, 
461-476  ;  glaciation,  461,  463  ; 
subdivisions,  463-464 ;  glacial 
evidences,  464-466  ;  local  glacia- 
tion, 466-470 ;  Continental  Europe, 
471-472;  Northern  Asia,  472; 
North  America,  473 ;  Southern 
Hemisphere,  473-475  ;  causes  of 
Glacial  Period,  475-476. 

Plesiosaurians,  283,  428. 

Plesiosaurus,  390,  392,  394,  396,  397, 

408,  421,  424,  481. 
Pleurotoma,  393,  396,  407,  413,  416, 

435,  436,  437,  456. 

Pleurotomaria,  321,  347,  354,  366, 
381. 

Pleurograptus,  330. 

Plication  of  strata,  142, 149. 

Pliocene  Period,  304,  432,  433,  453- 
460  ;  volcanic  activity,  234  ;  fauna 
and  flora,  454-455  ;  British  Isles, 
455-457  ;  Belgium,  457  ;  France, 
Italy,  458;  Vienna  basin,  458- 
459  ;  North  America,  459  ;  Aus- 
tralia, 459-460;  New  Zealand, 
460. 

Pliosaurus,  397. 

Plutonic  rocks,  237,  247,  248-263; 
sequence  of  crystallisation,  248  ; 


varieties  of  structure,  248-249 
granites,  249-251  ;  syenites,  261 
diorites,  252  ;  gabbro  type,  252 
peridotites,  253. 

Plutonists,  14. 

Pluvial  type  of  erosion,  486-488. 

Po  Series,  359. 

Pockets,  520. 

Polycotylus,  424. 

Polysynthetic  twinning,  193. 

Polyzoa,  271,  276,  365,  407,  456. 

Pomerania,  443. 

Pompeii,  221. 

Pondoland  Series,  422,  423. 

Pontian  Series,  448,  450. 

Popanoceras,  366. 

Poplar,  421,  436,  452. 

Porcellanite,  260. 

Porcupine,  455. 

Porifera,  271,  272. 

Porphyrite,  253,  254. 

Porphyritic,  250,  258. 

Porphyritic  structure,  240,  253,  254. 

Porphyritic  texture,  249. 

Porphyry,  253,  254. 

Porpoises,  440. 

Portland,  Isle  of,  398. 

Portland  sand,  398. 

Portland  stone,  210,  398. 

Portlandian,  391,  398. 

Portlock,  378. 

Portugal,  Devonian,  344 ;  Cretace- 
ous, 406,  418  ;  Cainozoic,  438,  448. 

Potomac  Series,  420. 

Potash,  as  base,  241. 

Potassium,  its  percentage  in  the 
earth's  crust,  176. 

Potassium  salts,  370. 

Pot-holes,  formation,  34. 

Potsdam  sandstone,  321. 

Pottsville  Series,  359. 

Pre-Cambrian  rocks,  307-316. 

Precious  stones,  506. 

Precipitation,  effect  on  glaciers,  57, 
58. 

Pre-glacial  drifts,  471. 

Prehnite,  242. 

Preservation  of  fossils,  268-269. 

Pressure  on  rocks,  125,  126  ;  effects 
of,  137-150,  171  ;  in  metamor- 
phism,  259,  260,  262,  263,  264,  267. 

Primary  era,  304. 

Primary  mineral,  191,  242. 

Primary  rock,  191. 

Principal  axis  of  crystals,  179. 

Principles,  1-12. 


588 


A   TEXT-BOOK    OF    GEOLOGY. 


Productus,  345,  347,  350,  354,  359, 

361.  362,  366,  368. 
Propylite,  257,  523  ;   veins,  519. 
Proterosaurus,  366. 
Protocardia,  378. 
Protozoa,  271,  272. 
Prussia,  Devonian,  344,  347. 
Psammobia,  422. 
Pseudomorphism,  130,  523. 
Pseudomorphs,  182. 
Pteraspis,  346. 
Pteria,  378. 
Pterichthys,  345. 
Pterinopecten,  351. 
Pterodactyls,  392,  401,  411,  421,  428. 
Pterodactylus,  397. 
Pterophyllum,  381,  382. 
Pteropod  ooze,  101. 
Pteropods,  102. 
Pterosaurus,  390,  408. 
Pterygotus,  281,  335,  338,  345. 
Ptilophyllium,  401. 
Pudding-stone,  202. 
Pug,  157. 
Pulverulent,  185. 
Pumice,  226. 
Punjaub,  439,  459. 
Pupa,  416,  472. 
Purbeck,  Isle  of,  398,  409. 
Purbeckian,  391,  397,  398,  399. 
Purpose  of  geology,  1. 
Pygaster,  389. 
Pyrenees,  147  ;    Carboniferous,  349  ; 

Mesozoic,    372,    373 ;     Cainozoic, 

431,  438,  447,  462,  471,  482. 
Pyrite,  17,  183,  186,  197,  198,  207, 

248,  264,  265,  266,  435. 
Pyroxene,  190,  195,  196,  241,  252. 
Pyroxene  diorite,  252. 
Pyrrholite,  197,  198. 

QUARTZ,  182,  188,  190,  191,  203,  204, 
205,  241,  242,  245,  247,  248,  249, 
251,  252,  254,  255,  257,  264,  265, 
266,  456,  517,  518,  520  ;  cleavage 
absent,  184 ;  position  on  scale  of 
hardness,  184  ;  general  properties, 
191,  192. 

Quartz-andesite,  257. 

Quartz-diabase,  255. 

Quartz-diorite,  243. 

Quartz-felsite,  253,  254. 

Quartzite,  260,  265,  266 ;  Algon- 
kian,  309 ;  pre-Cambrian,  313, 
320;  Ordovician,  326,  329,  330; 
Silurian,  340  ;  Mesozoic,  372. 


Quartz-mica-diorite,  252. 

Quartz-porphyry,  253,  254,  320. 

Quartz-schist,  265  ;  Dalradian,  312  ; 
Cambrian,  320.  * 

Quartzose-breccia,  201. 

Quarteose  conglomerate,  202. 

Quartzose  gritstone,  205. 

Queensland,  sandstone,  205 ;  Car- 
boniferous, 360,  361  ;  Trias,  381, 
383  ;  Jurassic,  402  ;  Cretaceous, 
423  ;  Cainozoic,  452. 

RADIAL  fold,  149. 

Radio-active  waters,  535. 

Radiolaria,  101,  103,  210,  271,  272; 
Cambrian,  320  ;  Jurassic,  389. 

Radiolarian  ooze,  103. 

Radium,  7. 

Rain,  action  of,  15,  30;  prints  of, 
120,  121. 

Rain,  work,  20-25 ;  chemical  effect 
of,  20-22  ;  effect  on  sulphides,  22, 
23  ;  mechanical  effects,  23  ;  earth- 
pillars  due  to,  23  ;  soil  produced 
by,  24,  25 ;  during  formation  of 
the  Flysch  series,  444. 

Raised  beaches,  132,  133,  482,  484. 

Rajmahal  Hills,  401. 

Rajmahal  Series,  380. 

Rajmahal  shales,  401. 

Ranikot  Series,  439. 

Rapid  earth-movements,  132,  134, 
135. 

Rate  of  denudation,  48,  49,  53. 

Rays,  389,  436,  442. 

Reading  Beds,  434,  435.     ' 

Recent  Period,  304,  432,  433,  476, 
481,  483,  484 ;  deposits,  476 ; 
fauna  and  flora,  477-478 ;  sub- 
divisions, 478-481. 

Recession  of  waterfalls,  38,  39. 

Recumbent  fold,  140,  149. 

Red  clay,  101,  102. 

Red  Crag,  455,  456. 

Red  Sea,  233. 

Reeds,  452. 

Refractory  clay,  207. 

Reindeer,  471,  480. 

Replacement  deposits,  511. 

Reptiles,  271,  281  ;  traces  of,  122  ; 
Palaeozoic,  317  ;  Permian,  366  ; 
Jurassic,  390,  392,  394,  395,  396, 
397,  398,  399,  401,  402;  Cain- 
ozoic, 477,  481. 

Reptilia,  271,  281,  283,  286,  383,  390, 
392,  394,  395,  396,  397,  398,  399, 


INDEX. 


589 


401,  402,  408,  409,  411,  421,  424, 

426,  436,  457. 
Residual  clays,  229. 
Residual  mountains,  492. 
Resinous  lustre,  186. 
Retreat  of  glaciers,  61,  62. 
Retreating  stage  of    Glacial  Period, 

463,  464,  468,  472,  475. 
Reversed  drainage,  75  ;    faults,  159, 

173. 

Rex,  brine  springs  at,  27. 
Rh»tic,  376,  377,  378,  380,  386. 
Rhine  Province,  Dyas,  368. 
Rhinoceros,  440,  448,  449,  450,  455, 

457,  469,  471,  472,  473. 
Rhizopoda,  271. 
Rhodesia,  sandstone,  205. 
Rhynchonella,    345,    347,    350,    366, 

392,  393,  394,  395,  407,  415,  477. 

Rhyolite,  241,  245,  246,  254,  256-257, 
258  ;  joints  in,  152. 

Richmond  beds,  330. 

Richter,  74,  75. 

Ridges  of  elevation,  233. 

Riebeckite-granite,  erratics,  470. 

Riesengebirge,  471. 

Rift- valleys,  495-496. 

Ripple-marks,  120,  323. 

River-drifts,  72. 

River-fans,  51. 

River-piracy,  47,  48,  53,  75. 

Rivers,  work  of,  15,  30-53  ;  de- 
velopment of,  41-46 ;  discharge 
of,  into  sea,  79  ;  effect  of  coastal 
recession  on  grading  of,  85,  108  ; 
deltas  of,  93  ;  Cainozoic,  471,  472. 

Road  metal,  510. 

Roches  Moutonnees,  68,  69,  78,  87, 
465. 

Rock-building,  111-124. 

Rock  crystal,  192. 

Rock-flour  below  glaciers,  67  ;  from 
glaciers,  71. 

Rock-forming  minerals,  188-189. 

Rock  salt,  26,  27,  99,  109,  177,  200, 
215,  216,  218,  339,  340,  368,  370, 
371,  372,  374,  375,  378,  384,  387, 

458,  508,  509,  535. 
Rock  structures,  125-131. 

JRocks,  texture  of,  4 ;  cementing 
^  medium,  4 ;  metamorphism,  5  ; 

action  of  rain  on,  20-25  ;  effects  of 
?  weathering  on,  22-25;  of  frosts, 
r  27,  28  ;  of  erosive  action  of  streams 
;  on,  35-49  ;  definition  of,  125,  188  ; 

composition  of,  241,  242;    Ordo- 


vician,  326,  327 ;  Silurian,  333, 
334;  Devonian,  344,  345;  Car- 
boniferous, 349,  350 ;  Jurassic, 
386 ;  Cainozoic,  429,  430,  433  ; 
temperature,  535,  536. 

Rocky  Mountains,  comparative 
youth,  147  ;  Cainozoic,  430,  448. 

Rodents,  448. 

Roestone,  210. 

Rolling  Down  Series,  423. 

Roof,  356. 

Roofing-slates,  509. 

Roses,  285. 

Ross,  475. 

Ross  ice  sheet,  475. 

Rostellaria,  407. 

Rotalia,  272. 

Rothliegende,  368. 

Rotomahana,  destruction  of  Pink 
and  White  Terraces,  222. 

Rotorua,  New  Zealand,  232  ;  sinters 
at,  27. 

Roxburgh  Flats,  45. 

Roxburgh  Gorge,  45. 

Ruahine  chain,  New  Zealand,  44. 

Ruapehu,  219,  226,  460,  475. 

Rubies,  506. 

Run-off,  25. 

Russia,  Cambrian,  320  ;  Ordovician, 
326  ;  Silurian,  333,  340  ;  Devon- 
ian, 342,  344  ;  Carboniferous,  349, 
350 ;  Permian,  363,  364,  368 ; 
Trias,  374 ;  Jurassic,  388,  400, 
403;  Cretaceous,  406,  411,  418; 
Cainozoic,  445,  471,  472. 

Rust,  20. 

Rutile,  203,  264,  265,  266. 

SADDLE-BEEFS, 332,513,514,515, 519. 
Sagenopteris,  382. 
Saghalien,  448. 

Sahara,  Cretaceous,  418,  422  ;  Cain- 
ozoic, 438. 

St  Cassian  beds,  379. 
St  David's,  pre-Cambrian,  311. 
St  Gallen  stage,  450. 
St  John  group,  321. 
St  Louis  Series,  359. 
St  Paul's  Cathedral,  210. 
St  Vincent,  221. 
Salina  Series,  338,  339. 
Salisbury  Plain,  434. 
Salix,  457. 

Salopian  beds,  336,  339. 
Salt,  439,  451. 
Salt  Lake  stage,  369. 


590 


A   TEXT-BOOK   OF   GEOLOGY. 


Salt  Range,  India,  Cambrian,  320, 
508 ;  Permian,  368,  370 ;  Cain- 
ozoic,  451. 

Salterella,  320. 

Saltholm,  417. 

San  Francisco  earthquake,  158. 

San  Juan  fault,  158. 

Sand,  rocks  worn  by  wind-driven,  19, 
20;  transportation  of,  88,  89; 
as  sorted  by  the  sea,  91-93  ;  as 
distributed  in  estuaries,  94 ;  on 
rising  sea-floor,  99  ;  deposition  of, 
111-113,115,117,118,119;  ripple 
marks  on,  120  ;  cementing  medium 
of,  126-128;  Jurassic,  387,  388, 
393,  397,  398;  Cretaceous,  405, 
411,413,414,425;  Cainozoic,  428, 
429,  433,  435,  436,  437,  441,  443, 
453,  456,  457,  458,  459,  469,  470, 
471,  476. 

Sandgate  beds,  411. 

Sandhills,  17. 

Sand-ripples,  18,  19. 

Sands,  32,  188,  198,  200,  203,  204, 
213,  215,  217,  347,  405,  410,  411, 
413,  418,  424,  425,  429,  433,  435, 
436,  437,  438,  439-441,  443,  450, 
451,  453,  456,  457,  458,  459,  463, 
466,  468,  469,  470,  471,  476,  510. 

Sands  as  moved  by  winds,  17,  18, 
118,  119. 

Sandstone-breccia,  201. 

Sandstone  conglomerate,  202. 

Sandstones,  5,  100,  113,  128,  143, 
189,  198,  203,  204,  205,  217,  228, 
231,  260,  266  ;  cleavages  in,  171  ; 
Algonkian,  309  ;  Torridonian,  312, 
313  ;  Palaeozoic,  317  ;  Cambrian, 
319,  320,  323  ;  Ordovician,  326, 
327,  328,  331  ;  Silurian,  333,  334, 
335,  336,  338,  340;  Devonian, 
342,  344,  346,  348  ;  Carboniferous, 
352,  353,  354,  355,  356,  360  ;  Per- 
mian, 363,  364,  366,  367,  368,  369, 
371  ;  Mesozoic,  372  ;  Trias,  374, 
375,  376,  377,  378,  380,  381,  382, 
383,  384,  385  ;  Jurassic,  386,  387, 
393,  395,  396,  400,  401,  402; 
Cretaceous,  418,  419,  422,  423, 
424;  Cainozoic,  429,  433,  434, 
438,  439,  441,  444,  449,  451,  452, 
456  ;  for  building,  509. 

Sanidine,  193,  257. 

Sarmatian  Series,  449,  450. 

Sassafras,  421. 

Satin-spar,  186. 


Saurians,  373,  374,  378,  381,  384, 
424. 

Savaii,  lava,  228. 

Saxony,  Dyas,  368 ;  Cretaceous, 
406,418;  lodes,  518. 

Saya  de  Malha,  107. 

Scandinavia,  glaciers,  59 ;  gneiss, 
264  ;  mica-schist,  265  ;  Eozoic 
rocks,  306,  313  ;  pre-Cambrian 
rocks,  313  ;  Cambrian,  319,  320  ; 
Ordovician,  326  ;  Silurian,  333  ; 
Pleistocene,  461,  462,  466,  467, 
469,  471,  475,  482. 

Scarborough  limestone,  394. 

Scarlet-red,  187. 

Sea wf ell,  331. 

Schist,  231,  307  ;  Algonkian,  309  ; 
pre-Cambrian,  311-315  ;  Lewisian, 
312  ;  Mesozoic,  372. 

Schist-conglomerate,  202. 

Schistose  rocks,  5,  264. 

Schizodus,  366. 

Schizoneura,  367,  382. 

Schwarz,  324. 

Scinde,  439,  444. 

Scope  of  geology,  13-29. 

Scoriae,  229. 

Scorpions,  271,  335. 

Scotland,  raised  beaches,  132,  133  ; 
overthrusts  in,  143,  149,  150  ;  Old 
Red  Sandstone,  205,  346  ;  till, 
207  ;  pre-Cambrian  of  North- West 
Highlands  of,  311,312;  Cambrian, 
319,  320,  321,  322  ;  Ordovician, 
326,  335  ;  Silurian,  333,  335,  340  ; 
Old  Red  Sandstone,  342,  344  ;  De- 
vonian elevation,  343  ;  Old  Red 
Sandstone  fossils,  345  ;  Carboni- 
ferous, 352,  353,  354,  356,  357, 
358 ;  Permian,  367 ;  Cainozoic, 
434,  437,  462,  466,  467,  469,  470, 
480,  481,  482,  483. 

Scott,  475. 

Screes,  368. 

Sea,  geological  work  of  the,  79-110  ; 
composition  and  volume,  80 ; 
effect  of  currents  of  the,  88  ;  sort- 
ing and  spreading  action,  91-93, 
113-115  ;  as  a  source  of  life,  97, 
109 ;  as  a  highway,  97,  109 ; 
variations  in  level  of,  98  ;  Cam- 
brian, 323  ;  Silurian,  339,  341  ; 
Devonian,  342,  343,  346,  347  ; 
Carboniferous,  351,  352 ;  Per- 
mian, 365;  Jurassic,  386,  387, 
388,  390,  394,  397,  401  ;  Cretace- 


INDEX. 


591 


cms,  405,  406,  407,  408,  410,  412, 
414,  415,  421,  422,  423,  426,  427  ; 
Cainozoic,  428,  430,  431,  433,  434, 
439,  440,  441,  444,  445,  446,  448, 
449,  453,  454,  455,  458,  481,  482, 
484. 

Sea-lilies,  271,  275. 

Sea-lions,  440. 

Seals,  271,  440,  457. 

Sea-mats,  271. 

Sea-saurians,  408. 

Sea-serpents,  408. 

Sea-snakes,  429,  436,  442. 

Sea-urchins,  271,  275,  334,  389,  407, 
414,  421,  422,  437,  438,  440,  443, 
446,  449,  453,  456,  477. 

Sea- weed,  285. 

Seas,  early,  3. 

Secondary  enrichment  of  veins,  520. 

Secondary  era,  304. 

Secondary  minerals,  191,  242. 

Secondary  rock,  191. 

Sectile,  185. 

Secular  movements,  4,  132. 

Sedges,  452. 

Sedgwick,  342. 

Sedimentary  rocks,  199-218. 

Sediment,  earth's  crust  mostly 
formed  by,  4,  111  ;  fossils  in,  97, 
98  ;  surface  markings  on,  119. 

Sediments,  folding  and  tilting  of,  5  ; 
alteration  of,  5,  11  ;  form  most  of 
the  earth's  crust,  4,  5  ;  fossils  in, 
97,  98. 

Sef strom  glacier,  470. 

Segregated  veins,  513,  514. 

Selenite,  189,  435. 

Semi-anthracite,  508. 

Senecan,  347. 

Senonian,  409,  413,  414,  415,  416, 
419,  421. 

Septaria,  129. 

Septaria  clay,  443. 

Septarian  boulders,  129. 

Sequoia,  436,  443,  452. 

Sericite,  194. 

Series,  303. 

Serpentine,  184,  190,  195,  242,  253, 
419,  525. 

Serpulites,  320. 

Shales,  5,  113,  115,  126,  143,  188, 
198,  206,  207,  215,  217,  260; 
cleavage  in,  170,  171  ;  Keewatin, 
307  ;  Algonkian,  309,  310  ;  Tor- 
ridonian,  312  ;  Palaeozoic,  317  ; 
Cambrian,  319,  320,  322;  Ordo- 


vician,  326,  327,  329,  330,  331, 
335  ;  Silurian,  334,  335,  336,  338, 
340  ;  Carboniferous,  350,  352,  353, 
354,  355,  357,  359,  360,  361  ;  Per- 
mian, 364,  365;  Mesozoic,  372; 
Trias,  374,  376,  380,  381,  382,  383, 
384 ;  Jurassic,  386,  387,  390,  391, 
393,  396,  397,  399,  400,  401,  402  ; 
Cretaceous,  405,  418,  420,  422, 
424;  Cainozoic,  429,  433,  434, 
441,  443,  444,  445,  451,  452. 

Sharks,  282,  298,  350,  436,  443 ; 
teeth  of,  102,  449. 

Shastan  Series,  420. 

Shear- breccias,  202. 

Shear-plane,  157. 

Shearing,  143,  149,  153,  155,  156, 
157. 

Sheets,  504. 

Shellfish.     See  Mollusca. 

Shelly  limestone,  208,  209. 

Shift,  160,  165. 

Shingle  deposits,  90-93,  113,  120. 

Shingle-flat,  91. 

Shingle-spit,  90. 

Shotover  River,  transporting  power, 
33. 

Shrinkage,  153. 

Shropshire,  205 ;  pre-Cambrian, 
311  ;  Ordovician,  326,  327,  331, 
332 ;  Silurian,  333 ;  Glacial 
Period,  467,  470. 

Siberia,  Ordovician,  326  ;  Devonian, 
344  ;  Carboniferous,  349  ;  Trias, 
378,  379;  Cainozoic,  445,  448, 
462,  472. 

Sicily,  raised  beaches,  132 ;  Per- 
mian, 363,  366  ;  Cretaceous,  407, 
418  ;  Cainozoic,  454,  458,  481. 

Sidlaw  Hills,  345. 

Sierra  Nevada,  401. 

Sierras,  Mesozoic  in,  373  ;  Cainozoic, 
430. 

Sigillaria,  351. 

Silica.     See  Siliceous  sinter. 

Silica,  215,  217,  241,  242,  255;  as  a 
cementing  medium,  127,  128,  217, 
266  ;  stability  of,  175,  176  ;  as  a 
pseudomorph,  183 ;  compounds, 
190  ;  in  organism,  210,  211  ;  in 
chert,  flint,  and  tripoli,  211. 

Silicate  of  alumina,  206,  207. 

Silicate  of  magnesia,  190. 

Silicates,  176,  189,  190. 

Siliceous  conglomerate,  128,  129,  202. 

Siliceous  glaze,  486. 


592 


A   TEXT-BOOK   OF   GEOLOGY. 


Siliceous  limestone,  208,  209,  210. 

Siliceous  quartzose  -  conglomerate, 
202. 

Siliceous  rocks,  200. 

Siliceous  sandstone,  129,  204. 

Siliceous  sinter,  27,  191,  192,  200, 
215,  217,  231,  232,  424,  525. 

Silicic  acid,  175,  176,  190. 

Silicified  trees,  452. 

Silicon,  affinity  of,  175  ;  its  percent- 
age in  the  earth's  crust,  176. 

Silky  lustre,  186. 

Sills,  236,  238,  245,  246,  255,  257. 

Silt.     See  Mud. 

Silures,  333. 

Silurian,  304,  317. 

Silurian  System,  333-341  ;  distribu- 
tion, 333  ;  rocks,  333-334  ;  fauna, 
334-335  ;  relationships,  335  ;  sub- 
division, 335-338 ;  Llandovery 
Series,  336 ;  Wenlock  Series,  336, 
338;  Ludlow  Series,  338;  North 
American  divisions,  338 ;  condi- 
tions of  deposition,  339,  340 ; 
Australasia,  340 ;  economic  pro- 
ducts, 340  ;  summary,  340,  341  ; 
Scotland,  346. 

Silurian,  Lower  (Murchison),  325. 

Silurian,  Upper  (Murchison),  333. 

Silver,  189,  323. 

Silver,  free,  as  solid,  174  ;  Palaeozoic, 
317;  infahlbands,  511. 

Simoceras,  404. 

Sinter,  27. 

Sirenians,  440. 

Siwalik  System,  459. 

Skates,  389. 

Skiddaw  slate,  329,  330,  331. 

Slates,  188,  198,  206,  207,  231,  260  ; 
crumbling  down,  32  ;  lamination, 
115  ;  cleavage  in,  170,  171,  173, 
260  ;  Palseozoic,  317  ;  Cambrian, 
320,  322,  323;  Ordovician,  330, 
331  ;  Silurian,  340 ;  Devonian, 
342,  346,  348;  Mesozoic,  372; 
Trias,  378. 

Slaty-breccia,  201. 

Slickensides,  152,  157,  164,  173. 

Slimonia,  338. 

Sloths,  474. 

Slow  earth-movements,  132-134. 

Smith,  W.,  14,  390. 

Snouts,  58. 

Snow,  action  of,  54-55. 

Snowdon,  332. 

Snowfields,  54. 


Snowline,  54. 

Soda,  as  base,  241. 

Soda,  orthoclase,  193. 

Sodalite,  251. 

Sodium,  its  percentage  in  the  earth's 
crust,  176  ;  a  constituent  of  salt, 
189. 

Soil,  formation  of,  23-25 ;  Pur- 
beckian,  399. 

Solenhofen,  oldest  bird,  284. 

Solfatara,  135,  231. 

Solfatara,  volcano,  231. 

Solfataric  stage,  231,  525. 

Solid  angle,  177. 

Somerset,  building  stone,  210  ;  De- 
vonian, 343  ;  Carboniferous,  353. 

Sorting  of  detritus,  111-113. 

South  Carolina,  Cretaceous,  420. 

South  Orkney  Islands,  Ordovician, 
326. 

South  Pacific  Islands,  coral,  103. 

South  Victoria  Land,  58,  59  ;  Sand- 
stone, 205  ;  Cambrian  in,  320  ; 
Cainozoic,  475. 

Southern  Alps,  New  Zealand,  rivers 
from,  32. 

Southland,  molluscs  of,  96. 

Spain,  raised  beaches,  132 ;  Cam- 
brian in,  319  ;  Ordovician,  326  ; 
Devonian,  344  ;  Trias,  378  ;  Cre- 
taceous, 406,  418  ;  Cainozoic,  448, 
454. 

Species,  270. 

Specific  gravity,  185,  186,  233. 

Specific  gravity  bottles,  186. 

Specular  iron,  265. 

Speeton,  412,  414,  469. 

Speeton  clay,  412. 

Spencer,  H.,  476. 

Sperenberg,  salt,  370. 

Sphene,  248. 

Sphenodon,  283,  401. 

Sphenophyllum,  351. 

Sphenopteris,  351,  381,  382,  394,  402, 
410,  422. 

Spheroidal  weathering,  22,  228,  229. 

Spherulites,  240. 

Spiders,  271,  334. 

Spindle  trees,  436. 

Spirifer,  345,  347,  350,  354,  366. 

Spiriferina,  350,  366,  375,  376,  379, 
381,  384,  389,  392. 

Spirula,  477. 

Spitzbergen,  Trias,  378  ;  Cretaceous, 
406  ;  Cainozoic,  448. 

Splintery  fracture,  184, 


INDEX. 


593 


Spondylus,  407. 

Sponges,  271,  272  ;    Cambrian,  320  ; 

Jurassic,  389  ;    Cretaceous,  407. 
Spongiae,  271,  407. 
Spotted  slate,  266. 
Springs,  action  of,  25-27  ;    kinds  of, 

25,  27;    hot,  216,  231,  232,  242, 

525,  529  et  seq. 
Spruces,  452. 
Spy  man,  480. 
Squid,  279. 
Squirrels,  450. 
Staffordshire,   160  ;    coalfields,  357  ; 

Glacial  Period,  467,  470. 
Stages,  303. 
Stalactites,  26,  196. 
Stalagmites,  26,  196. 
Stampian,  443. 
Stanley  sandstone,  205. 
Star  Series,  361. 
Starfishes,     275  ;      Cambrian,     320  ; 

Silurian,     334 ;       Jurassic,     393  ; 

Cretaceous,  407  ;    Recent,  477. 
Stars,  2. 

Station,  influence  of,  on  fauna,  95, 
'    96. 

Staurolite,  260. 
Steam    from    volcanoes,    231,    525 ; 

its  action  in  geysers,  232. 
Steatite,  187. 
Stegosaurus,  401. 
Stellaster,  393. 
Steneosaurus,  397. 
Step-fault,  167,  173. 
Stigmaria,  351. 
Stilbite,  242. 
Stinkstone,  210. 
Stockton  group,  383. 
Stockwork,  511,  513. 
Stone  age,  483. 
Stonesfield  slate,  394. 
Stormberg  Series,  382,  383. 
Strassfurt,  370. 
Strata,  111  ;  tilting  of,  135-137,  149  ; 

plication  of,  142  ;  measuring  thick- 
ness of,  545,  546. 
Stratification,  111-119,  122-124. 
Stratified  mineral  deposits,  505,  508- 

511. 

Stratigraphical  geology,  14. 
Streak,  187. 

Streams,  work  of,  4,  10,  15,  30-53. 
Stress  on  rocks,  125  ;  in  beams,  154  ; 

horizontal     shearing,     154,     155 ; 

cause  of  joints,  faults,  and  folds, 

156. 


Stricklandia,  334,  336. 

Strike,  136,  137,  537  et  seq. 

Strike  fault,  160,  173,  524 ;  effects 
of,  162. 

Stringocephalus,  345,  347. 

Stromboli,  219,  226. 

Strophomena,  328,  336. 

Structural  geology,  13. 

Stuben  sandstone,  376. 

Sturgeon,  282,  389,  457. 

Sub-aerial  denudation,  15. 

Sub-Apennine  Series,  458. 

Sub-glacial  debris,  63. 

Sub-kingdoms,  270,  271. 

Sublimation,  crystals  formed  by,  117. 

Submarine  volcanoes,  230. 

Submerged  coal,  133. 

Submerged  forests,  133,  134. 

Subsequent  rivers,  46. 

Subsidence,  4,  300,  301  ;  effect  on 
river  erosion,  42,  43  ;  on  sea-level, 
98  ;  deposition  during,  100  ;  earth 
movements,  132-150  ;  during  coal 
formation,  213 ;  Carboniferous, 
351;  Jurassic,  388,  389;  Creta- 
ceous, 406,  410,  420,  426  ;  Cain- 
ozoic,  441. 

Subsoil,  24. 

Succinea,  472. 

Sulphate  of  barium,  128. 

Sulphate  of  lime,  79,  138. 

Sulphate  of  magnesia,  79. 

Sulphate  of  potash,  79. 

Sulphates,  176,  200,  215,  216. 

Sulphides,  effect  of  rain  on,  22,  23. 

Sulphides  of  iron,  197. 

Sulphur,  free,  as  solid,  174  ;  crystals, 
177,  231  ;  in  fumaroles,  511,  525, 
526. 

Sulphuretted  hydrogen,  231. 

Sulphuric  acid,  175. 

Sulphurous  acid,  231. 

Sumatra,  232,  439. 

Summer  stage,  369. 

Summit  glaciers,  59. 

Sun,  2. 

Sun-cracks,  120,  323. 

Sunday's  River  beds,  422,  423. 

Superficial  mineral  deposits,  505- 
508. 

Surface  features,  development  of, 
485-503. 

Surface  markings,  119. 

Surf  ace -stress,  20. 

Surrey,  Cretaceous,  410  ;  Cainozoic, 
435. 

38 


594 


A   TEXT-BOOK   OF   GEOLOGY. 


Suru  Valley,  flood,  34;  ice-dammed 
lake  in,  76,  77. 

Sussex,  Cretaceous,  410. 

Sutherland,  Jurassic,  391. 

Sutton,  456. 

Swabian  Stone,  210. 

Sweden,  raised  beaches,  132  ;  eleva- 
tion of,  133  ;  Silurian,  340  ;  Creta- 
ceous, 417. 

Switzerland,  149. 

Switzerland,  Cretaceous,  406,  418; 
Cainozoic,  443,  450. 

Syenite,  238,  243,  245,  251,  258; 
Lewisian,  312. 

Syenite -porphyry,  254. 

Symbols,  geological,  216,  217. 

Symmetrical  folds,  139. 

Synclinal  axis,  139. 

Syncline,  138,  139,  149 ;  effect  of 
dip-fault  on,  166,  167. 

Synclinoria,  141. 

Syria,  Cretaceous,  406,  419,  422 ; 
Cainozoic,  438,  448 ;  rift-valley, 
495. 

Syringopora,  350. 

Systems,  303. 

TABLE  Cape,  452. 

Table  salt,  189. 

Tabular  crystals,  182. 

Tachylite,  226,  245,  246,  257. 

Tachylite  glass,  245. 

Tseniopteris,  381,  394,  401,  402. 

Tahiti,  volcanic  rocks,  244. 

Taigas,  472. 

Taireri  Moraine,  474,  475. 

Talc,  in  scale  of  hardness,  184  ;  feel 
of,  187  ;  composition  of,  190  ;  in 
chlorite-schist,  265  ;  in  talc-schist, 
265  ;  in  marbles,  266. 

Talc-schist,  265. 

Talchir  beds,  368,  370,  380. 

Talus,  62. 

Tapes,  448. 

Tapir,  436,  440,  450. 

Taranaki,  244. 

Tarannon  shales,  336. 

Tarawera,  New  Zealand,  116,  219, 
220,  221,  222. 

Tarns,  73. 

Tarr,  R.  S.,  62. 

Tasersuak  lake,  76. 

Tasman  glacier,  58,  59. 

Tasmania,  ice  in,  54 ;  Cambrian, 
320,  321  ;  Ordovician,  326  ;  Silu- 
rian, 333,  340 ;  Carboniferous, 


360  ;  Trias,  383  ;  Cainozoic,  440, 
452,  453,  473,  474,  481. 

Taupo,  244. 

Taxites,  394,  422. 

Tectonic  veins,  519. 

Teleosaurus,  394,  397. 

Teleostei,  281,  282,  390,  408. 

Tellina,  448,  457. 

Temperature,  effects  of  changes  of, 
20 ;  effect  on  glaciers,  57,  58 ; 
gradient  in  the  earth,  203  ;  Ter- 
tiary, 431,  436,  446,  448,  452,  455, 
456,  457,  461,  462,  463,  464,  466- 
476,  482,  483  ;  of  rocks,  535,  536. 

Tenacity,  185. 

Tennessee,  Cambrian  in,  319  ;  Ordo- 
vician, 327,  332  ;  phosphates  in, 
348  ;  Cretaceous,  420. 

Tension,  153,  154,  155,  172. 

Tension  cracks,  154. 

Terebra,  448,  456. 

Terebratella,  407. 

Terebratula,  350,  354,  366,  375,  376, 
379,  381,  384,  389,  393,  394,  395, 
404,  407,  411,  413,  417,  477. 

Terebratulina,  415. 

Terraces,  50,  69,  70,  77. 

Terrestrial  deposit,  133. 

Tertiary,  304 ;  volcanic  activity, 
234. 

Tethys,  430,  444-446,  447,  448,  451, 
481. 

Tetragraptus,  328,  331. 

Tetrahedral  hypothesis,  9. 

Texas,  gushers  in,  27  ;  Permian, 
366,  369  ;  Trias,  384  ;  Cretaceous, 
419,  420  ;  Cainozoic,  439,  459. 

Texture  of  rocks,  4,  239,  240,  241, 
245,  246,  247. 

Thalassic  zone,  96. 

Thanet  Sands,  434,  435. 

Theriodontia,  383. 

Thermal  springs.     See  Springs. 

Thian  Shan,  147,  148. 

Thickness  of  marine  deposits,  96,  97. 

Thinnfeldia,  381,  382. 

Throw,  160,  161,  162,  165. 

Thrust-blocks,  492. 

Thrust-planes,  157,  162,  164. 

Thuringia,  Dyas,  368. 

Tibet,  Jurassic,  389,  401  ;  Cretace- 
ous, 406,  419. 

Tides,  action  of,  82,  83,  113,  120. 

Tierra  del  Fuego,  232,  473. 

Till,  65,  78,  207,  465,  469. 

Tillite,  369. 


INDEX. 


595 


Tilting  of  sediment,  5,  125,  135-137, 

340. 

Timaru,  86. 

Time.     See  Geological  Time. 
Time-plane,  95,  303. 
Tin  ore,  196,  261  ;    Palaeozoic,  317  ; 

in  deposits,  506,  507,  513,  517. 
Tinds,  75. 
Tip,  118. 
Titanic  iron,  203. 
Titaniferous  iron,  197. 
Titanite,  197,  198,  525. 
Tomago  Series,  361. 
Tongariro,  460. 
Tongrian,  443. 

Topaz,  in  scale  of  hardness,  184. 
Topography,  influence  of  the  sea  on, 

84-88. 

Torbane,  oil  shales,  357. 
Torrent  tract,  497. 
Torridonian  Period,  304,  312  ;   rocks, 

313. 

Tortoises,  401,  436,  450. 
Tortonian,  450. 
Tough,  185. 
Tourmaline,  185,  190,  196,  203,  251, 

261,  264,  265. 
Tourmaline-granite,  250. 
Trachyloid  phonolites,  257. 
Tourmaline-rock,  196. 
Trachyoceras,  379,  384. 
Trachyte,  245,  257. 
Trachyte  glass,  245. 
Trails,  of  animals,  120, 122,  374,  375; 

in  Algonkian  rocks,  310  ;   in  Cam- 
brian sediment,  323. 
Transition  beds,  364. 
Transporting    power    of    rivers,    33  ; 

of  glaciers,  62,  67,  68-72  ;    of  sea 

currents,  89. 
Transvaal  plateau,  46. 
Transylvania,  brown  coals  of,  214  ; 

andesites,  257. 
Travertine,  26,  196,  215. 
Tread  well  mines,  511. 
Tremadoc  slates,  321,  322. 
Trematosaurus,  375. 
Tremnocidaris,  417. 
Tremolite,  266. 
Trenton  limestone,  330,  332. 
Triassic  System,  344,  363,  372-385, 

386,    387,    390 ;     mammals,    285 ; 

rocks  and  distribution,  373-374 ; 

German   facies,  373-378 ;    Bunter 

Series,   375 ;    Muschelkalk  Series, 

375,    376 ;     Keuper    Series,    376 ; 


Great  Britain,  377-378;  Alpine 
or  marine  beds,  378-380  ;  India, 
380  ;  Australasia,  381  ;  Antarctic 
Continent,  381,  382  ;  South  Africa, 
382,  383  ;  North  America,  383  ; 
surface  features,  384 ;  economic 
products,  384,  385. 

Triclinic  System,  181,  182. 

Trigoiiia,  366,  381,  393,  394,  395, 
396,  407,  423,  460. 

Trilobites,  279,  280,  373;  Palse- 
qzoic,  317;  Cambrian,  320;  Ordo- 
vician,  327,  328,  329,  330,  331  ; 
Silurian,  334,  335,  336,  338,  340  ; 
Devonian,  342,  345,  348  ;  Carboni- 
ferous, 350,  354  ;  Permian,  366. 

Trimetric  System,  180,  182 

Trinucleus,  328,  329,  330. 

Tripoli,  211. 

Triton,  416. 

Trochocyathus,  407. 

Trochus,  394,  423. 

Trough-fault,  167,  168,  173. 

Trough-limbs,  139. 

True  bottom,  506. 

Tuatara,  283,  401. 

Tufaceous  sandstones,  231. 

Tuff  cones,  224. 

Tuffs,  230,  327,  329,  331,  332,  345, 
350,  358,  361,  453,  458. 

Tunbridge  Wells  Sand,  411. 

Tundras,  472. 

Tunis,  elevation  of,  135;  Cretace- 
ous, 406,  418. 

Turbo,  380,  393,  422,  423. 

Turkestan,  Trias,  378 ;  Cainozoic, 
472. 

Turkey,  Devonian,  344. 

Turonian,  409,  413,  414,  415,  419. 

Turrilites,  408,  481. 

Turtles,  408,  421,  436,  442. 

Tuscaloosa  Series,  420. 

Twin  crystals,  183,  193. 

Tyrol,  dolomite,  209  ;  Trias,  379. 

Tyrone,  Old  Red  Sandstone,  344 ; 
Permian,  367. 

UINTA  structure  of  mountains,  490, 

491. 

Uitenhage  Series,  422. 
Ulster,  Ordovician,  326. 
Ulsterian,  347. 

Ultra-basic  group  of  rocks,  242. 
Umzamba,  423. 
Uncites,  345. 
Unconformity,  291-294,  296,  297,  299. 


596 


A    TEXT-BOOK    OF    GEOLOGY. 


Under-clay,  207,  213,  356. 

Undulations,  125. 

Uneven  fracture,  184. 

Ungulates,  429,  448,  457,  459. 

Unio,  393,  399,  410,  442,  450. 

United  States,  specific  gravity  ob- 
servations, 233  ;  andesites,  257  ; 
Calciferous  Series,  322 ;  Ordovi- 
cian,  326,  327  ;  Carboniferous, 
349,  350;  Cretaceous,  406,  417, 
420,  421,  425,  439. 

Univalves,  271,  278. 

Unoxidised  zone,  520. 

Unstratified  mineral  deposits,  505, 
511-527. 

Uplift.     See,  Elevation. 

Upper  Marine  Series,  Austrian  Car- 
boniferous, 361. 

Upthrow,  161. 

Urals,  Mesozoic,  372,  373 ;  Cairi- 
ozoic,  471. 

Urgonian,  416. 

Utah,  rock  salt,  216 ;  Ordovician, 
326 ;  Jurassic,  401  ;  Cretaceous, 
417. 

Uticah  beds,  330. 

VALENTIAN  beds,  336. 

Valley  glaciers,  59,  73,  75. 

Valley  protected  by  basalt,  48. 

Valley  tract,  497. 

Valley  trains,  71,  72,  78. 

Valleys,  3,  73  ;  drowned,  84  ;  for- 
mation of,  487  et  seq.  ;  494-498. 

Vancouver  Island,  Senonian,  423. 

Variscan,  a  pre-Pliocene  mountain 
chain,  147,  148. 

Vein-cavities,  origin  of,  517,  518. 

Veins,  filling  of  mineral,  517  ;  age 
of  rilling  of,  518,  519  ;  formation, 
528. 

Veldt,  46,  485. 

Velocity  of  rivers,  33. 

Ventral  valve,  276. 

Venus,  413,  450. 

Vertebrata,  271. 

Vertical  dip,  136. 

Vertical  planes,  180. 

Vesuvian  type  of  volcanic  eruption, 
220,  235. 

Vesuvius,  219,  220,  221,  222,  226, 
243. 

Victoria,  Australia,  47  ;  outliers  in, 
146  ;  sandstone,  205  ;  lava-flow, 
220  ;  basaltic  plateau,  220  ;  Ordo- 
vician, 326,  330 ;  Silurian,  333, 


340 ;  Carboniferous,  360,  361  ; 
Permian,  369  ;  Jurassic,  402  ; 
Cainozoic,  452,  453,  460;  gold, 
507  ;  saddle-reefs,  513,  514. 

Victoria  Land,  382. 

Vienna  basin,  449,  450,  458. 

Vines,  436,  442. 

Virginia,     Cambrian,     319 ;      Ordo 
vician,  327  ;   Cretaceous,  420. 

Vitreous  lustre,  186. 

Viviparus,  410,  442. 

Volcanic  action,  219-258;  Car- 
boniferous, 358,  360. 

Volcanic  activity,  148,  327,  331,  332, 
334,  345,  361,  373,  406,  419,  424, 
430,  434,  436,  437,  451,  452,  453, 
454,  458,  459,  460,  461,  476,  481. 

Volcanic  agglomerate,  230. 

Volcanic  ash,  451,  473. 

Volcanic  bombs,  229. 

Volcanic-breccias,  202,  230,  474. 

Volcanic  glass,  226. 

Volcanic  neck,  238. 

Volcanic  rocks,  255-258  ;  Cambrian, 
320  ;  Carboniferous,  358,  360,  361. 

Volcanic  tuff,  230,  474. 

Volcanoes,  219-258,  489,  490  ;  active, 
219-238  ;  dormant,  219,  234,  235  ; 
extinct,  219,  223,  233,  234,  235; 
sites,  220  ;  eruptions,  220-223  ; 
volcanic  plug,  223 ;  craters,  223  ; 
cones,  223-225 ;  lava  streams, 
225-226  ;  columnar  structure,  226- 
228  ;  pillow  structure,  228  ;  spher- 
oidal weathering,  229 ;  amyg- 
daloidal  structure,  229  ;  frag- 
mentary or  pyroclastic  ejecta,  229, 
230  ;  steam  and  gaseous  emana- 
tions, 231  ;  expiring  volcanic 
activity,  231  ;  thermal  springs, 
231,  232  ;  geysers,  232  ;  distribu- 
tion of,  232,  233  ;  origin  of,  233  ; 
former  activity,  233-234  ;  old  land 
surfaces,  234,  235. 

Voltiza,  375. 

Voluta,  436. 

Vosges,  471. 

Vughs,  183. 

Vulcanists,  14. 

WADHUBST  clay,  411. 

Waipara,  conformity  at,  294. 

Waipara  Series,  424,  425. 

Waitomo  Caves,  New  Zealand,  26. 
!   Wakatipu  glacier,  56. 
!  Walchia,  366,  394. 


INDEX. 


597 


Wales,  slates,  171  ;  anthracites, 
212 ;  Cambrian  rocks  of  North, 
319,  322  ;  Ordovician,  326,  327, 
331  ;  Silurian,  333,  335,  340 ; 
Old  Red  Sandstone,  342,  344; 
Devonian  elevation,  343 ;  Car- 
boniferous, 352,  353  ;  coalfields, 
357  ;  Jurassic,  391  ;  Glacial 
Period,  467,  470. 

Walnut,  285,  452. 

Walrus,  457. 

Wanganui  plain,  43,  44. 

Wash,  84. 

Washington,  lava-flow,  220;  Cre- 
taceous, 420  ;  Cainozoic,  434,  439. 

Wash-outs,  97. 

Water,  action  of,  3,  52,  242,  520; 
chemical  effects  of  rain-,  20-22, 
520  ;  freezing  of,  27,  28  ;  solvent 
power  of,  30,  31  ;  influence  of  rapid 
flow  of,  31-35  ;  a  liquid,  174,  176  ; 
use  of,  in  specific  gravity  test,  185, 
186 ;  magmatic,  233 ;  in  meta- 
morphism,  259,  261,  262;  from 
bore -holes  in  Rolling  Downs  Series, 
423;  meteoric,  527  ;  deposits  from, 
527  ;  supply,  529-535. 

Waterfalls,  37,  380;  recession  of, 
38,  39. 

Water-fleas,  281. 

Water-lilies,  452. 

Waterlime  hydraulic  limestone,  339. 

Water-surface,  1. 

Waves,  action  of,  82,  83 ;  from 
Krakatoan  explosion,  222. 

Weald,  410. 

Weald  clay,  409,  410,411. 

Wealden,  409,  410,  414. 

Weathering,  15,  21,  22,  23  ;  zone  of, 
520. 

Weka  Pass  stone,  424. 

Wenlock  limestone,  336. 

Wenlock  Series,  336. 

Wenlock  shale,  336. 

Werner,  A.  G.,  14. 

West  Virginia,  Permian,  366. 

Western  Australia,  marine  erosion, 
87. 

Westland,  New  Zealand,  gold,  460. 

Westmoreland,  Glacial  Period,  467, 
468. 

Westphalia,  coals,  213  ;  Devonian, 
344;  Carboniferous,  349;  Creta- 
ceous, 406,  413. 

Weybourn  Crag,  457. 

Weymouth,  409,  410. 


Weymouth  Crag,  455. 

Whales,    271,   440,   442,    448,    453, 

457. 

Whetstones,  510. 
Whitby,  jet,  213. 
White  Island,  New  Zealand,  gypsum, 

216,  226. 

White  Stone,  210. 
Wianamatta  shales,  381.      > 
Wieliczka,  458,  509. 
Wilckens,  453. 
Wild  duck,  457. 
Wild  goose,  457. 
Willow,  421,  427,  436,  457. 
Wiltshire,  Middle  Oolite,  396. 
Wind,  work  of,  17,  20,  113,  118,  119, 

120. 

Winding  of  streams,  39-41. 
Windsor  Castle,  205. 
Wiry  fracture,  184. 
Wisconsin,  ores,  332. 
Wolf,  457. 
Wolfram,  261. 
Wood  beds,  422. 
Woodocrinus,  350. 
Wood-opal,  192,  235. 
Woolhope,  336. 
Woolhope  limestone,  336. 
Woolwich  and   Reading    beds,   434, 

435. 
Worcester,  South  Africa,  conformity 

at,  294. 

Worms.     See  Annelids. 
Wrekin,  pre-Cambrian,  311. 
Wiirtemburg,  Swabian  Stone,  210. 
Wyandotte  Caves,  Indiana,  26. 
Wynyardia,  452. 

Wyoming,  Ordovician,  326  ;  Juras- 
sic, 401  ;  Cretaceous,  417,  425  ; 

Cainozoic,  434. 

YAKUTAT  Bay,  468. 

Yang-tse-kiang,  32  ;  plains  near  the, 
51. 

Yellow  ground,  22. 

Yellowstone  National  Park,  mineral 
springs,  27,  232  ;  silicified  trees, 
452. 

Yilgarn  goldfield,  46. 

Yoldia,  457,  471. 

Yoredale  beds,  35,  51. 

Yorkshire,  master-joints,  152  ;  sand- 
stone, 205 ;  Carboniferous,  354, 
357  ;  Permian,  367  ;  Cornbrash, 
392  ;  Inferior  Oolite,  393  ;  Middle 
Oolite,  396  ;  Cretaceous,  410,  411, 


598 


A    TEXT-BOOK    OF    GEOLOGY. 


412,  413 ;  Glacial  Period,  468,  469, 
470. 
Y-Tryfaen,  332. 

ZAMITES,  394,  422. 
Zechstein,  368. 
Zeolites,  242. 


Zeuglodons,  440,  453. 

Zinc-blende,  189. 

Zinc  ores,  332,  517. 

Zircon,  203,  248,  257. 

Zones   in   marine   deposits,   96,    111, 

303. 
Zoology,  270. 


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